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PLAGUE

[PLAGUE]

ITS CAUSE AND THE MANNER OF ITS EXTENSION—ITS MENACE—ITS CONTROL AND SUPPRESSION—ITS DIAGNOSIS AND TREATMENT

BY
THOMAS WRIGHT JACKSON, M.D.

MEMBER AMERICAN RED CROSS SANITARY COMMISSION TO SERBIA, 1915; LATELY CAPTAIN AND ASSISTANT SURGEON, U. S. VOLUNTEERS; LATELY LECTURER ON TROPICAL DISEASES, JEFFERSON MEDICAL COLLEGE; MEMBER OF MANILA MEDICAL SOCIETY AND PHILIPPINE ISLANDS MEDICAL ASSOCIATION; AUTHOR OF A TEXT BOOK ON TROPICAL MEDICINE; DIRECTOR, DEPARTMENT OF SANITATION AND EPIDEMIOLOGY FOR H. K. MULFORD COMPANY

WITH BACTERIOLOGIC OBSERVATIONS
BY
DR. OTTO SCHÖBL
BUREAU OF SCIENCE, MANILA

ILLUSTRATED

PRESS OF
J. B. LIPPINCOTT COMPANY

COPYRIGHT, 1916
BY THOMAS WRIGHT JACKSON, M.D.

THIS BOOK IS [DEDICATED] BY THE AUTHOR TO
DR. ALDO CASTELLANI

REGIUS PROFESSOR OF TROPICAL DISEASES, UNIVERSITY OF NAPLES. EMINENT IN MEDICAL RESEARCH, MY FRIEND, COLLEAGUE AND COMRADE DURING STRENUOUS DAYS IN SERBIA.

[CONTENTS]

[ILLUSTRATIONS]

PLAGUE

ITS CAUSE AND THE MANNER OF ITS EXTENSION—ITS MENACE—ITS CONTROL AND SUPPRESSION—ITS DIAGNOSIS AND TREATMENT

[INTRODUCTION]

The question of the need for new books upon medical topics must ever remain undecided, by general agreement, in the medical profession.

There is no such thing in medical literature as an insistent demand from the profession for new volumes upon old topics.

Authors need not hope, therefore, to create the impression that they are meeting long-felt though unexpressed wants of medical readers in launching new books.

On the other hand, the creator of a new volume upon an old subject should seek justification for literary paternity in the progressive changes in the status of our knowledge of disease, its causes, prevention, and cure. Such changes are admittedly going on with a certain degree of constancy and at such a rate of frequency that new presentations of old themes, are both justified and desirable from time to time.

With this idea in mind and with the desire to present, in useful and practical form, a work which shall contain at least some unhackneyed material and which shall represent modern studies and a record of actual control work done in this justly-dreaded disease, the following pages are submitted to the medical profession and to sanitarians generally.

With a profound respect for the laboratory worker and his work and with a profound conviction that to him belongs the greater measure of credit for real accomplishment in connection with plague up to the present time, I desire to insist that the true utility of knowledge gained within laboratory walls lies in its intelligent application in the outer world and that ofttimes this application must be made by men who are themselves without extended laboratory training. An appreciation of principles—with an intelligent ability to accept, to appropriate, to apply and, most of all, to refrain from entering without due preparation the domain of the laboratory worker—is an indispensable requisite in the equipment of the practical sanitarian, upon whom must fall the responsibilities of success or failure in combating the disease we are now to consider.

During the past fourteen years it has been my privilege to observe two epidemics of plague in the Philippine Islands. Some of these observations were made in the capacity of a military medical officer, but my later observations, upon which this report and study are chiefly based, were made from the view-point of a civil health officer. At different times I have been called upon to deal with the disease both as sanitary officer and clinician, and from October, 1912, to July, 1914, I had charge of all plague suppressive measures in Manila. In 1914 I was also in charge, as acting chief, of the San Lazaro Hospitals Division of the Bureau of Health, Manila, where all cases of plague are brought, either for treatment or autopsy.

As some of the material which I have collected for text-book articles during the past eight years bears directly upon the present discussion and presentation, I have ventured to quote from it, sometimes without rephrasing, such parts as are accurate at the present time. I am also quoting freely from the records and from the experiences of my predecessors and colleagues in the work in Manila.

It should be understood that the pathology of the disease has been practically omitted from consideration as out of place in an epidemiologic investigation and report. The pathologic side of the work during the Manila epidemic of 1912–1914 was covered in a masterly manner by Dr. B. C. Crowell and his associates at the Medical School of the University of the Philippines, and I have no doubt that the record of the work done and studies made will appear in appropriate form in due time and will hereafter be referred to as among the most valuable pathologic studies ever made during a plague epidemic, on account of their accuracy and completeness.

I have included, as of great value and directly related to the epidemiologic phase of this study, reports of some of the bacteriologic work done in connection with this epidemic at the Bureau of Science, Manila, by Dr. Otto Schöbl. I am sure that the value of his studies as reported in part here, with his permission, will be apparent to every careful reader. I am greatly indebted to him for his permission to make use of this portion of his studies. Having been in daily touch with Dr. Schöbl during the year and a half of the continuance of this epidemic, I can appreciate to the fullest extent the painstaking and accurate character of his work and findings, of which the part here presented is by no means the greatest.

I am quite aware of the fact that there are those who view with some question the practicability of controlling plague by the measures applied in Manila, as recited here; but American plague workers are likely to meet this unbelief by pointing to the accomplished fact, in San Francisco, in Honolulu, in Porto Rico, as well as in Manila; and before long, as we confidently expect, in New Orleans.

These exponents of the school which contends that plague epidemics are little affected by rat-excluding, rat-destroying and rat-proofing efforts, believe that the waning and disappearance of epidemic plague in a given place depend in chief part upon the exhaustion of susceptible material among the rodent population. However appealing this argument may be, it is impossible for its exponents to duplicate American results with equal results in the cities of China, India, Java and elsewhere, where governmental control and adequate financial ability to carry out campaigns have been lacking, from one cause or another. Wherever our methods have been followed, at home and in the insular possessions of the United States, we have terminated human epidemics of plague and have apparently put an end to rat plague in comparatively short campaigns. So long as this discrepancy in results continues we shall favor the American plan. When we review the work and results of Blue and his fellows of the United States Health Service and the officers of the Bureau of Health of the Philippine Islands, we find little reason for us to favor a change to the expectant plan of waiting for an epidemic to run its course.

While speaking of the Philippine Islands, the admirable work of Strong in Manila, covering years of study of the immunity problem, and his dangerous and highly valuable work as a member of the Commission which studied the Manchurian epidemic of pneumonic plague in 1911, must be mentioned.

Some years ago I called attention to the fact that few, if any, American cities were prepared to meet an outbreak of plague with an adequate supply of antipest serum and that the preparation of antiplague serum was a neglected or overlooked branch of serum manufacture in the United States. Since that time, in the midst of a plague epidemic in Manila, where, for a time, the supply of locally prepared (Bureau of Science) serum threatened to become exhausted, I looked into the possibilities of getting a supply elsewhere and found that, to do so, in anything like a reasonable length of time, was impossible. Fortunately the threatened serum famine did not occur, the local supply in Manila proving adequate, although for a few weeks we were obliged to make use of a stock of Japanese serum which had been on hand for several years. Since the warning of some years ago, at which time the plague danger was an anticipated one, bubonic plague has actually appeared in the United States (New Orleans), the cases being sufficiently numerous to cause grave concern and to call forth the utmost repressive efforts of the authorities. The possibility of plague appearance in the coast cities of the United States, at any time, cannot be disregarded and provision for the treatment of human cases, as well as repressive (antirat) measures, is imperative. Antiplague serum is not producible upon a few hours' notice, nor is it manufactured in the United States. In view of present war conditions the difficulty of securing serum from overseas sources is greatly increased, so that we are well-nigh compelled to depend upon home-produced serum. In view of the uselessness of drug treatment it is plainly the duty of national, state and municipal authorities to keep on hand a reasonable supply of antipest serum to meet any outbreak. Manufacturers of biological products realize that the preparations for producing, storing and marketing antiplague serum are expensive and that the maintenance of immunized animals and the employment of expert serologists call for expenditures which are unlikely to be recovered from any demand for serum and that, moreover, the government is doing and will do all that lies within its power to make the serum unnecessary, by excluding plague. These are not encouraging conditions to lead American serum producers to add antiplague serum to the list of their products. If, under these adverse conditions, any producer of biologic products shall undertake to produce and maintain an adequate supply of antiplague serum, he will merit credit for a truly philanthropic service and will deserve the support of governments, national, state and municipal, as well as that of the medical profession.

[CHAPTER I
ITS HISTORY AND ITS EXTENSION]

In plague there exists the most intimate relationship between cause and prevention. We will therefore set forth here, as briefly and concisely as their importance will permit, the principal facts related to the causation of the disease. Without an understanding of this relationship there can be no rational preventive treatment.

These facts constitute one of the interesting stories of modern medicine: the story of the arrangement and interpretation of certain apparently unrelated facts, some of them long known to men, in the clear light of modern method; the story of the application of analysis, synthesis, logic and experiment, all leading to the creation of an understanding which permits us to battle successfully with pestis bubonica, one of the most ancient of human plagues.

History.—This disease has an historic interest, most engaging and fascinating, which one finds it difficult to pass over with mere mention.

I venture to recall, therefore, that plague almost certainly dates back to the pre-Christian era, the earlier record naturally being lacking in sufficient accuracy of description to enable us to identify the recorded epidemics, definitely and positively, with true bubonic plague.

An epidemic of the second century b.c., as described, seems to have been one of true plague, while the pandemic which began in Egypt in the sixth century a.d., thence extending to Constantinople, Europe and the British Isles, was certainly the disease known in modern times as the plague. This pandemic, beginning as the plague of Justinian, was probably followed by the continuous presence of the disease in Europe, marked by many local outbreaks and periods of quiescence and extending down through the centuries to the period of the Crusades. In the eleventh and twelfth centuries the returning Crusaders spread the plague widely through Europe, which country it ravished from the eleventh to the fourteenth centuries, reaching its climax of intensity in the "Black Death" of Europe of the Middle Ages. The disease thereafter continued to devastate Europe, the great population centres, Paris and London, suffering especially from its visitations and its more or less constant presence. The Great Plague of London, the last important epidemic of the disease in that metropolis, began in 1664 and lasted five years. With less than half a million of inhabitants it is estimated that London gave one of every six or seven of her citizens to the Black Death during the first year of the epidemic. Then followed a remarkable disappearance of the disease from Western Europe. The eighteenth century was marked by few epidemic appearances of plague.

At the end of the first half of the nineteenth century it had practically disappeared from Egypt and from European and Asiatic Turkey, formerly its favorite haunts. In interior Asia it has probably existed for centuries, the non-emigrating character of the people limiting and confining its devastations.

To these centres and to the commercial invasion of China, we must probably trace the beginning of the present pandemic of plague, which exists to-day, a menace to the civilized and uncivilized world. In the days of the Crusades a religious invasion of the infected centres caused the disease to spread throughout Christendom, while in the present day a commercial invasion has caused it to spread completely around the world.

That this is a truth and not a fanciful statement is shown by the appearance of plague in the following countries since 1894, when it spread from interior China. In every case it has followed those sanitary lines of least resistance, the paths of commerce.

Extension.—To the eastward, from China, it spread to Japan, the Philippines, Australia, the Hawaiian Islands, Alaska, California, Mexico, Peru and the western coast of South America. To the westward, it invaded India, Mauritius, Egypt, Suez ports, Eastern, Central and South Africa, Mediterranean ports, Great Britain (Scotland), the West Indies and Brazil. In the last twenty years plague has caused millions of deaths, and, during a single week in April, 1907, it destroyed more than 75,000 lives in India, a number about equal to the deaths of a year in London during the Great Plague of 1665. In contrast with India the rest of the world has suffered little during the present world-epidemic, but this loss, while relatively small, is enormous when translated into lives and dollars. The figures for India are simply huge.

Mortality.—The official lists of deaths in India for the last twenty years include some in which the number of reported deaths per year exceeded one million, and it has been estimated that the actual number of persons dead from the plague during this period approximates 8,000,000.

It is gratifying to note a marked decrease in the total mortality in the reports of the last few years, but so long as the annual death list, year after year, was measured by hundreds of thousands, rather than thousands, the situation could not be considered as anything but grave.

Widespread Dissemination in Recent Years.—Without going into statistics deeply we may consider also the list of countries, states and islands from which plague cases have been reported officially during the last few years.

My purpose is to invite attention to the continued existence of various plague foci, any one of which might serve to extend the infection further, were governmental quarantine and public health supervision relaxed.

During August, September, October, November and December, 1909, plague cases occurred in India, Mauritius, China, Japan, Egypt, Turkey, Russia, British East Africa, the Azores, Venezuela, Ecuador, Peru, Chili, California (two cases), and the Hawaiian Islands.

During the first half of 1910 no very marked variation in the distribution of plague occurred, cases being reported from practically all of the foreign countries just named.

A year later the situation, so far as the distribution of plague cases is concerned, was not greatly changed, as may be seen from the following tabulation, which I have abstracted from the British Medical Journal of September 16, 1911.

India.—Deaths from plague in India during the first six months, 604,634. Most prevalent (1) United Provinces, 281,317; (2) Punjab, 171,084; (3) Bengal, 58,515; (4) Bombay Presidency, 28,109. Deaths in July, not included above, 8990.

Hong Kong.—April 24 to August 21, 255 cases, 194 deaths.

China.—January 1, 1911, plague was reported in varying intensity in (provinces and towns) Manchuria, Peking, Tien-tsin, Chefo, Shantung, Shanghai, Amoy, Foochow, Swatow, Canton, Pakhoi and Laichow.

Indo-China.—At Saigon, in March and April, 1911, many cases reported. April 17 to May 7, 56 cases; 17 deaths. May 22 to May 28, 37 cases; 12 deaths.

Siam.—In Bangkok plague was more severe during 1911 than in any previous year. March 15 to April 15, 33 cases and 29 deaths.

Java and Sumatra.—In Java, May 25 to June 3, 105 cases and 62 deaths (one province). In Sumatra plague was present, no statistics.

Straits Settlements.—A few cases, mostly imported, reported in 1911.

Japan.—A few cases at Kobe in 1911. In Formosa, from April 2 to April 15, 31 cases; 24 deaths.

Egypt.—Plague reported from Port Said, Suakin (on board ship), Cairo and Alexandria; also from 11 provinces. The province of Kena had a severe outbreak, May 5 to May 31, 51 cases and 49 deaths.

Persia.—Several cases reported from ports on the Persian Gulf.

Turkey in Asia.—A few cases at Muscat, Basra and at Port of Jeddah.

British East Africa.—Kismayu and Port Florence reported a few cases in April, 1911.

Mauritius.—January 1 to April 11, 110 cases and 70 deaths.

Portuguese East Africa.—Plague was reported present at Nahoria in May, 1911.

Russia.—In the Kirgis Steppe in the Astrakan Government in January, 50 cases; 30 deaths.

South America.—Plague prevailed during 1911 in Peru, Ecuador, Brazil, Chile and Venezuela. No severe outbreak except in Peru, where from February to May many cases occurred and died. At Libertad, in March, were reported 60 cases and 23 deaths.

Appearance of Plague in Porto Rico, New Orleans and Manila.—The developments of 1912, which most concern us, were the appearance of human plague and the discovery of plague-infected rats in Porto Rico, Cuba, and the Philippines, and the discovery of infected rats in New Orleans. Thus the Atlantic cities of the United States were for the first time seriously threatened, and the menace of the pestilence at home loomed up on our horizon with sufficient prominence to excite public concern. Our protectors and guardians of the United States Public Health Service, to whose watchfulness we must credit our prolonged escape from the plague, are carrying out all the protective measures at their command with the utmost activity.

At the present time we find Porto Rico freed from the disease. New Orleans has undergone and is still undergoing treatment which may be expected, most confidently, to clear it of both human and animal plague.

Of Manila and the work there, much will be found in the following pages, but as both rat plague and human plague have been absent for more than a year we may fairly look upon the epidemic as ended. After so long an interval as this any reappearance of plague may fairly be viewed as a new epidemic, although it is not humanly possible to say that rat plague has entirely and permanently disappeared from the city of Manila, as yet.

[CHAPTER II
THE CAUSE AND THE MENACE OF PLAGUE]

The foregoing facts are quite sufficient to make us realize both the possibility and the danger of a world-epidemic; a danger which has existed for some years and which recently has been especially menacing to the United States.

Causation of the Disease.—Plague is an acute infectious epizoötic disease, caused solely by Bacillus pestis, a bacterial organism. The disease is common to man and to a number of the lower animals and fowls.

Prominent among the animals susceptible to the disease is the rat, and from this animal, through the intermediation of the flea, by far the most cases of human plague arise. In California the ground squirrel (Citellus beecheyi), a rodent closely related to the marmots of Asia, plays a similar rôle. Of the Asian marmots, the tarbagan, a large rodent, also commonly suffers from subacute chronic plague, which is transmissible to man as an acute disease by the fleas which the animal harbors.

Its Conveyance.—Although conveyance of plague through rats by contact alone—that is to say without the medium of the flea—is denied by modern experimenters, it is perhaps wiser and safer to consider the disease infectious, inoculable and contagious in the common medical meaning of these terms. While it is usually conveyed to man by the flea, it may be acquired by the inhalation of plague bacilli and, according to some authorities, by ingesting or swallowing the bacilli.

When infection takes place through the digestive tract, or in other words, by the ingestion of bacilli, either the flesh of plague-infected animals or fowls, or food superficially contaminated with plague bacilli by rats, cockroaches or other carriers, serves as the medium.

Speaking practically, the possibility of infection through ingestion is nearly negligible. Indeed, the conclusion of Simpson in regard to this possibility has been disputed and denied. However, the recent occurrence of plague in a cat in Manila, in my own experience, observed with me and carefully worked out by Dr. Otto Schöbl, points strongly to the possibility of ingestion plague, the cat in this case apparently having acquired plague from eating rats dead from plague.

A full account of this case appears in the bacteriologic observations of Dr. Schöbl and in my recital of the history of the Manila epidemic.

Types of Plague.—Plague in man may be of several types and these are designated by names descriptive of the symptoms or of the regions of the body most affected. Thus we have bubonic, septicæmic and pneumonic types. As both mild and virulent cases occur, we also use terms descriptive of the severity and course of the cases. Thus we describe certain cases as ambulant, abortive, larval and fulminant. In the rat the evidences of plague are less striking in life than they are at the post-mortem table. Indeed plague-stricken rats, either naturally or artificially (experimentally) infected, often show very slight evidences of disease before death. Chronic plague in rats and a relative immunity to inoculation in certain wild rats are fairly well recognized phenomena.

Flea Conveyance of Plague Bacilli.—Both male and female fleas convey plague, but the exact method of carrying the plague bacilli from diseased rats to man, while fairly well determined, is of such recent decision as to leave room for further experimentation. At present it is believed that the flea deposits plague bacilli, at the time of biting, upon the skin, by ejecting the contents of its rectum and by regurgitation of its stomach contents. At least the flea is known to perform these acts at the time of biting, and the rubbing or scratching of the flea bite with the hand may easily introduce the bacilli into the skin at this spot.[1]

[1] Acknowledgment is hereby made to the Contributors to "The Rat and Its Relation to Public Health" by various authors, prepared by direction of the Surgeon-General, P. H. and M. H. S., for numerous facts utilized in the preparation of this article. The particular contributors whose valuable chapters have been drawn upon for information are D. E. Lantz, C. W. McCoy, D. H. Currie, Carrol Fox, Rupert Blue, W. C. Rucker, R. H. Creel, M. J. Rosenau, V. C. Heiser, W. C. Hobdy, and J. W. Kerr.

The possibility that the flea introduces the plague bacilli upon his mandibles, or the skin-piercing armament with which he is provided, is also to be considered. However, the following facts support the first proposition. It has been experimentally shown that the average capacity of a flea's stomach is about one-half of a cubic millimetre and that thousands of plague bacilli may be ingested by the flea during the biting of a plague-diseased rat; that the plague bacilli multiply enormously and for many days in the flea's stomach and that the bacilli are found only in the insect's digestive tract; that plague bacilli are regurgitated from the stomach and are voided from the rectum with the digested blood.

It has also been proved that almost all varieties of rat fleas, under favorable circumstances, will bite man and that the most common human flea (Pulex irritans) is frequently found upon rats, the flea, generally speaking, being much less particular in his choice of hosts and in his permanence of residence than most insects and ectoparasites in general.

Of the rat fleas, Pulex pallidus (Lœmopsylla cheopis) is common under various names in India, the Philippines, Australia, Italy, Brazil and in tropical countries generally. It bites both rat and man. Ceratophyllus fasciatus, the common rat flea of Great Britain and the United States, also bites both rat and man. In North America and elsewhere certain other fleas of the genus Ceratophyllus have been found upon ground squirrels, cats, rats, sparrows and in chicken yards.

Dog fleas and cat fleas (genus Ctenocephalus) also infest rats, and fleas of other genera are found upon mice, rats and ground squirrels rather indiscriminately.

The significance of these facts in connection with prevention of plague is apparent and it is plain that our warfare against fleas must be made upon all fleas and not upon a single variety. In this connection the possibilities of the conveyance of plague bacilli by other suctorial parasites and by insects which are not parasites, must be borne in mind.

Thus the bed-bug, the louse, the tick and the mosquito must be suspected as possible intermediaries and the fly and the cockroach as possible food contaminators. Indeed, laboratory experiments have already incriminated bed-bugs, flies and lice as potential vectors of plague bacilli.

Experiment and observation have demonstrated, however, that above all other parasites and insects, the flea is most likely to convey the plague germ from rat to man, by reason of his frequent excursions from rat-host to human-host, his taste for blood from either host, his enormous activity and his ability to jump. After a searching inquiry into the plague question the Indian Plague Commission came to the conclusion that contagion plays a very minor part in the spread of the disease, less than three per cent of human cases being so acquired.

This commission also decided that infection is conveyed from rat to rat and from rat to man solely through the agency of fleas. While these conclusions are probably true—and therefore of the utmost importance from the standpoint of practical prevention—I should question whether the other possibilities, however remote, are entirely negligible.

Seasonal conditions may affect the course of an epidemic in various ways. (a) By effect upon flea prevalence, cold weather greatly lessening the number of insects. (b) By effect upon rats, cold weather and rains either driving them from overground to underground, or vice versa, or from their principal avenues of travel in cities (the sewers), into houses and buildings. (c) By effect upon the plague germ, Bacillus pestis. The resistance of this organism is very variable, sunlight and drying being its greatest enemies, while darkness and dampness are its chief allies. So far as temperature is concerned, the plague bacillus is not likely to be seriously affected by natural temperatures, as it is not destroyed by heat below 150 degrees Fahrenheit, nor by cold measured by zero Fahrenheit, which means that it survives freezing, generally speaking.

It is probable that the periods of greatest seasonal prevalence of plague will be found to correspond generally with increased prevalence of rat fleas. During the periods when rat fleas are absent or least prevalent, the disease is perpetuated in the form of chronic (subacute) rat plague in a small number of the rodents. The India Plague Commission made and verified this observation.

Cholera epidemics often abate spontaneously and this is believed to be due in part to attenuations of virulence and changes in the cholera organism which may be demonstrated in the laboratory. We can hardly hope for such spontaneous abatements in plague epidemics, as it has been found difficult to attenuate or to intensify cultures of plague bacilli permanently in laboratory experiments with animals. If it is true that plague epidemics are often marked by a preponderance of mild cases in the early days and a gradual subsidence of intensity of the cases as the epidemics wane, we probably will have to look to the susceptibility of our patients for our explanation of this phenomenon, rather than to variations in the virulence of the plague bacilli. If plague bacilli continue to be distributed to susceptible people the disease should continue with a general stability of virulence.

Stability of Virulence of B. Pestis.—According to Strong, stability of virulence is a marked characteristic of B. pestis, it having been shown by him that it is difficult to increase the virulence of a very virulent strain or to intensify an attenuated one in laboratory animals, working with monkeys, rats and guinea-pigs.[2] If his observations are correct (and they seem to correspond with the findings of other observers), the oft-recorded occurrence of a preponderance of mild cases of plague in the early days of an epidemic and the gradual subsidence in intensity of the disease as the epidemic approaches its close will have to be explained upon other grounds than those of variability of virulence by attenuation of virulent strains alone. While he admits that B. pestis may become attenuated under certain conditions many times during the course of an epidemic, it may also regain its virulence, he contends, under other conditions.

[2] "Studies in Plague Immunity," R. P. Strong, Philippines Journal of Science, June 1907, No. 3. Frequent reference has been made to these studies in the preparation of this article, for which acknowledgment is hereby made.

With these facts concerning the cause and the manner of extension of plague and its menace before us, we are in position to approach the problem of prevention intelligently, and in the case of plague prevention is preëminently preferable to cure, as well as decidedly more practicable.

I think we may be permitted here to sum up the problem of plague prevention thus: Without fleas, without rats, or without human plague cases, there can be no extension of plague, practically speaking.

Therefore the destruction of both rats and fleas, the isolation of human plague cases, and the exclusion from them of all suctorial parasites and insects, will provide practical security for mankind generally.

A word concerning pneumonic plague may be permissible. This form of plague occasionally occurs in epidemics of great fatality, as, for example, the epidemic in Manchuria, North China, a few years ago.

The mystery of this outbreak was largely dispelled by the work of the Americans, Strong, Teague and Barber, of the Bureau of Science of Manila.

The occurrence of secondary pneumonia in bubonic or septicæmic plague is rather common and it is likely that such secondary plague pneumonias are the starting points of epidemics of pneumonic plague, i.e., of cases of primary plague pneumonia, the point of infection being in the respiratory organs and the infection being acquired through the inspiration of plague bacilli.

The principal prerequisites seem to be an extremely moist atmosphere under confined conditions and a low temperature; conditions most unfavorable to evaporation and ventilation. Under these conditions the pneumonic patient sprays plague bacilli into the air while coughing and droplet infection follows.

It is therefore apparent that epidemic pneumonic plague is controllable by sanitary and hygienic measures and, furthermore, that in the absence of original cases of bubonic and septicæmic plague, with secondary plague pneumonias which give rise to primary plague pneumonia in the manner explained, respiratory plague in epidemic form will not occur.

There is no evidence pointing to the conveyance of respiratory plague by insects or other carriers.

[CHAPTER III
ITS CONTROL AND SUPPRESSION]

Plague Prevention.—At present the most promising and the most rationally based phase of plague control is that of prevention. The reason for this is plainly apparent. If the facts in the case are as stated and if the conclusions of the Plague Commissioners and students of epidemiology the world over are correct, to eradicate plague we need only to control its carriers.

To exterminate the rat (and perhaps the marmot and ground squirrel), to prevent the transportation of rats or of infected rat fleas in ships, trains, clothing, merchandise and upon the bodies of men and animals from the numerous foci or plague centres of the world to non-infected localities, is a beautiful plan indeed.

Restricted to single communities, even where the intelligence, patriotism, effort and wealth of the whole people are enlisted, the undertaking is formidable, with obstacles to its execution, and discouragement must often be expected. Extended in its application to the whole plague-infected world it becomes an undertaking seemingly impossible of accomplishment.

Yet we are encouraged to face the situation by a glance at what has been accomplished. The United States, perhaps, presents the highest examples of achievement in the cases of San Francisco and Manila. The work in San Francisco is too recent and has been too well published to require detailed review here. A successful campaign against rats in 1907 practically terminated an epidemic of considerable proportions well within a year. Behind this movement, however, were the powerful machinery of the Federal Government, money in generous amount and a considerably aroused public, resentful of the mismanagement of the 1903 epidemic, whereby, through pure fear of financial loss to commercial interests and by a disgraceful suppression of the truth, California was made, permanently perhaps, one of the world's plague centres.

It has been estimated that the rat population of the world is equal to the human population, and this estimate does not appear to be unreasonable when one considers as indices the destruction of the rodents in cities by the hundreds of thousands, upon single farms by the thousand, and the wonderful procreative powers of the rat.

Economic Importance of Rat Destruction.—It is certain that the economic importance of rat destruction upon grounds other than those purely sanitary must be impressed upon the public wherever a rat campaign is to be carried on.

The absolute inutility of the rat, its enormous destructiveness to crops, to merchandise in warehouses and in transit, to poultry, eggs, fruits and vegetables, to buildings and furniture, and its incendiary habits causing annual fire losses of considerable magnitude, must be emphasized in season and out of season. Such items as the value of the grain consumed by a single rat per year, as estimated by the experts of the Agricultural Department, are convincing arguments in the case. At a daily consumption of two ounces, the ration for a full-grown rat, this grain value varies from sixty cents per year, for wheat, to two dollars per year, for oatmeal, for each rat subsisted. Similar data in great variety, relating to direct and indirect losses, are available for the purpose of making impressive the economic need for rat destruction.

Accumulated experience from various countries and cities shows plainly that there is no single method of rat destruction to be depended upon to the exclusion of all others and it also shows that without governmental direction and supervision, backed by ample authority and the ability and willingness to expend considerable money, neither single nor combined methods will be successful. Moreover in the countries where special effort is most needed there is often distrust on the part of the natives, religious prejudice against the destruction of animal life and frequently open opposition to the authorities in their efforts to destroy rats. The same superstitions and religious beliefs which prevent the killing of venomous snakes in India, at the annual cost of thousands of human lives, operate against most measures of rat destruction proposed by the Government.

Extermination Methods.—The plans and weapons of warfare against rats include the use of poisons; traps; starvation; rat-proof construction of buildings, wharves, bakeries, stables, granaries, etc.; the introduction of diseases among the rat population by bacterial viruses and the conservation of the natural enemies of the rat, such as the cat, the dog, the ferret, the mongoose, and certain wild animals and birds of the woods and fields.

Among the most widely used and most effective poisons is arsenous acid boiled with rice, or mixed with cheese or cornmeal in the form of a paste, or placed upon sweets and fruits.

Crude phosphorus is chiefly used in similar pastes. When mixed with glucose its inflammable properties are said to be lost. Its inflammability is, of course, a serious obstacle to its general use.

Strychnine, owing to its bitter taste, is of little value in poisoning rats, and when used is best combined with glucose and one per cent. of cyanide of potassium. Soaked wheat, bread or similar food is then treated with this mixture and placed where rats may eat it. It is said to be eaten readily by ground squirrels with fatal effect. It is, however, expensive and apt to be taken by domestic fowls. Most rat poisons have the disadvantage of being dangerous to human life and must be used with caution wherever children and ignorant native persons are about.

Trapping.—Trapping has been found to be a very effective means of rat destruction in cities. (See later pages for relative efficiency of traps.) Rat traps are of several varieties and are constructed upon various principles. It is sometimes desirable to catch the rats alive and uninjured, and for this purpose barrel traps, wire cage traps and similar devices are placed in the rat highways. These highways are readily discovered in the cities. Considerable care must be taken to overcome the natural caution of the rat, and this includes judgment in the use of attractive bait, the concealing and smoking of traps after handling and perhaps the use of some scent, such as the oil of anise, of which rats seem to be fond. As a general rule bait should differ from the food naturally supplied by the locality. For example, about granaries and stables fresh animal food should be used for bait, while about slaughter houses, meat-markets, fish-markets and similar places, where animal offal is abundant, the rat should be tempted with vegetable bait.

Where the circumstances will permit, and this is apt to be so for ground-squirrel destruction, the burrows may be filled with some asphyxiating or poisonous gas. In this manner whole families of rodents, and their fleas as well, are destroyed.

The system is not often applicable in houses, but aboard ships it is found most effective, the holds of ships being flooded with sulphur dioxide, developed by burning sulphur in a special furnace provided with a pumping and piping system for delivering the gas at distant parts of the ship. In empty ships' holds and elsewhere the simple burning of sulphur in open vessels effects the same results, provided sufficient sulphur and a sufficient number of vessels be used and further provided that the generation and confining of gas be sufficiently prolonged. In San Francisco harbor, where for more than a year nine vessels were disinfected per day, this method was adopted as more effective, speedy and economical than any other system. It has the disadvantage, in the case of laden ships, of affording some danger of fire.

Carbon bisulphide has been extensively used in California in the burrows of ground squirrels. Its fumes, being heavier than air, penetrate the burrows and promptly poison or asphyxiate all living animals and fleas. Absorbent material of some kind is saturated with the liquid and placed in the entrance of the burrow, which is then quickly sealed to confine the gas.

It will be seen that, in common with other methods of rat destruction, fumigation has a limited application and a number of serious objections. It is particularly useful aboard ships.

The method should never be employed by unskilled persons or those unacquainted with the dangers to human life from noxious or asphyxiating gases.

Starving Rats.—The subjects of the starvation of rats and rat-proof construction may be considered together.

Just as the pig in the Philippine Islands and elsewhere in the Orient must give place as a scavenger of human excreta to modern and decent methods of waste disposal, so must the rat, a garbage scavenger the world over, give place to systematic garbage collection and removal, with temporary storage of garbage in covered metal cans (rat proof).

Incidentally it may be mentioned that the effect of such measures upon the prevalence of flies and the transmission of disease by these insects will be very great and very beneficial to the public health.

Food must be kept from rats and rats must be kept from the food. Perhaps the greatest resorts of rats are the places where cattle are fed, where grain is stored and where animals are killed. Slaughter houses, markets, grocery stores, restaurants, bakeries, wharves and warehouses must be regulated by ordinances duly enforced. Much can be done with screens of heavy iron wire with a mesh of less than one inch.

When concrete and metal have displaced wood and plaster as construction materials; when plank sidewalks and refuse piles are no more and when the catch basins of sewers have been made rat-proof the subsistence problem for the rat will be greatly increased in difficulty, and starvation should then begin to lessen the rat population, at least in the cities.

Rat-proofing.—Municipal authorities should take up the matter of rat-proof construction for new buildings and the rat-proofing of old ones by approved alterations. In Manila, Hong Kong and elsewhere these methods are receiving attention and encouraging reports are recorded, more particularly with regard to the disappearance of plague in districts so treated than in the disappearance of rats. This is most important, for if the rat and his fleas are excluded from houses and therefore from intimate association with man (an apparently feasible matter through the rat-proof construction of buildings), protection against human plague is in great measure accomplished.

RAT PROOF STRUCTURE WITH SOLID CEMENT BASE, SOLID CONCRETE POSTS, AND UNBOARDED CEILING

In Manila the disappearance and continued absence of human plague in previously infected localities goes hand in hand with the introduction of systematic rat-proofing in sections where cases of human plague occur.

These measures were first instituted in 1906 and plague disappeared from Manila in the same year and did not reappear until 1912.

From 1900 to 1905, $15,000 was paid in rat bounties and $325,000 was paid for salaries, wages and expenses in rat catching, with little appreciable effect upon the number of rats and without causing the plague to entirely disappear. It must be admitted, however, that practical control of the disease was attained during this period.

Rat-proofing of dwelling houses is less expensive than perpetual wholesale rat destruction and is a perfectly effective measure against human plague. In the suppression of the San Francisco epidemic in 1907 rat-proofing was also extensively resorted to.

The expense of rat-proofing has been generally considered as prohibitive, but if the work be confined at first to the vicinity of infected centres and if it be carried on subsequent to rat-destruction in corresponding areas the expense need not always be prohibitive—at least in American governed cities. The Manila plan of plotting the city into "plague-infected" areas corresponding with the capture of plague-diseased rats and systematically working within geographic boundaries in which rat plague exists or is likely to spread, as determined by rat captures and examinations of the rats for signs of plague, has proved to be a good plan.

To prevent the transportation of rats in ships, trains and merchandise is an undertaking of difficulty as well as of importance. In the case of vessels it involves an understanding of the manner by which rats gain ingress to the ship and the ways of preventing them from entering. Few facts are better known, perhaps, than the fact that all ships harbor rats, but, except to the initiated, the extent to which some ships are infested is by no means understood. I have made voyages upon steamships, which upon alternate trips carried forage for animals in the holds, when the conditions were, to say the least, uncomfortable. To have one's state-room taken possession of by rats, his clothing carried away, or to awake with a rat in his berth are unpleasant, but not uncommon, experiences. I personally know of a woman, prostrated with sea-sickness, who was obliged to remain in her berth and see four large rats disport themselves about her room, and in another case, on the same ship, a rat jumped from the washstand into the berth of a sleeping woman, running across her exposed face and arm.

In travelling upon small dirty steamers in the Orient I have often slept on deck, quite as much to avoid the rats and vermin in the state-rooms as for better ventilation. In a certain ship in which I travelled some of the ship's officers amused themselves by shooting rats with an air-rifle in the lower decks, quietly hiding themselves in dimly-lighted places and shooting the rats as they crossed the lighter spaces.

In many ships the rat population far exceeds the human population. In San Francisco 310 rats were destroyed by a single fumigation on a vessel of only 260 tons burden. In Bombay 1300 rats were destroyed at one time upon a single ship and in London 1700 were secured at one fumigation.

The ease with which rats adapt themselves to new environment is shown by the fact that they live, when permitted to do so, in cold storage and refrigerating rooms where they grow heavy coats of fur for protection against the cold.

They gain ingress to ships in three principal ways: (1) By coming overside upon gang-planks, wharf stringers, etc. (2) By passing along the lines by which the ship is made fast to the dock, through hawse holes, the rat being an expert rope walker. (3) By coming aboard in the cargo.

By the latter method rats are often brought aboard by whole families, their fleas included. Many styles of packages such as barrels, bales, crated goods, grain in sacks and matting in rolls present the rat with abundant opportunity to take passage and it is probably thus, as stowaways, that rats go to sea in the largest number. Plainly, then, the placing of rat-funnels upon all lines from ship to wharf, the use of special fenders, the raising of gang-planks and even anchorage in the stream will not prevent rats from getting aboard ships unless cargo disinfection be practised before loading the vessel. The ship itself should be fumigated every three months if possible.

Rats are doubtless carried in considerable numbers upon railway cars, both freight and passenger.

While riding in a street car in Manila in 1908 I saw a rat run along the window ledge, to the mingled fright and amusement of the passengers.

The same principles which apply in the case of ships apply to cars and trains as well. Grain cars in particular should receive especial attention.

Rat Destruction by the Spread of Rat Diseases.—The proposal to destroy rats by wholesale, by spreading epizoötic diseases among them, through feeding them bacterial virus, has received much attention in the last ten years. In 1900 Danysz isolated a bacillus from field mice suffering an epidemic disease communicable to rats, and great hopes were entertained that by means of this method decided reductions in the rat population would result. Indeed the results in Cape Town, South Africa, in 1901, and in Odessa, Russia, in 1902, seemed to justify the hope to some extent and certain observers still believe the method to be effective. Experience with the Danysz and other organisms has shown, however, that introduced epidemic diseases do not destroy rats in sufficient number to do much good and that nearly all the viruses experimented with are more or less unreliable.

Most of the organisms are apparently related to the colon, typhoid or hog-cholera groups. The mouse-typhoid bacillus (B. typhi murium) was originally isolated by Loeffler in 1899. The paratyphoid bacillus and Gärtner's B. enteritidis correspond closely with the Danysz organism and can scarcely be separated culturally. In rodents they produce enteritis, sometimes hemorrhagic in character, and they are by no means to be regarded as harmless for man, as originally supposed. In Japan, in particular, serious and fatal cases of diarrhœal disease have followed the accidental eating by man of food treated by these bacterial poisons.

On account of the natural resistance of rats to diseases of bacterial causation (plague being the most notable exception to this rule), and the clinical fact that no sufficient death rate among rodents is produced by feeding them upon bacterial viruses, as well as on account of the dangers to man just mentioned, this method of rat destruction is not in favor at present.

Poisoning rats and ground squirrels by chemical poisons seems to be a preferable method, at least equally effective and without most of the disadvantages of uncertainty and danger which attach to the bacterial viruses.

Rat Destruction by Domestic Animals.—Concerning the utility of such domestic animals as are natural enemies of the rat, in the warfare against the offending rodents, there is considerable difference of opinion, based upon varying experiences. I leave out of consideration all but the cat and dog.

It will be found that wherever cats and dogs are well housed (indoors) and well fed they are apt to be fat, lazy and inefficient. House cats of this class will catch mice but will often leave rats alone, but half-wild cats, obliged to forage for their own subsistence, are often excellent rat-catchers. Small, active dogs, particularly of the terrier breeds, will often keep houses practically free from rats and upon farms they are especially valuable, particularly if the construction of buildings is such as to permit them to get beneath the floors. The employment of these animals will necessarily be confined to individuals for the freeing of individual premises from rats.

A fact to be borne in mind is one already cited, viz.: that cats and dogs sometimes harbor the same fleas as the rat. Infected rat-fleas often leave dead rats for other animals and, all things considered, there are many other objections to the intimate house dog and house cat which find comfortable resting places impartially upon the beds of adults or the cribs of babies and children.

Furthermore, my personal observations have been such as to cause me to place small reliance in the value of the ordinary dogs and cats found about habitations wherein the construction is favorable to rat-harboring.

Summary of Prevention for the Community.—Before passing to the consideration of other matters I would sum up the measures of preventive treatment for the community. There must be (1) Active warfare against rats and other plague-affected rodents and their fleas; (2) Modified quarantine—detention or disinfection applied to persons, goods and animals; (3) Disinfection of cargoes shipped from infected ports; (4) Isolation of the sick and proper disposal of the dead; (5) International notification between governments of the occurrence of plague within their respective territories; (6) Lastly,—but we might say first in importance,—the early recognition of the presence of plague and the rapid diagnosis in individual cases, both of which are dependent upon laboratory workers.

All of these measures must be fostered, directed and aided in every possible way by competent authority (national if possible), whose officers must be men of great moral courage and of unselfish purpose. Behind all of this must be generous financial support.

I can best emphasize the importance of the observance of the principles I have laid down by introducing personal experiences in the conduct of the antiplague campaign in Manila during 1912, 1913 and 1914.

I therefore present here the following account of the epidemic, the campaign of suppression and the various lessons learned.

It should not be difficult for the reader to make applications of the principles already set forth and to confirm by the reported facts the assertion that methods based upon these principles are effective.

If repetitions of any of the foregoing principles occur it is hoped that, when taken in connection with concrete applications cited, they will not appear as redundant.

The Manila Epidemic of 1912 to 1914.—The chronologic facts concerning the development and extension of plague in Manila in 1912, 1913 and 1914 are as follows:

The disease made its reappearance in Manila, after an absence of six years for the human disease and five years for rodent plague, two verified human cases having been recorded in June, 1912.

Preceding the appearance of the first Manila cases there occurred upon incoming ships a number of cases of plague during the Spring of 1912, detected at quarantine. Although there is no conclusive evidence which connects these imported cases, originating in Hong Kong, China, with the epidemic which broke out in Manila a few months later, the fact of their occurrence and recognition is interesting enough for us to consider before taking up the study of the Manila epidemic. Concerning these imported cases Dr. Victor G. Heiser, then Director of Health for the Philippines, wrote as follows in the Philippine Journal of Science, in February, 1914.

Unusual Character of Plague at Quarantine.—It is perhaps worthy of note that, prior to the appearance of plague in Manila a number of cases of the disease were found on incoming steamers. For instance, on April 6, 1912, a death was reported on the steamship Zafiro, which had arrived the day previous from Hongkong and had been in the harbor for twenty-four hours at the time of the death. At the medical inspection of the vessel, which was made the day previous, no illness was detected. An investigation showed that the victim had been on deck on the night of April 5, 1912, in apparently good health. The next morning, at 6 o'clock, he was found dead in his bunk. The necropsy and subsequent biological findings reported by Dr. R. P. Strong of the Bureau of Science showed that death was due to pneumonic plague.

On April 7, 1912, the steamer Loongsang arrived in Manila from Hongkong, and the captain reported that a death had occurred the day previous in a Chinese member of the crew. Upon investigation of this case, the captain stated that the man was apparently in good health, but that while hauling on a rope he fell over in an apparent faint and was placed in a chair and in the course of a few hours expired. The necropsy and animal inoculations showed that he had died of plague and probably of the pneumonic variety.

Beginning April 7, 1912, the temperature of all members of the crew and of the passengers that arrived in vessels from foreign ports was taken with a view to detecting any possible cases of plague.

On the arrival of the steamship Taisang from Amoy at the Mariveles Quarantine Station at about 6.30 A.M. on April 30, 1912, the entire personnel was carefully examined and found free from sickness of a suspicious nature and from elevations of temperature. Seventy-three persons were detained to serve a quarantine detention of seven days. On the evening of April 30, a Chinese passenger, aged fifty-one years, was found to have a temperature of 39° C. with a pulse of 100. He was placed in the hospital, but protested vehemently that he was not sick. He was carefully watched from the first; there was a slight cough; physical examination of the chest revealed a few râles; smears made of the sputum and stained for plague bacilli were negative. On the fifth day, the fever still persisted, but the patient stated that he did not feel ill and demanded to be released from the hospital. On this day, the expectoration was blood-stained, but no suspicious organisms could be found in the smears nor could any physical signs of pneumonia be detected. Furthermore, there were no palpable glands. On the morning of the seventh day, the temperature and pulse dropped and the general condition was distinctly worse. The patient now admitted that he felt ill. Several hours later, he flinched when pressure was made in the right axilla. Lymphatic enlargement was now made out, and by the evening of the seventh day the bubo in the axilla had increased markedly in size, the swelling approximating 3 by 7 centimetres. Glands now became palpable in other portions of the body, particularly in the cervical region, and a few hours later there were inguinal and femoral buboes. The patient became rapidly worse, and died at 7 o'clock on the morning of the eighth day of his illness. At the necropsy, the glands of the right axilla and those of the right side of the neck were found enlarged; the other lymphatic glands were also enlarged, but to a lesser degree. There was consolidation of the lower lobe of the right lung, and the spleen was about twice its normal size. In brief, the necropsy findings of a typical case of septicæmic plague were present. Smears from the spleen and the right axillary gland showed immense numbers of bipolar-staining organisms. Cultures made from fresh pieces of tissues and later inoculated into animals gave positive results for plague.

Beginning of the Manila Epidemic.—Proceeding with the Manila epidemic inaugurated with the two cases referred to as recorded in June, 1912, we find that the total number of cases recorded from the time of the outbreak in 1912 until the last case in 1914 was 90. (This includes none of the imported cases from China which developed en route to Manila from Chinese ports.)

Of these 90 human cases, 76 were fatal and autopsies were performed in all instances. Fourteen persons recovered. The number of cases of animal plague up to July, 1914, was 53. This refers only to laboratory-proven cases of rat plague. As a matter of fact, hundreds of dead rats, almost certainly plague rats, were found in the course of rat-proofing operations.

Although the period covered by this epidemic approximates two years, it must not be supposed that the progress and extension of the epidemic was an uninterrupted or unobstructed one.

On the contrary, such extension as occurred was made in spite of the most active suppressive effort, and it is believed that this effort brought about a creditable result, as indicated by the accompanying record.

When one considers the favorable conditions for the natural spread of plague, both in Manila and throughout the Philippine Islands, and realizes the interposed difficulties and obstructions, natural and unnatural, geographic, human and domestic, which confront us at every turn of the path to correction, removal and reformation, our success in checking the spread of plague appears as a real achievement, especially when contrasted with the results of effort during the same period in a British city of similar size but a few days' sail from Manila, where the cases were numbered by thousands and where the infection still persists.

First Manila Cases.—The first case of plague (June 12, 1912) occurred in a resident of Tondo, 920 Calle Antonio Rivera, and in the light of subsequent developments it may perhaps be grouped with the October cases traced to the Manila Railway Company's freight station and yard, as 920 Calle Antonio Rivera is but a stone's throw from the Manila Railway property. The connection, however, is not clear, and, on the other hand, it is not wholly inconceivable that the rat epidemic and human plague cases at the railway station in October may have been secondary to this June case. Such speculation is fruitless, however, so far as establishing facts is concerned.

The second case of human plague occurred 13 days later, June 25, in a resident of a district somewhat removed from the first case, but in the same general section of the city.

Then came a lull of more than a month, until August 4, during which time no case of plague occurred; or at least none was reported.

August brought forth five cases on the fourth, eighth, fifteenth, and twenty-first days of the month, in residents of the Quiapo and Binondo districts.

These cases were unrelated to the preceding ones so far as could be ascertained.

Another lull of a month, until September 24, now occurred without a reported case of human plague. During this time, however, the first cases of rat plague were discovered, one on August 30 and two on September 6, all of them in the Quiapo district.

From this time (September 24) on, however, human cases occurred at intervals of a few days until Christmas Day, 1912, the longest plague-free period being one week; the number of cases by calendar months being distributed as follows: September, 3 cases; October, 22 cases; November, 12 cases; and December, 6 cases.

Geographic Grouping.—Not until October 21 was there any apparent geographic grouping of cases indicating a well localized infected centre. Upon this date there began the outbreak of plague among the employees of the Manila Railway Company, laborers at the freight station and yard of the company. This freight station and yard is located between Calle Azcarraga, Calle Dagupan and Calle Antonio Rivera. The outbreak totalled 17 human cases, all fatal, and extended into November. Indeed, the last case traced to this focus occurred on December 7, 1912.

During the present epidemic of plague in Manila this focus was the only one to which a larger number of cases than five could be traced, and in all the other instances where multiple cases were traced to an infected centre, the foci were all single buildings.

The locations giving rise to multiple infections and the number of cases of plague developing at each address, with months of incidence, are as follows: Calle San Fernando (804–814), November, 1912, 4 cases; Calle Teodoro Alonzo (518), November and December, 1912, 2 cases; Calle Cabildo (Intramuros), November and December, 1912, 2 cases; Calle Comercio (1028), February, 1913, 2 cases; Calle Sande (1364), April, 1913, 5 cases; Calle Juan Luna (1226), May, 1913, 2 cases.

Returning to the Manila Railway outbreak, it is necessary to state that a well-defined epidemic among rats preceded this outbreak, resulting in the death of a large number of rodents (undoubtedly from rat plague). This epidemic was not reported by the railroad company until the outbreak of human plague had begun. It was then too late to identify plague in the dead and mummified rats found under floors, platforms and elsewhere, but the fact that large numbers of rats had recently died here was established by the unanimous testimony of the employees at the freight station and the finding of rat cadavers.

As stated, the human outbreak here occurred upon October 21, and fifteen cases developed within 3 days.

This indicates an extensive desertion of fleas from plague rat cadavers and an attack upon human beings, after a fasting period, on the part of the fleas, of several days. The human outbreak at the station and the death of a large number of rats at the same place, just previous, correspond to a nicety and establish to a moral certainty the connection necessary to explain the epidemic.

After the railway epidemic of human plague, cases continued to occur through November and December, without apparent relation to each other, except in the following instances, which have already been mentioned:

Four cases under one roof on Calle San Fernando (November 12, 13, 16 and 22); 2 cases in one house on Calle Teodoro Alonzo (November 26 and December 2); and 2 cases in the same house on Calle Cabildo (Intramuros), November 23 and December 11.

These multiple cases will be referred to elsewhere.

The other cases during October, November and December were apparently sporadic and unrelated, either to the other human cases or to the few scattering cases of rat plague discovered from time to time. Without doubt, however, all were actually related to preceding cases of rat plague, i.e., to undiscovered rat cadavers, dead from plague and deserted by infected fleas.

In the following plague houses (see list of cases) dead rats were actually found, although the advanced degree of desiccation and mummification defeated the biologic determination of the cause of death: 518 Calle Teodoro Alonzo; 973 Calle Azcarraga; 282 Estero de Binondo.

In other plague houses the recent finding of dead rats was alleged by the occupants, but rather too indefinitely to record positively.

A study of the maps and lists showing the localities in which cases of rat plague had been found up to this time (December 26, 1912), in connection with the location of plague houses, was much less suggestive than a similar study of the lists and maps covering the cases of 1913.

However, the existence of concurrent rat plague and human plague, in corresponding sections of Manila, had been well established already by bacteriologic studies of captured rats, made at the Bureau of Science.

Of nearly equal weight was the observation concerning the two epidemics, rat and human, at the Railway Station, which I have already described.

The year 1912 closed, then, with a recorded total of 50 human cases and 7 verified cases of rat plague.

January, 1913, saw but a single case of human plague. This occurred on January 24, just a month from the last previous case, that of Christmas Day. During this month no case of rat plague was reported.

In February, 3 human cases occurred and in March, 4 cases were recorded.

Early in March, 1913, cases of rat plague began to occur in the Tondo district in a section lying between Manila Bay and the Estero de la Reina and extending northward from Calle Moriones. This was a new district for rat plague and as the cases increased in number we were able to foresee and predict the appearance of human plague in the same district, which in point of congestion of population, poverty of its residents and in the matter of dilapidation of its light material houses and shacks, is about the worst locality in Manila.

From March 22 to September 20, 1913, all the cases of human plague, 11 in number, occurred in the midst of this district. During the same period 25 cases of rat plague were reported from the same section, and a glance at a map of this part of Tondo instantly shows the relationship existing here between rat plague and human plague.

This relationship is additionally emphasized by referring to the memoranda concerning certain overcrowded houses, in the midst of the rat plague district, where multiple human cases occurred. (See memoranda in re 1226 Calle Juan Luna and 1364 Calle Sande.)

CLEANING AND RAT PROOFING IN BASEMENT OF 1226 CALLE JUAN LUNA IN WHICH TWO CASES OF PLAGUE OCCURRED. RAT CADAVERS FOUND UNDER BROKEN FLOORS (MANILA PLAGUE CAMPAIGN)

The human cases in April were 5 in number, all originating in the same house, and the May cases numbered 4, two of which occurred in the same house.

It may be explained, in passing, that two cases of human plague, discovered in Malolos, 25 miles from Manila, on March 23 and March 26, respectively, were definitely traced to the same house in Manila, number 12 Calle Aguila, Tondo, both patients having lived in the basement of this house until within 48 hours of the development of the disease. These persons were unrelated and were two of a large number of people who lived in a tenement at this address. Both patients were detected, while still alive, in Malolos, where they were living in different and widely separated houses. One of the patients died in Malolos but the other one was brought to Manila by train and died at San Lazaro Hospital. Fortunately no infection was transferred to Malolos by these two persons. In this connection it is interesting to note that no other cases have been reported from outside of Manila, except the small outbreak in Iloilo in the southern islands, where the antiplague work was successfully directed by Dr. Carroll Fox. Concerning this outbreak, Dr. Heiser, then Director of Health for the Philippines, writes as follows (Philippine Journal of Science, February, 1914):

Plague in Iloilo.—In Iloilo, a case suspicious of plague was reported on July 5, 1912, and this diagnosis was subsequently confirmed by the laboratory. It occurred in the person of a Chinaman who was reported to have come from Bais, Oriental Negros, but later investigation showed that he had been a resident of Iloilo at least since February, 1912. The next case was reported August 18, and the last case, September 17, 1912. There was a total of 9 cases. All of the cases were confined to two houses. During July, August, September, and October, 1146 rats were caught in the vicinity of the houses in which the human cases had occurred, along the water front, and in the places which were regarded as suspicious, but in not a single instance was an infected rat found.

Directed to Take Charge of Plague Suppressive Measures.—Upon my arrival in Manila from the United States, on October 23, 1912, I received orders from the Director of Health to take charge of all plague suppressive measures in Manila and I remained in charge of this work continuously until July 11, 1914.

Plague Fighting Organization.—The plague fighting organization was composed of three American Sanitary Inspectors and from ten to fifteen native Assistant Sanitary Inspectors of the Bureau of Health, rat catchers and laborers of the Bureau and laborers of the City of Manila supplied by the Department of Sanitation and Transportation. The combined force varied in numerical strength from 100 to 150 men and was usually divided into three parties, distributed in various parts of the city according to the local indications and needs from time to time.

After the invasion of Tondo by rat plague we made special effort to rat-proof the light material houses of that section, in the course of our cleaning operations, by the closure of the open ends of bamboo timbers with cement and with tin cans, in the manner shown in photographs herewith. In addition to this, special attention was given to the repair of broken cement work, and hundreds of Bureau of Health orders, verbal and written, were issued to owners, at my request, in the rat plague districts.

The number of houses in which bamboo timbers were closed by cement or tin exceeded a thousand.

In addition to these means, the very important matter of depopulating the insanitary basements of the light material houses in squares where plague has occurred was given attention, with the result that hundreds of families were moved from these insanitary and dangerous ground-floor rooms to quarters well above ground and measurably removed from the rats, which roam over the ground from house to house, foraging for food under kitchens and in ground-floor storerooms, tiendas and eating places. The fish packing factories afford them abundant food and a number of cases of plague have occurred adjacent to these fish-drying establishments.

Rat-proofing and Rat Destruction.—While it is frankly admitted that rats may not be completely exterminated by poisoning and trapping, the statement, so frequently repeated of late, that destructive measures really increase their number, is unwarranted and unsustained by facts, at least in Manila. It seems to be the common practice for disbelievers in trapping and poisoning to array the methods of rat-proofing and rat destruction as alternative policies, whereas everyone practically familiar with the work in such cities as Manila—or even in the United States—knows that there is often no choice permitted. Rat-proofing is highly desirable, permanent in its results, and in every respect the "method of election." On the other hand, it is entirely inapplicable at certain times and in certain localities where poverty, lack of interest of property owners, and ofttimes lack of interest and of money on the part of municipalities, absolutely preclude its immediate application. It is therefore unfortunate that the statement, that rat poisoning and trapping are ineffective, either in controlling plague or in reducing the numbers of rats, is circulated. It may be shown easily, by the daily records, that within a few weeks after extensive rat poisoning and trapping (with the breaking up of nests) is pursued in a given locality, the rat catch drops in the most decided manner.

Individual premises may be practically cleared of rats by continued intelligent rat catching and poisoning, and while the normal rat birth-rate may keep pace with the normal rat death-rate it will not keep pace with the normal death-rate plus the poisoning and trapping death-rate in any given locality, provided that the poisoning and trapping, with the destruction of nests, be intelligently and continuously carried out.

Rat-proofing and rat destruction, then, should not be contrasted as alternative procedures or policies. Both are valuable and each has a proper place. In communities non-infected with plague and unexposed to infection it will probably be found that rat-proofing, carried out in connection with the repairs of old buildings and the erection of new ones, will meet the requirements. On the other hand, in cities exposed to plague infection or already infected, rat destruction is bound to be necessary for years to come.

In emergency, the removal of people from intimate relationship with rats (so far as is possible), as practised recently in Tondo district, Manila, will often have to take the place of rat-proofing; and rat destruction and expulsion will be found, in the last analysis, to be the methods upon which success or failure in fighting plague during epidemic time will depend.

In this connection I quote correspondence which passed between the Director of Health and myself in 1913.

Upon March 22, 1913, I directed the following letter to the Director of Health:

Sir: I have the honor to state that Estaban Masibac, aged twenty-two, laborer, who died at 140 Perla of bubonic plague, slept upon the ground floor of this house upon a bamboo bed. All these basement dwellers in this district now infected with rat plague are in considerable danger.

The roving rats which wander over these ground surfaces from house to house come into pretty close contact with these basement dwellers, and it would appear that they visit the upper stories of the houses rather infrequently, unless food is stored there. Upon the ground they forage upon the food dropped there by the residents of the houses.

I would like to have authority to order the vacation of these basement rooms which are almost invariably unfit for human habitations.

I look upon this measure as an important one at this threatening time and believe it should be enforced in every square or block where plague rats have recently been found. If this authority is granted it will be used judiciously.

Very respectfully,

[Signed] T. W. Jackson,

Medical Inspector in Charge of Plague Suppression.

Upon March 24 I received the following letter of authorization:

Sir: Confirming my verbal instructions of yesterday I have to request that, in accordance with the recommendation contained in your letter of March 22, that on account of the danger of the spread of plague in the district in which plague has appeared extensively, the basement dwellers in blocks, or squares, in which plague has been found, should be ordered to vacate.

Very respectfully,

[Signed] Victor G. Heiser,

Director of Health.

Upon November 26, 1912, five dead rats were reported from the U. S. Army Commissary Warehouses on the Pasig River near the Malecon. They were found dead by workmen there and were thrown into the river by the finders and thus, unfortunately, examination for plague was prevented.

Upon November 27, a cat, known to have caught and eaten rats recently at the same place, was reported to be sick. I took the cat to the Bureau of Science where she was observed until she died, three days later.

At autopsy, typical bubonic plague (cervical) was disclosed, and several guinea-pigs inoculated from the spleen and bubo died from the same disease. A guinea-pig, inoculated from a swab introduced into the cat's rectum, also died from plague (see report of Dr. Schöbl).

Four kittens, recently born of this plague cat, were observed for two weeks but showed no sign of the disease.

Subsequently about 80 rats were caught at these warehouses and in the vicinity, but none of them showed post-mortem signs of plague. The Medical Department, U. S. Army, then took up the matter of rat catching on all military reservations in Manila and in all buildings thereon, but no more cases of animal plague were discovered.

Fleas and Their Habits.—In "Observations Upon the Bionomics of Fleas Bearing Upon the Epidemiology of Plague in Eastern Java," by N. H. Swellengrebel, Ph.D., published by the government at Batavia, Dutch India, in 1913, some interesting facts, developed by study and experimentation, are presented. Some of these facts have a bearing on the plague problem in the Philippines, for it should be borne in mind that certain climatic similarities and racial similarities pertain commonly to the Javanese and Filipinos and their respective countries.

While we are not prepared at present to make general application of the Javanese findings to the Philippine Islands, for lack of parallel or confirmatory studies in the Philippines, we may state some of the conclusions of the Java workers with propriety, and we may also point out similarities in the construction of certain Filipino and Javanese habitations in their relation to rat harboring.

Swellengrebel, in Java, noted the number of fleas per rat, dealing with Xenopsylla cheopis (the commonest rat flea in Java) almost exclusively. This flea, it will be remembered, is also the common rat flea of India, the Philippines, Australia, Italy, Brazil and tropical countries generally, being variously known as Lœmopsylla cheopis, Pulex pallidus, P. brasiliensis, P. philippinensis, and (in Italy) P. murinus.

It would not be unreasonable, therefore, to expect to find at least some of his observations applicable to the Philippine Islands.

Swellengrebel failed to find Ctenocephalus canis (dog flea), C. felis (cat flea) and Ceratophyllus fasciatas (the common rat flea of the United States and Europe) upon Javanese rats. In attempting to determine the normal flea census he found that field rats, and field rats caught indoors, as well, generally carry fewer fleas than house rats and that the number of fleas per house rat varies in different districts from .02 per rat to 2.3 or 4 per rat and that this variation is not invariably constant with the presence or absence of rat plague. Concerning the question whether or not a high flea census may indicate rat plague, Swellengrebel offers the reasonable opinion that there is little doubt that plague in rats increases the number of fleas per rat above normal and that, consequently, a sudden or marked increase in the number of fleas per rat, without a known normal cause, indicates increased rat mortality and probably rat plague.

As to the influence of temperature and humidity on the hatching of larvæ, he concludes from experimentation that the duration of development of the egg varies under various hygrometric conditions, the general rule being, "the lower the humidity the longer the development period."

As to the influences of temperature and humidity upon the transition of larva to imago he finds that if humidity diminishes, a smaller number of larvæ reach the adult stage; and also that a saturated humidity (in artificial cultures), causing condensation of water in the substratum, is very fatal to larvæ. He offers the thought that this, perhaps, explains why only small numbers of fleas are found on field rats which live in holes in rice fields which are necessarily damp, especially in the rainy season.

His experiments to determine the duration of life of fasting fleas were made with laboratory-bred fleas which had never fed on blood and with fleas which had already sucked blood.

The duration of life was variable, but of those fleas already fed with blood three-quarters (¾) perished within 10 days and the remainder lived from ten to twenty days, only one-tenth, however, surviving for 13 days, if moist conditions were maintained. High temperature was determined to be an unfavorable condition.

If from these findings one should attempt to predicate or predict the extension of plague in house rats—based on flea prevalence—and this with relation to climatic conditions, we should be led to the conclusion that the rainy season, with its greater humidity, would be quite the most favorable time of year for rat plague extension in Manila and, upon the contrary, that the hot dry season through its unfavorable influence upon flea breeding would be the least favorable season for rat plague in Manila.

The hot months of 1913 did not bear out this reasoning, however, for during these months rat plague was at its height.

That increased prevalence of human plague has not gone hand in hand with increased prevalence of rat plague in Manila, may be explained, I feel sure, by the activity of our efforts to destroy rats and to remove the people from close relationship with them.

Another factor of possible explanation of the greatest prevalence of human plague in Manila during the late rainy season of 1912 (October), is the fact that rats are certainly driven above ground into houses and therefore into closer relationship with man by heavy rainfall and the consequent flooding of their subterranean homes.

It appears, therefore, that the seasonal explanation of greater plague prevalence, rat or human, is susceptible of several interpretations and I feel sure that in countries like the Philippines seasonal variations in heat do not suffice to rid the rats of fleas during any months of the year. If, then, conditions of rainfall serve to drive the rats above ground and indoors during certain months, it would be reasonable to expect more human plague from closer relationship of rat and man,—provided that no special measures were carried out.

Such, however, is not invariably the rule, if statistical studies are to be taken as evidence, and so we are reminded that generalizations for countries of different climates and seasons are not wholly reliable.

Rat breeding, as well as flea breeding, is influenced by climate, but as the reproductive activity of the rat is most retarded by cold weather—an unknown condition in the Philippines—and as the climate of Manila is fairly equable so far as heat and cold are concerned, the only factor which needs to be considered is that of rainfall. As already mentioned, rainfall doubtless serves to drive rats above ground and so, to a certain extent, away from their nests in burrows and underground.

Their well-known adaptability to changing conditions, however, permits them to house themselves comfortably above ground when driven out of these burrows and holes.

Javan Observations.—The following conclusions were reached by Dr. J. J. van Loghem in a report upon "Some Epidemiological Facts Concerning the Plague in Java" (published by Civil Medical Service in Netherlands India-Batavia, 1912):

1. In plague-infected villages, as distinguished from plague-free villages, there exists a considerable mortality among house rats.
2. Rats in plague houses and plague quarters have repeatedly died from plague. Fresh plague rats appear more often in the houses adjoining plague houses than in the houses themselves.
3. The house rat exists even in the immediate vicinity of man.
4. The ordinary parasite of the house rat is Xenopsylla cheopis, which experimentally is known to choose man as a host when starving.
5. Fresh plague rats have repeatedly been found to harbor a great number of fleas.
6. Virulent plague bacilli have been demonstrated in the stomachs of such fleas.

Concerning the prevention of plague by improving the native dwellings, the same observer says: "Obviously an increase in the distance between man and rat becomes an important factor as a means of preventing the disease."

Conditions of Manila Habitations Favorable to Rats and Plague.—As shown by our own experiences in Manila, this end, the separation of rats and men, is not obtainable by destruction of rats by poison, traps and rat catchers. Rats dying of plague in their nests furnish the greatest danger to man. The plague problem, therefore, where rats are already infected, from the stand-point of direct prophylaxis, is the problem of dwellings. It was from this stand-point that we attacked the problem in the Tondo (Manila) campaign in 1913.

Manila Verification of Javan Observations.—Having in mind the experiences of the plague investigators in Java during the recent epidemics there (1911–1912), we sought, from the time the Manila outbreak occurred, to verify some of the findings of the Java investigators, at least with special reference to the nesting of rats in close proximity to human beings and the consequent exposure of these persons to the infected fleas which desert the rats dying from plague in these nests.

Not until rat plague invaded the special district of Tondo, in Manila, in March, 1913, did the opportunity present itself. Theretofore the Manila cases had generally appeared in houses of the so-called "hard material districts," where house construction is entirely unlike that with which the Java workers dealt. With the invasion of Tondo, however, the Java and Manila conditions became similar. I quote the descriptions of Javanese house construction from the report of Dr. J. J. Van Loghem, "Some epidemiological facts concerning the plague in Java," Batavia, 1912.

The Javan Village House.—In substance, he says that the Java village house, as a general type, is a one-storied structure with its roof sloping to the front and back, i.e., with its ridge parallel with the front and back aspects of the building. It is not elevated above the ground by supports or palisades and has no separate floor, the earth serving as the floor.

The outer frame is of strong bamboo poles and the inner frame is also constructed of bamboo. These bamboo timbers are perforated at various points to permit of framing with other pieces of bamboo and for the entrance of pegs, etc.

The roofs of these houses are often made of tiles, but at times the familiar thatched roof is seen. In both cases the supports or rafters are bamboo poles. The principal piece of furniture is the "bale bale," or bedstead, usually made of bamboo, except in the houses of the well-to-do. Small storerooms are often located in the houses, and stables are sometimes built against them. In many cases the family provisions are kept in the house and the cattle are housed here as well.

Manila Light Material Houses.—If, now, we turn our attention to the average Tondo (Manila) light material house it will be apparent that the description given for the Java village house fairly describes the Tondo house, except that the Philippine house is commonly elevated 2 metres or more above the ground upon bamboo supports (see photographs). The basement is usually enclosed in a manner similar to the principal room of the Java house and the basement room may fairly be compared, structurally and in the matter of its floor, with the one-story Java house. In the Manila house, however, the floor of the upper room takes the place of the roof of the Java house and like it is supported by bamboo timbers.

Here, then, in our enclosed basement story, we have a practical replica of the one-storied Java house.

Here, also, the principal piece of furniture is often a bamboo bed, practically identical with the Java "bale bale," if we may judge from photographs.

In the Java houses the favorite nesting places for rats were found to be the interiors of horizontal bamboo pieces of the roof, house frame and bedstead.

The rat usually gains entrance by gnawing through the natural partitions between the bamboo sections near the outer end of the pole. Our Manila photographs show both the natural open ends of such timbers and the rat-gnawed perforations in the partitions.

In Java, rats also nest in the thatched roofs, as they occasionally do in the Philippines.

Nest Materials.—The materials utilized for nests by rats in Manila and Java seem to be identical also. Straw, dry leaves and pieces of cotton are mentioned in the Java reports. The same materials and additional ones will be found mentioned in our reports upon nests.

BAMBOO HOUSE SUPPORTS NOT SEALED WITH CEMENT. NOTE HOLES GNAWED IN BAMBOO ENDS. RATS FREQUENTLY MAKE NESTS IN THESE HOLLOW BAMBOO RAFTERS.

The presence of food was also noted in the bamboo nests in Java and we often find articles of food in our Manila nests.

Dr. Korn, P. H. Service, and the writer (T. W. J.) investigated a good many of these bamboo house-timbers and we not only found such evidences of rats as food, rat fæces and nest materials, but in one case a rat was actually driven out of a bamboo nest by introducing a long thin strip of wood. The evidence of similar conditions then is complete.

We also duplicated the experiences of the Java workers in finding dead rats inside of the bamboo house timbers in close proximity to patients sick (or dead) with plague (see memoranda in the case of Esteban Masabik, of 140 Calle Perla, March 22, 1913).

Very extensive rat destruction and cleaning operations, covering a large portion of the city of Manila and including all sections where cases of rat plague or human plague developed, were undertaken and this work was carried on without interruption for about two (2) years. City laborers to the number of 60 to 150 were used and the work was supervised by Sanitary Inspectors Brantigan and Searcy, of the Bureau of Health. During a part of the time a flying column of 50 men, under Sanitary Inspector Hunniecutt, was detached from the main party and employed at placing rat poison.

The total amount of accumulated dirt removed from houses and yards approximated 5250 tons (for 17 months ending November 1, 1913).

Without doubt this general cleaning campaign and the removal of this enormous accumulation of dirt and rubbish was of great value as an antiplague measure.

The rat catch will always be found to depend upon several factors, viz.: the number of persons employed; the number of traps and portions of poison placed; the location of the operations and the length of time a given locality is trapped, poisoned and cleaned. The variety of baits and poisons will also affect the results.

In addition to these factors certain others are found to operate in reducing the rat catch, as, for example, weather conditions and the occurrence of Sundays, holidays and the days just preceding and following holidays.

Upon rainy days and the days just mentioned the rat catch almost invariably falls off.

From statistics collected by me in connection with this work, Dr. V. G. Heiser, then Director of Health for the Philippine Islands, published the following memorandum in 1914. As it is a correct transcript of my records I introduce it here in its entirety.

Comparative Statistics in Rat-catching Methods.[3]—With a view to ascertaining which type of rat trap was most effective and also the average number of rats that are caught by a given number of poisoned baits that are set out, statistics were kept during the antirat campaign in Manila. The ratio maintained in catching rats with two types of traps is indicated in the following [table], a perusal of which will show that for the three months ended June 30, 1913, there were 120,565 spring or snap traps set and that for every 100 of this type of trap set there were caught 6.9 rats. During the same period there were 47,075 wire cage traps set; the total number of rats caught was 339; which gives 0.72 rat caught for each hundred traps set. For the quarter ended September 30, 130,627 spring or snap traps were set and 9,753 rats were caught, which gives 7.47 for each 100 traps set. During this period 40,621 wire cage traps were set and 395 rats were caught, which gives 0.97 rat caught for each 100 wire cage traps set.

[3] Reprint from the Public Health Reports, Vol. 29, No. 6, February 6, 1914.

No accurate record was kept of the number of each kind of rat bait set. Only the total of all was recorded. Bacon or coconut with strychnine and rice with arsenic were used. For instance, for the quarter ended June 30, 1913, there were 166,237 poison baits set in new territory and the rats found poisoned average for each 100 baits 0.72. During the next quarter there were 177,309 baits set in territory that had been worked over, and only 216 rats, or 0.12 rat per 100 baits, were killed. From the foregoing it appears that the rat poison ranks lowest in efficiency but perhaps highest in economy. In view of the fact that the original cost of the cage trap is many times more than that of the spring trap, and the cost of maintenance is very high, it will be apparent that the spring trap is by far the more economical as well as more effective of the two.

Generally speaking, however, the number of rat catchers engaged and the location of their operations has the largest influence upon the total catch of rats. For the fiscal year July 1, 1912, to June 30, 1913, inclusive, the total catch was 55,101 rats (Manila only); to December 1, 1913, 79,676.

The most natural explanation of the general correspondence between the highest rat catch and the highest incidence of human plague would be upon grounds of greater activity in rat catching effort at times of greatest plague prevalence, but from the inauguration of general systematic rat catching there was no cessation of effort, even during the abatement of plague, and in consequence this explanation does not apply strictly.

It is true, however, that whenever plague occurred in districts theretofore free from the disease, rat catching was pushed vigorously in the surrounding localities.