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
[Illustration: [Logo]]
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.
[Illustration:
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.
[Illustration:
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).
[Illustration:
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.