Introductory material

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THE RAT AND ITS RELATION TO THE PUBLIC HEALTH.

By Walter Wyman,

Surgeon-General of the Public Health and Marine-Hospital Service.

INTRODUCTION.

The science of bacteriology has elucidated many facts with respect to the causation of disease, and with this advance in knowledge, old theories regarding the miasmatic and humoral origin of human ills have been abandoned.

Epidemiological studies have likewise determined the methods of transmission of many of the infectious and contagious diseases, thus eliminating erroneous conceptions that they are attributable to some mysterious condition of the atmosphere or soil, or to a visitation of the wrath of the Almighty.

Both these sciences have contributed to our knowledge of the relationship of living things, particularly with respect to their influence upon each other in relation to health and disease. It is now known, for instance, that mosquitoes are the pests of man, not only because of their bites, but because they at times transmit malaria, dengue, filariasis, and yellow fever. So, too, it is known that rodents are the enemies of man, not only because of the toll exacted from him, but because they are the principal agents in the propagation and spread of bubonic plague.

Ancient writings abound in allusions to pestilences and their connection with epizootics among rats and mice.

In the Book of Samuel there is reference to a pestilence having relation to mice, and that it might be stayed the Philistines made offerings of golden images of the mice that marred the land.

During the centuries that have intervened rats have migrated to practically every quarter of the earth, causing untold losses on account of their depredations. They have also, in all probability, been the primary agents of transmission in the pandemics of plague which have visited the earth. The fact that plague is due to a specific microorganism, and that its presence in man is also associated with epizootics in rats, has led to a more careful study of this animal, particularly in relation to his habits, the diseases from which he suffers, and the methods necessary to his control. Prior to the 10beginning of the present pandemic of plague which had its origin in China, interest in the rat was almost wholly an economic and financial one. Since that time evidence has been rapidly accumulating which proves that this animal and his parasites are responsible for the transmission of plague and that plague itself is essentially a disease of the rat.

A knowledge of this animal on the part of the sanitarian therefore becomes essential. During the enforcement of antiplague measures in California, Hawaii, the Philippine Islands and elsewhere, observations of great value have been made and their practical application has resulted in better directed efforts for the elimination of the disease.

In studies of plague and leprosy with the view to their diagnosis and control, it is not enough now to isolate the microorganisms responsible for these diseases, but the sanitarian must be able to recognize the pathological conditions present in animals affected, and to do so he must have practical knowledge of this subject in order that he may differentiate between the various diseases from which these animals suffer.

Opportunity for observation and study of the diseases of rats and the methods necessary to their eradication has been afforded to the officers of the Public Health and Marine-Hospital Service who are constantly stationed on the outposts in the warfare against exotic diseases. The results of these observations have been utilized by officers of the service, and some of them have been published for the benefit of others.

The rat has received much attention of late in other parts of the world. In Denmark, for instance, a legalized warfare against rodents has been begun, principally on account of their influence in the transmission of trichinosis. In England there exists The Incorporated Society for the Destruction of Vermin, and in other places rat destruction is being agitated both from economic and public health standpoints.

In view of the great importance of the rat in relation to the public health, it has been thought advisable to collect and publish all pertinent information on the subject, in order that public health officials who should be on the lookout for the appearance of plague among rodents might have available a reliable treatise on the subject.

Studies of rodents from a biologic and economic standpoint come within the province of other departments of the public service, and the cooperation of the Biological Survey and Bureau of Entomology of the Department of Agriculture was therefore requested and received.

The subjects dealt with in this publication have been prepared by those having wide experience.

11In the chapter on natural history by Mr. David E. Lantz there is given a classification of rats as well as the distribution of the genus Mus in America. An interesting and important fact is mentioned that the Biological Survey has no records of the presence of the brown rat in Nevada, Utah, Wyoming, Idaho, and the greater part of Montana. Mr. Lantz also describes the different species in America, and refers to their habits as to breeding, feeding, migrations, invasions, and ferocity. The facts presented by him emphasize the great difficulty of ridding cities of these pests.

Passed Assistant Surgeon McCoy discusses plague infections in rats and describes the methods of examination. He also describes the gross lesions found in plague rats, gives the bacteriologic diagnosis of rat plague and the cultural characteristics of the plague bacillus on various media. He gives the methods of artificial infection of rats with plague, and reviews the recent work of Ledingham in relation to the histology of rat plague. Finally, he presents results of his own investigations to show that the wild rat is not especially susceptible to plague infection, and that a certain percentage of such animals enjoy a natural immunity to plague.

Doctor Brinckerhoff discusses rat leprosy; states that it is very similar to human leprosy, and that it is caused by a bacillus which closely resembles the bacillus of Hansen. He describes the pathological changes found, and expresses the hope that the disease will receive further earnest study, in order that additional information may throw light on the problems presented by leprosy in man.

Passed Assistant Surgeon Currie briefly outlines the bacterial diseases of the rat, other than plague and leprosy. He mentions the great utility that would follow the discovery of a rat destroying bacterium, but states that it appears now more than probable that few such natural diseases of rats exist.

In a chapter on organic diseases of the rat, Doctor McCoy summarizes the results of his observations made during examinations of these animals in the Federal laboratory of the service at San Francisco. These observations are of interest, and will assist those engaged in such work to further classify the pathological changes noted as well as differentiate them from plague.

The ectoparasites of the rat are classified and described by Mr. Nathan Banks, and he has presented in condensed form information of much practical value upon the subject.

Dr. Ch. Wardell Stiles discusses the internal parasites of rats and mice in relation to the diseases of man. He regards the rat as a permanent reservoir for trichinosis, and states that this disease will probably never be eradicated from man until rats and mice are practically eradicated, and a national campaign directed against trichinosis must take the rat into consideration.

12A compendium of animal parasites reported for rats and mice is presented in a chapter by Ch. Wardell Stiles and Albert Hassall. While, as the authors state, no list of this kind can ever lay claim to being complete, it represents the present knowledge of the subject.

In a discussion of the flea and its relation to plague, Passed Assistant Surgeon Fox summarizes the theories as to the transmission of this disease. He also mentions the insects that have been suspected of transmitting plague and presents accumulated evidence that fleas actually convey the infection. He then gives the anatomy of the mouth parts of the Ceratophyllus Fasciatus, the common rat flea of North America. He also enumerates the fleas that have been found on rats, and gives the results of identifications of 19,768 fleas in San Francisco and Oakland, Cal. The plates accompanying this article, and their description should be of great value to those engaged in antiplague measures.

Surgeon Blue briefly discusses the subject of rodents in relation to the transmission of bubonic plague. He discusses the theories as to the cause of seasonal prevalence of this disease and presents a table showing the number of rats examined during the different months of the year, the number found infected, the average temperature and rainfall for those months and the character of the days, as to the number clear, partly cloudy, or cloudy. He refers to plague infection in ground squirrels in California and warns against the possibility that this animal may become responsible for the establishment of a permanent focus of plague on the Pacific coast of the United States, as the marmots are so concerned with regard to India.

The all-important subject of rodent extermination is considered in detail, various phases of the subject being dealt with by different authors.

Passed Assistant Surgeon Rucker discusses the destruction of these animals by trapping, poisoning, cutting off of food supply, and destroying of existing nests and at the same time preventing the making of new ones. He describes the methods of use of the various mineral poisons, but finally states that rodents must be builded out of existence; in other words, habitations must be rendered rat proof.

Mr. Lantz, in discussing the natural enemies of the rat, mentions the animals that destroy these pests. He concludes that on account of this function bounties for the destruction of small animals that prey on rodents can not be justified and that they should in the future be protected in every way possible.

Passed Assistant Surgeon Creel discusses rat proofing as an antiplague measure, and gives in detail the principles of construction necessary. He concludes that rat proofing is the most valuable antiplague measure, and that it should precede auxiliary measures such as trapping and placing of poisons.

13Surgeon Rosenau discusses the bacterial viruses in relation to rat destruction. As a result of his investigations in the hygienic laboratory and the reports of investigations and practical use elsewhere, he concludes that the bacterial viruses have signally failed to accomplish the mission for which they were intended, and that they are not entirely harmless to man, as has been stated.

Passed Assistant Surgeon Heiser briefly outlines the measures recommended for the eradication of plague in cities by means of sectional extermination of rats and general rat proofing. He gives results following this method of procedure in Manila, and presents charts showing how to deal with infected city districts.

Passed Assistant Surgeon Hobdy, in a chapter on the rat in relation to shipping, refers to the voyage-making tendencies of the rodent, its destructiveness aboard ship, and its power of adapting itself to unusual conditions and surroundings. In one small lumber vessel fumigated by Doctor Hobdy at the Angel Island quarantine station there were collected 525 dead rats. Mention is also made of another vessel on which were collected 1,700 rats after fumigation. He discusses the methods by which it gains access to vessels, and outlines the practices that should be observed to keep it off. He also describes in some detail the measures to be adopted for its destruction after it has gotten aboard ship, and mentions the different methods of fumigation.

Mr. Lantz, in a third paper, discusses the rat as an economic factor, and states in his paper that they do not serve any useful purpose. On the other hand, they cause enormous loss through damage to grain, merchandise, poultry and eggs, game and wild birds, fruit and vegetables, and flowers and bulbs. They also cause damage by setting fire to buildings and destroying furniture. He refers to various estimates made of the losses in the United States from rats, and they vary from $35,000,000 to $50,000,000 a year; but at the same time he states that, with present information, any attempt to state the amount of loss from rats would be largely guesswork.

Assistant Surgeon-General Kerr refers to the rat as a factor in international sanitation, and briefly outlines the provisions contained in international sanitary agreements for their eradication. He reviews the efforts being made at the more important seaports to exterminate rats, as well as the methods being employed to that end. The information presented is, in part, compiled from consular reports received through the Department of State. There are given, so far as obtainable, copies of laws and ordinances enacted for the destruction of rats and the different methods practiced in ports where plague has prevailed, and the facts presented indicate that a more or less widespread crusade against rats is being carried on. He expresses the belief that it is too much to expect that the rat population can ever 14be exterminated from any city, but that it is not too much to expect that ocean carriers can be freed from rodents and kept so, which action would confine plague within continental boundaries.

Epidemiological studies made of plague since the adoption of the International Sanitary Convention of Paris and the International Sanitary Convention of Washington have proven that the rat and its parasite, the flea, are the agents of transmission of the disease. In other words, where rats go plague will go. I believe, therefore, that in order to stop the further progress of plague, radical measures should be adopted, and in a communication of February 26, 1909, addressed to the Secretary of State, I suggested the advisability of submitting the question of a systematic destruction of rodents aboard ship to an international sanitary conference, with the view to the adoption of an international sanitary regulation on the subject. The adoption of such a regulation would undoubtedly lessen quarantine restrictions, prevent the destruction of cargo by rodents, and obviate the danger of the further spread of plague.

Until ships are freed from rats, each country must take all necessary precautions, consistent with international agreements, to destroy rats; and the sanitary authorities of infected localities must, at great expense, determine the extent of infection among rodents, with the view to its elimination. This problem when it presents itself in a community is of great magnitude, and those responsible for its solution should be familiar with all its phases.

It is with the view to supplying the necessary information in one treatise that this publication is issued. In its preparation the bureau has had the cooperation of the Department of Agriculture and acknowledgements are due, and here made to, the officers of that department for their hearty cooperation in contributing some of the chapters which follow.

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