Chapter I: Part 1

TREASURY DEPARTMENT

Public Health and Marine-Hospital Service of the United States

THE RAT AND ITS RELATION TO THE PUBLIC HEALTH

BY VARIOUS AUTHORS

PREPARED BY DIRECTION OF THE SURGEON-GENERAL

]

WASHINGTON
GOVERNMENT PRINTING OFFICE
1910

TABLE OF CONTENTS.

Page.

_Introduction_ (Walter Wyman) 9

_Natural History of the Rat_ (David E. Lantz) 15

Classification of rats 15

Distribution of the genus _Mus_ in America 17

History of the brown rat 19

General description of the species in America and key to the
species 20

Habits of rats 22

Breeding habits 22

Abundance 23

Migrations and invasions 24

Food 26

Feeding habits 26

Ferocity 27

_Plague Infection in Rats_ (George W. McCoy) 29

Mode of examination 30

Gross lesions of natural rat plague, acute 32

Subcutaneous injection 32

The bubo 33

The granular liver 34

The spleen 35

Pleural effusion 35

Gross lesions of natural rat plague, chronic 36

Rat plague without gross lesions 37

Microscopical examinations 37

Bacteriological diagnosis of rat plague 38

Pest-like bacteria found in rats 41

Artificial infection of rats with plague 41

Modes of infection 42

Local reaction 43

The bubo 44

The liver and spleen 44

Chronic plague due to artificial inoculation 44

The histology of rat plague 45

Natural rat plague 46

Immunity of rats to plague 46

References 48

_Rat Leprosy_ (Walter R. Brinckerhoff) 49

Introduction 49

Review of literature 49

Description of disease 50

Etiology 52

Summary 52

Bibliography 53

_Bacterial Diseases of the Rat other than Plague_ (Donald H.
Currie) 55

Danysz bacillus or bacillus typhi Murium of Loeffler 55

Pneumonia 55

Staphylococcus abscesses 56

Bacillus pseudo-tuberculosis rodentium (Pfeiffer) 57

Toyama’s bacillus 57

Infections of mice 57

_Organic Diseases of the Rat, Including Tumors_ (George W. McCoy) 59

Usefulness of wild rats for laboratory purposes 59

Circulatory apparatus 60

Pulmonary apparatus 60

Digestive tract 61

Cirrhosis of the liver 61

Fatty degeneration of the liver 61

Hernia 61

Genito-urinary tract 62

Nephritis 62

Abscess of the kidney 62

Atrophy of the kidney 63

Vesical calculi 63

Tumors 64

Metastases 67

Histological structure 67

Lipomata 67

Fibromata 67

Sarcomata 67

Adenomata and Carcinomata 67

_Ectoparasites of the Rat_ (Nathan Banks) 69

Fleas—Siphonaptera 69

Lice—Anoplura 77

Mites—Acarina 80

_Internal Parasites of Rats and Mice in Their Relation to
Diseases of Man_ (Ch. Wardell Stiles and Charles G. Crane) 87

Summary 87

Introduction 87

Protozoa 88

Cestoda 95

Nematoda 101

Acanthocephala 108

_Compendium of Animal Parasites Reported for Rats and Mice—(Genus
Mus)_ (Ch. Wardell Stiles and Albert Hassall) 111

_The Flea and Its Relation to Plague_ (Carroll Fox) 123

Theories as to the transmission of plague 123

Insects that have been suspected in the transmission of plague 124

Experiments proving that fleas can transmit plague 125

The bacillus in the flea 126

How the flea clears itself of bacilli 127

Regional distribution of fleas on rats 127

Anatomy of the mouth parts of the Ceratophyllus Fasciatus 128

Outside the head 128

Inside the head 129

The act of biting 131

How the flea infects its host 132

Enumeration of fleas that have been found on rats 133

Results of identification of fleas in California 135

Synopsis of fleas commonly found on rats 136

Ceratophyllus Fasciatus, Bosc 136

Lœmopsylla Cheopis, Rothschild 138

Ctenopsyllus Musculi, Dugés 140

Pulex Irritans, Linnæus 142

Ctenocephalus Canis, Curtis 143

References 144

_Rodents in Relation to the Transmission of Bubonic Plague_
(Rupert Blue) 145

Epidemiological observations in San Francisco 147

Theories as to the cause of seasonal prevalence 149

The occurrence of plague in the marmot of Asia and ground
squirrel of California 150

Plague infection in ground squirrels 150

The natural habitat of plague 151

References 152

_Rodent Extermination_ (Wm. Colby Rucker) 153

Trapping 154

Poisoning 156

Natural enemies 159

Cutting off of the rat’s food supply 160

Building the rat out of existence 161

_Natural Enemies of the Rat_ (David E. Lantz) 163

Animals that destroy rats 163

Hawks 163

Owls 164

Wild mammals 166

Skunks 166

Weasels 166

Minks 167

Domestic animals 167

Dogs 167

Cats 167

Ferrets 168

Other animals 168

Mongoose 168

Alligators 168

Snakes 169

Bounties on predatory animals 169

_Rat-Proofing as an Antiplague Measure_ (Richard H. Creel) 171

Rat-proofing of primary importance 173

Rat-proofing is expensive 174

Methods of rat-proofing 175

Rat-proofing ordinances should be specific 177

Choice of architecture and building materials 178

_Inefficiency of Bacterial Viruses in the Extermination of Rats_
(Milton J. Rosenau) 179

Introduction 179

Experiments upon rat virus in the Hygienic Laboratory 183

Experiments with microorganisms for destroying rats by the U.
S. Biological Survey 186

Experiments during the San Francisco plague outbreak 188

Opinions of others 190

Pathogenicity for man 193

References to the literature 201

Résumé 204

_Plague Eradication in Cities by Sectional Extermination of Rats
and General Rat-Proofing_ (Victor G. Heiser) 205

_The Rat in Relation to Shipping_ (Wm. C. Hobdy) 207

Adaptability of the rat to his surroundings 208

Damage to cargo 209

Fumigation 211

Summary 213

_The Rat as an Economic Factor_ (David E. Lantz) 215

Introduction 215

Utility of the rat 215

Destructiveness of the rat 216

Grains 216

Merchandise in stores and warehouses 218

Merchandise in transit 219

Poultry and eggs 219

Game and wild birds 220

Fruit and vegetables 221

Flowers and bulbs 221

Fires 222

Buildings and furniture 222

Miscellaneous 223

Amount of losses caused by rats 224

Indirect losses 225

_The Rat in Relation to International Sanitation_ (John W. Kerr) 227

International sanitary regulations 228

Inquiry into the crusade against rats throughout the world 230

Rat extermination in United States ports 231

Rat extermination in Chinese cities 232

Rat extermination in Madras, Bombay, and Calcutta, India 234

Rat extermination in Yokohama and Nagasaki, Japan 235

Rat extermination in East Africa 237

Rat extermination in Cape Town, South Africa 238

Rat extermination in Alexandria and Cairo, Egypt 238

Extermination of rats at the port of Constantinople 238

Rat extermination in Russian ports 239

Destruction of rats in Trieste, Austria 240

Destruction of rats in Genoa, Italy 240

Destruction of rats in Barcelona, Spain 241

Rat destruction in French ports 241

Ministerial decree relating thereto 242

Destruction of rats in German ports 243

Measures against rats in Rotterdam, Holland 245

Destruction of rats at Antwerp, Belgium 245

Destruction of rats in Denmark 245

Danish law of March 22, 1907 245

Collection and destruction 247

Destruction of rats in Swedish ports 248

Destruction of rats in English ports 249

Measures against rats in Australian ports 250

Measures against rats in South American ports 252

Measures against rats in West Indian ports 252

Destruction of rats in Panama 253

Measures against rats in Vancouver, B. C. 253

Necessity of concerted action of nations 254

LIST OF ILLUSTRATIONS.

Page.

Fig. 1a. Upper molars of the brown rat (_Mus_): tubercles in 16
three rows

Fig. 1b. Upper molars of the rice rat (_Oryzomys_): tubercles in 16
two rows

Fig. 2a. Right hind foot of brown rat, showing long sixth foot 17
pad

Fig. 2b. Right hind foot of house mouse, showing round sixth 17
foot pad

Fig. 3a. Ears of brown rat and black rat, showing relative size 21 fig. 3b.

Fig. 4. Necropsy appearance of normal rat 48

Fig. 5. Necropsy appearance of plague-infected rat 48

Fig. 6. Flea, showing the various parts 70

Fig. 7. Louse—_Polyplax spinulosus_ 78

Fig. 8. Mite—_Lælaps echidninus_ 81

Figs. 9 Internal parasites of rats and mice 90–109 to 58.

Fig. 59. Isolated plague-infected center, Manila, P. I. 206

Fig. 60. Scheme for testing rat-plague infection, Manila, P. I. 206

Plate I. Mouth parts of _Ceratophyllus fasciatus_ 130

II. _Ceratophyllus fasciatus_ 136

III. _Lœmopsylla cheopis_, Rothschild 138

IV. _Ctenopsyllus musculi_, Duges 140

V. _Pulex irritans_, Linnæus 142

VI. _Ctenocephalus canis_, Curtis 144

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 beginning 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.

In 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.

A 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.

Surgeon 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 be 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.

NATURAL HISTORY OF THE RAT.

By DAVID E. LANTZ.

_Assistant, U. S. Biological Survey, Department of Agriculture_.

INTRODUCTION.

The extermination of rats has become one of the serious problems of modern times. That such noxious animals should have flourished so long is not creditable to our civilization. While no kind of rat can be regarded as harmless, the various species differ greatly in harmfulness. In comparison with the cosmopolitan species that have reached our shores from the Old World, our native rats do little damage. It is important, therefore, to be able to recognize the introduced forms, to understand their habits, and to concentrate efforts for their extirpation.

CLASSIFICATION OF RATS.

Rats and mice belong to the _Rodentia_, an order which comprises more than a third of all living species of mammals. Also, it exceeds any other mammalian order in the number of its individuals.

Rodents are mainly herbivorous mammals, mostly of small size, having a furry, sometimes a spiny, integument, clawed digits, and usually plantigrade feet. The most important distinguishing character of the order is its dentition. This is marked by the absence of canine teeth and the presence of strongly developed incisors growing from permanent pulps. The incisors are never more than two in the lower jaw and usually but two in the upper. They are elongated, curved, chisel-like in shape, and continue to grow throughout the life of the animal. Only the front of these teeth is covered with enamel, a provision which keeps them sharp by the more rapid wearing away of the softer dentine in the body of the tooth, as the upper and lower pairs meet in gnawing. Between the incisors and the cheek, or molar, teeth of rodents there is a wide, vacant space, marking the entire absence of canines.

The most extensive family of rodents is the _Muridæ_, a name which applies to rats and mice in the widest sense of those terms. It is difficult to characterize the family, since its members differ widely. However, most of them are rat-like in form and light and active in movements. None of the family have premolars; and, except in a single genus (_Hydromys_), the number of molars is three. Oldfield Thomas, the eminent English zoologist, includes in this family no less than 77 genera, or almost half the total of 159 which he ascribes to the whole order _Rodentia_.[A] He further subdivides the _Muridæ_ into a dozen subfamilies, of which the _Murinæ_ and the _Sigmodontinæ_ are the most extensive. The name _Cricetinæ_ is now generally used instead of _Sigmodontinæ_, though not always with the same limitations.

Footnote A:

Proc. Zool. Soc., pp. 1012–1028, 1896.

The _Murinæ_ comprise only Old World rats and mice, while the _Cricetinæ_ are, in the main, American forms. In the _Murinæ_ the cusps, or tubercles, of the unworn upper molars are arranged triserially, or in three longitudinal rows; in the _Cricetinæ_ they are arranged biserially, one row on the outer and one on the inner margin (fig. 1). The wearing away of these cusps leaves characteristic curved lines of hard enamel surrounding areas of dentine. When the cusps are in pairs the worn pattern looks somewhat like the Greek letter sigma (Σ), whence the name sigmodont, often applied to native American rats and mice.

Fig. 1a. Fig. 1b.

FIG. 1a.—Upper molars of the brown rat (_Mus_): tubercles in three
rows.

FIG. 1b.—Upper molars of the rice rat (_Oryzomys_): tubercles in two
rows.
]

The _Murinæ_ are the true rats and mice, typified by the genus _Mus_, which contains by far the largest number of species. Trouessart, in his Catalogus Mammalium, enumerates 260 species of _Mus_ described before 1905. Since that date a number of new forms have been described.

The genus _Mus_ is characterized by narrow, ungrooved incisors; three small, rooted molars; soft fur mixed with hairs, sometimes with spines; a rudimentary pollex having a short nail instead of a claw; a long tail bearing rings of overlapping scales and often naked or nearly so. The ears are rather large, the eyes bright and prominent, and the muzzle somewhat pointed. The members of the genus are natives of the Old World, throughout which, with the exception of Madagascar, they are quite generally distributed. Nearly seven-eighths of the whole number of species are commonly called rats.

The distinction between rats and mice is arbitrary and based on size. Exclusive of the tail, rats may be said to vary in length from 4½ to 10 inches or more, while mice measure from 2 to 4 inches. With few exceptions, rats have six well-defined footpads (plantar tubercles), the last on the hind foot being elongated in shape; the last hind-foot pad of mice is usually circular (fig. 2).

Of the many species of _Mus_ only three or four have developed the ability to adapt themselves to such a variety of conditions as to become cosmopolitan. Four have found lodgment in America: The common house mouse (_Mus musculus_); the old English black rat (_Mus rattus_); the Egyptian, or roof, rat (_Mus alexandrinus_); and the brown rat (_Mus norvegicus_), known also as the gray rat, barn rat, wharf rat, sewer rat, and Norway rat. The black rat and the roof rat differ from each other chiefly in color. Indeed some zoologists regard them as races of the same species, and the trinomial _Mus rattus alexandrinus_ for the roof rat is now in use among zoologists.

FIG. 2a.—Right hind foot of brown rat, showing long sixth foot pad.

FIG. 2b.—Right hind foot of house mouse, showing round sixth foot pad.
]

DISTRIBUTION OF THE GENUS MUS IN AMERICA.

The common house mouse (_M. musculus_) found its way to America soon after the first settlement by Europeans. It now inhabits all settled parts of North and South America, as well as nearly the entire Old World; but in very cold regions it does not always survive the winters, and is therefore comparatively scarce or local. It almost always reaches a new settlement sooner than the rat.

The black rat (_M. rattus_) has been known in Europe since the twelfth century. It was carried to South and Middle America about three and a half centuries ago (1554). The time of its arrival in the English colonies of North America is not known with certainty, but it was well established in the settled parts by the beginning of the eighteenth century. Soon after the arrival of the brown rat, the black species began to decrease in numbers, and has become extinct in most localities. At present it is not uncommon in some parts of the South, and still occurs in scattered colonies in Canada and some of the States east of the Mississippi, and also on some of the coastal islands. It is occasional in many of our seaports, being apparently brought from the Far East in merchandise. Except in a few ports like San Francisco, where new arrivals are probably rather frequent, these introduced individuals are often destroyed before they multiply. The history of the black rat in Europe and its disappearance before the brown rat is an exact parallel to its history here, and the animal is now comparatively rare north of the Alps, except in the Channel Islands.

The Biological Survey has specimens of the black rat from Massachusetts, New Hampshire, Georgia, Florida, Alabama, California, and Washington, and also from Mexico, Honduras, Nicaragua, and Hawaii. There are authentic records of its recent occurrence in Newfoundland, Quebec, Nova Scotia, New York, North Carolina, Tennessee, West Virginia, and Mississippi. In parts of South and Middle America it is abundant.

The roof or Alexandrian rat (_M. alexandrinus_) is similar to the black rat in form and general habits, though not in color. Little is known of its history, but it is thought to be a native of Egypt, where it is still abundant. It has established itself in many parts of the world, mainly in warm climates, and is common near the coast in the southern parts of the United States.

The Biological Survey has specimens of the roof rat from North Carolina, Georgia, Florida, Alabama, Mississippi, Texas, Arizona, and California. In the last-named State it is abundant in the Sacramento Valley. It is known also from Dismal Swamp, Virginia, and from Cuba, the Bermudas, Trinidad, San Domingo, Costa Rica, Nicaragua, Mexico, and Hawaii. Also, it inhabits many parts of South America, where in places it is the dominant species.

The most destructive of the rat family is the brown rat (_M. norvegicus_). In most parts of the United States it is the common rat about houses and barns in the country and about markets, wharves, and warehouses in cities. It is larger and more robust than either the black or the roof rat, and differs from both in habits. It is more of a burrower, and lives in excavations which it makes under buildings and in loose soil along hedges and river banks. This habit, combined with its greater strength and ferocity has enabled it to supplant the other species in temperate latitudes; but in the warmer parts of America and the Old World it has not been able to drive out the others. The house mouse everywhere holds its own against the brown rat by its ability to escape into retreats too small for the rat to follow.

The brown rat inhabits most of the thickly populated parts of America. North of Panama it occurs generally except in the arid interior, from the Isthmus to the Yukon Valley and southern Greenland. In the Great Basin it is practically unknown, and in New Mexico and Arizona it is confined chiefly to towns along the railroads. The Biological Survey is without records of its presence in Nevada, Utah, Wyoming, Idaho, and the greater part of Montana. The reason for its absence in that region is not understood, but its ability to withstand extreme cold is proved by the fact that it flourished in latitude 78° 37′ north on board Doctor Kane’s ship _Advance_, in the Second Grinnell Expedition, during the two winters when that vessel was icebound. It has also adapted itself to the continuous low temperatures of cold-storage warehouses, in which it appears to breed freely.

HISTORY OF THE BROWN RAT.

We know little of the history of this species. Greek and Roman writers make no mention of rats of any kind, but possibly knew the animals and included them in their frequent references to mice. Pallas, in 1778, described the brown rat under the name _Mus decumanus_, and this was generally used until it was found that Erxleben had called it _M. norvegicus_ in 1777. Previously, the common name Norway rat had often been used for this species.

The brown rat is generally supposed to be of Asiatic origin. Various modern writers have asserted that it came originally from Persia or India; but W. T. Blanford states that the species is at present unknown in Persia, and that in India the black rat is the generally distributed species, while the brown rat occurs only along the coast and the navigable rivers.[B] This implies that the latter species is a comparatively recent immigrant into India.

Footnote B:

Fauna of British India. Mammals, p. 409, 1891.

As regards the arrival of the brown rat in Europe, two facts are known. The species reached England from some eastern port about 1728 or 1729, and according to Pallas, a little earlier, 1727, crossed the Russian frontier from Asia and soon spread over the greater part of that country.[C] This statement, taken in connection with that of Blanford, makes it highly probable that before this migration the Asiatic home of the species was north, rather than south, of the high mountains of northern India. This view, which has been adopted by several naturalists, is further strengthened by the fact that the animal flourishes better in temperate than in tropical climates.

Footnote C:

Zoographica, Rosso-Asiatica, vol. 1. p. 165, 1831.

Possibly earlier and unrecorded westward migrations of the brown rat took place. A few years ago Professor Waile, the archeologist, while making excavations at Cherchell on the coast of Algeria, dug up the skull of a rat, which he stated was contemporary with the Roman occupation of the country under the Cæsars. The skull had but one molar, much worn, but the cranial bones were intact, and French zoologists pronounced the remains as undoubtedly those of the “surmulot,” or brown rat.[D] This shows that we have little more than conjectures for the early history of this species.

Footnote D:

Comptes Rendus des Séances de L’Académie des Sciences, Paris, vol.
116, p. 1031, 1893.

The brown rat is said to have first appeared in Paris in 1750. It was brought to the United States, probably from England, about the beginning of the Revolution, 1775. According to Audubon, it was unknown on the Pacific coast of the United States in 1851; but Dr. J. S. Newberry thought it must have arrived at San Diego, Monterey, and San Francisco at a much earlier date.[E] Doctor Cooper recorded its arrival at Fort Steilacoom, Washington, as occurring about 1855.

Footnote E:

Pac. R. R. Reports, Vol. 6, Zoological Report, pt. 2, p. 60, 1857.

GENERAL DESCRIPTION.

The brown rat differs considerably from the black rat and the roof rat. It is larger, has a shorter head, a more obtuse muzzle, smaller ears, and a relatively shorter and stouter tail. The general color is grayish-brown above and whitish below. The over hairs of the upper parts have black tips. The tail is usually shorter than the head and body combined. The average measurements of adult specimens of the brown rat in the Biological Survey collections are as follows: Total length, 415 millimeters (16.3 inches); tail, 192 millimeters (7.1 inches); hind foot, 43 millimeters (1.7 inches). This species sometimes attains a total length of 19 to 20 inches, and has been known to weigh 24 to 28 ounces and even more. The average weight of an adult brown rat is considerably less than a pound.

The black rat is less robust than the brown rat. It has a longer head, a sharper muzzle, and larger and broader ears (fig. 3). The tail is longer than the head and body combined. The fur is of a sooty, or plumbeous black, color, paler on the underparts. It is much softer and denser than that of the brown rat, and the mixture of very dark and lighter over hairs gives it a peculiar shining appearance. The average measurements of 20 apparently adult specimens in the collection of the Biological Survey are as follows: Total length, 379 millimeters (14.9 inches); tail, 207.4 millimeters (8.1 inches); hind foot, 35.8 millimeters (1.4 inches).

The roof rat in general resembles the black rat, except as to color and texture of fur. Above it does not greatly differ in color from the brown rat, but its underparts are usually more yellowish. The fur is commonly shorter and harsher in texture than that of the black rat, but this difference might not always be apparent in specimens of the two forms from the same latitude. The average measurements of 50 adult specimens of the roof rat in the collections of the Biological Survey are as follows: Total length, 393.3 millimeters (15.5 inches); tail, 212.8 millimeters (8.4 inches); hind foot, 36.2 millimeters (1.4 inches).

FIG. 3A, FIG. 3B.—Ears of brown rat and black rat, showing relative
size.
]

Both albinism and melanism are frequent among rats, and pied forms also are common. It has been claimed that all the white rats (albinos) of the bird stores are _Mus rattus_, but albinism is by no means confined to this species. Doctor Hatai found that all the colonies of white rats maintained at the neurological laboratories of Chicago University and the Wistar Institute of Anatomy, Philadelphia, were of the _M. norvegicus_ species.[F] The same is true of all the albino rats in the collections of the National Museum and the Biological Survey. These collections contain also several spotted rats (gray and white) and sooty-black specimens indistinguishable in color from _M. rattus_, all being undoubtedly of the _M. norvegicus_ species.

Footnote F:

Biological Bulletin, vol. 12, pp. 266–273, March, 1907.

KEY TO THE SPECIES OF MUS IN AMERICA.

Size small. Total length of adult less than 200
millimeters _Mus musculus_

Size large. Total length of adult exceeding 300
millimeters.

Ears moderate, when laid forward barely or not
reaching eye; tail shorter than (rarely equal
to) the length of head and body, darker above
than below; color of body normally gray-brown
above, white below; hind foot 38–46 millimeters _Mus norvegicus_

Ears larger, when laid forward reaching at least
to middle of eye; tail longer than head and
body, dusky all around; hind foot 33–37
millimeters.

Color grayish-brown above, white or yellowish _M. rattus
white below. alexandrinus_

Color blue-black above, slaty below _M. rattus_

HABITS OF RATS.

BREEDING HABITS.

Both climate and food supply affect the rate of multiplication of most rodents. The rat probably increases more rapidly in a temperate and equable climate than in one of great variability. Extremes of heat and cold retard multiplication, decreasing both the number of litters in a year and the number of young at a time. In northern latitudes, apparently, more or less interruption of breeding occurs in the winter months.

Where the country is well settled the food supply of rats is not likely to be deficient; and when the animals have access to stores of grain, the young mature very quickly and probably reproduce earlier than when grain is absent.

The brown rat is more prolific than either the roof rat or the black rat. The female brown rat has usually 12 mammæ—3 pairs of pectoral and 3 pairs of inguinal—although these numbers are not constant, one or more teats frequently being undeveloped. The black rat and the roof rat have only 10 mammæ—2 pairs of pectoral and 3 pairs of inguinal—with but little tendency to vary. Records of actual observations on the number of young confirm the deductions that might be drawn from the above facts. At Bombay, India, during the recent investigations made by the India Plague Commission, 12,000 rats were trapped and examined. The average number of embryos found in pregnant brown rats was 8.1; the highest number, 14. The average for the black rat was 5.2; the largest number, 9.[G]

Footnote G:

Etiology and Epidemiology of Plague, p. 9, Calcutta, 1908.

In temperate latitudes the average number of young produced by the brown rat is undoubtedly greater. Instances of very large litters observed in England are recorded in The Field (London). In two instances 22 and 23 young, respectively, were found in a single nest, though no evidence is offered that these were the progeny of a single female; but in two other cases 17 and 19 embryos were found in gravid females. A dealer in feedstuffs in Washington, D. C., relates that he found 19 young rats in a single nest in his store. Within the past few months the writer has examined four pregnant brown rats taken in traps. The numbers of embryos they contained were 10, 11, 11, and 13, respectively. While we have not enough data for definite conclusions, we may safely state that the average litter for this latitude is not less than 10.

Frank T. Buckland, in Curiosities of Natural History, relates that a white rat which he kept in captivity gave birth to 11 young when only eight weeks old. As gestation in rats occupies three weeks, this animal must have bred when only five weeks old.

The number of times rats breed in a year is not definitely known, and probably varies considerably with local conditions. Kolazy makes the almost incredible statement that two female white rats, kept by him in confinement and well fed, within thirteen months gave birth to 26 litters of young, numbering 180 in all. One of them produced young regularly at intervals of 25 days.[H]

Footnote H:

Verh. Zool. Bot. Gesel. Wien., pp. 731–734, 1871.

The writer recently kept two young female brown rats with a male in a large open cage for several months. One of the females gave birth to young on April 15; the other on April 17. The number in these litters was not observed, as some were devoured soon after birth, and all within three days, presumably by the male rat. On May 23 both females gave birth to young, 24 in number, all in one nest.

The known facts concerning the breeding of the brown rat may be briefly summarized as follows: The animals breed from three to five times a year, each time bringing forth from 6 to 19 young. After a gestation period of twenty-one days, the females give birth to their young in nests built in underground burrows or under floors, stacks, lumber, woodpiles, or other shelter. The young are blind and naked when born, but grow rapidly, and young females are capable of breeding when less than three months old.

Early spring and summer are the periods of greatest reproductive activity among rats. Young, however, are to be found every month of the year.

The above statements apply in the main also to the black and the roof rat, but the number of young in a litter is somewhat smaller. The newly born young of the black rat have not the bright pink color of those of the brown and the roof rat, but are bluish, especially on the upper parts. Black-and-white spotted rats are at first bluish-and-red spotted, the red areas representing the white of the adults.

ABUNDANCE OF RATS.

From the foregoing account of the breeding habits of rats, the great difficulty of ridding cities or large areas of the animals may be readily understood. Ordinarily, they breed more rapidly than they are destroyed. Although few are seen in daytime, at night they fairly swarm along river fronts and wharves, as well as in sewers, stables, warehouses, markets, and other places where food is abundant. Their real numbers may sometimes be discovered when any such harbor is demolished.

An ordinary farm sometimes supports an astounding number of rats. In 1901, an estate of 2,000 acres near Chichester, England, was badly infested with the pests. They were systematically destroyed by traps, poisons, and ferrets, under the supervision of the proprietor. In this way 31,981 were killed, while it was estimated that tenants at the thrashing had destroyed fully 5,000 more. Even then the property was by no means free from rats.[I]

Footnote I:

The Field (London), vol. 100, p. 545, 1902.

During a plague of rats on the island of Jamaica in 1833, the number of rats killed on a single plantation in a year was 38,000. The injury to sugar cane on the island caused by the animals was at that time estimated at half a million dollars a year.[J]

Footnote J:

New England Farmer, vol. 12, p. 315, 1834.

The report of the Indian Famine Commission presented to the English Parliament in 1881 affords one of the best illustrations of the number of rats that may infest a country. An extraordinary number of the animals at that time inhabited the southern Deccan and Mahratta districts of India. The autumn crop of 1878 and the spring crop of 1879 were both below the average, and a large portion of each was destroyed by rats. The resulting scarcity of food led to the payment of rewards for the destruction of the pests, and over 12,000,000 were killed.[K]

Footnote K:

British Medical Journal, Sept. 16, 1905, p. 623.

MIGRATIONS AND INVASIONS.

Migrations of rats have often been recorded. The brown rat is known in Europe quite generally as the migratory rat. The Germans call it the Wanderratte. Pallas narrates that in the autumn of 1727 this species arrived from the east at Astrakhan, southeastern Russia, in such great numbers and so suddenly that nothing could be done to oppose them. They crossed the Volga in immense troops. The cause of this general migration was attributed to an earthquake; but since similar movements of the same species often occur without earthquakes, it is probable that only the food supply of the animals was involved in the migration which first brought the brown rat to Europe.

A seasonal movement of rats from houses and barns to the open fields takes place in spring when green and succulent plant food is ready for them. The return movement takes place in the autumn. This seasonal migration is noticeable even in large cities.

But more general movements of rats frequently occur. In 1903 a multitude of migrating rats spread over several counties in western Illinois. They were noticed especially in Rock Island and Mercer counties. For several years previous no abnormal numbers of the animals were seen, and their coming was remarkably sudden. An eyewitness to the occurrence informed me that as he was returning to his home one moonlit night he heard a general rustling in a nearby field, and soon a great army of rats crossed the road in front of him, all moving in one direction. The host stretched away as far as they could be seen in the dim light. These animals invaded the farms and villages of the surrounding country and caused heavy losses during the winter and summer of 1904. A local newspaper stated that between March 20 and April 20, 1904, Mr. F. W. Montgomery, of Preemption, Mercer County, killed 3,435 rats on his farm. He caught most of them in traps.[L]

Footnote L:

Moline (Ill.) Evening Mail, Apr. 25, 1904.

In 1877 a similar migration of rats into parts of Saline and Lafayette counties, Mo., took place.[M] Also, one came under my own observation in the Kansas River valley in 1904. This valley, for the most part, was flooded by the great freshet of June, 1903, and for about ten days was covered with several feet of water. Probably most of the rats in the valley at the time perished in the flood. Yet in the fall of 1903 much of the district was visited by hordes of rats, which remained during the winter and had so increased by the following spring that serious losses to grain and poultry resulted.

Footnote M:

Forest and Stream, vol. 8, p. 380, July 12, 1877.

No doubt most of the so-called migrations of rodents, were all the facts known, could be accounted for as instances of abnormal reproduction or of failure of food supply in one place, compelling change of habitat. In England a general movement of rats inland from the coast occurs every October. This is known to be closely connected with the closing of the herring season. During the fishing the rodents swarm to the coast, attracted by the offal left in cleaning the herring; and when this food fails, the animals troop back to the farms and villages.

In South America plagues of rats are often periodical, occurring in Parana, Brazil, at intervals of about thirty years and in Chile at intervals of from fifteen to twenty-five years. It has been discovered that these plagues in the cultivated lands follow the ripening and decay of the dominant species of bamboo in each country. The ripening of the seed furnishes for two or more years a favorite food for rats in the forests, where the animals multiply greatly. When this food fails, they are forced to the cultivated lands for subsistence. In 1878 almost the whole crops of corn, rice, and mandioca in the State of Parana were destroyed by rats, causing a serious famine.[N]

Footnote N:

Nature, vol. 20, p. 65, 1879.

An invasion of rats (_Mus rattus_) in the Bermuda Islands occurred about the year 1615. Within two years they had increased so alarmingly that none of the islands was free from them. The rodents “devoured everything that came in their way—fruits, plants, and even trees”—so that for a year or two the people were nearly destitute of food. A law was passed requiring every man in the islands to keep 12 traps set. In spite of all efforts the animals continued to increase, until finally they disappeared so suddenly that they must have been victims of a pestilence.[O]

Footnote O:

Popular Science Monthly, vol. 12, p. 376, January, 1878.

FOOD OF RATS.

Instead of being strictly herbivorous, as might be inferred from their dentition, rats are practically omnivorous.

The bill of fare of the rat includes grains and seeds of every kind, flour, meal, and all food products made from them; fruits and garden vegetables; mushrooms; bark of growing trees; bulbs, roots, stems, leaves, and flowers of herbaceous plants; eggs, chicks, ducklings, squabs, and young rabbits; milk, butter, and cheese; fresh meat and carrion; mice, rats, fish, frogs, mollusks, and crustaceans. This great variety of food explains the ease with which rats maintain themselves in almost any environment.

FEEDING HABITS.

Rats resemble squirrels in the manner of holding food while eating. As soon as they have separated a small portion of food from a larger mass, they sit up, arching the back and holding the morsel in the paws and turning it as a squirrel does. After eating, they brush the mouth and fore parts, including the whiskers (vibrissæ), with the paws until all are clean. Rats drink much water, a habit often taken advantage of in placing traps or poisons for them.

Rats generally feed after sunset, but in places where they are not often disturbed they come out and feed in broad day and even in the sunshine.

The roof rat and the black rat are more expert climbers than the brown rat, which is larger and clumsier. In buildings, the brown rat keeps mainly to the cellar and lower parts, where it commonly lives in burrows. From these retreats it makes nightly excursions to the upper parts of the house in search of food. The roof rat and the black rat live in the walls or in the space between ceilings and roofs. They nest in any of these places.

Rats readily climb trees to obtain fruit. In the Tropics the roof rat and the black rat habitually nest in trees and spend much of their time in these arboreal retreats, while the brown rat makes only occasional excursions into the branches in search of food.

In the open, rats seem to have defective vision by daylight. They move slowly and uncertainly. On the contrary, at the side of a room and in contact with the wall they run with great celerity. This fact suggests that the vibrissæ serve as feelers and that the sense of touch in them is extremely delicate. The animals always prefer narrow spaces as highways—another circumstance which may be made use of in placing traps.

FEROCITY OF RATS.

The ferocity of rats has been grossly exaggerated. The stories of their attacks upon human beings, sleeping infants especially, have but slight foundation. If attacked, nearly all rats defend themselves with the teeth; and no doubt a horde of rats, if hungry, would be formidable. Ordinarily the probability of being bitten by rats is remote, and the bite is not poisonous.