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.
The ferocity of rats is mainly exercised against members of their own
order. The brown rat is undoubtedly the most formidable of the genus in
America, and possibly in the world; yet when captured it adapts itself
readily to confinement, and in a few days will take food and water
whenever offered. The enmity of this species toward other rats and mice
is well known. It is supposed to have destroyed the black rat over the
greater part of Europe and America, although it is possible that disease
carried by the brown rat was a factor in the disappearance of the other
species. That the black and the roof rat in tropical countries have not
been displaced by the brown rat is probably owing largely to their more
arboreal habits. It is not uncommon in the Far East to find two species
of rats living side by side in the same locality. An example is _M.
imperator_ and _M. rex_ living on one of the Solomon Islands. The first
is a burrowing species; the other arboreal. In 1877 two native species
of rats, _M. macleari_ and _M. nativitatis_, were found living together
in amity on Christmas Island, in the Indian Ocean.[P] About ten years
ago the brown rat was accidentally introduced, and it is now thought
that both the native species are extinct.
Footnote P:
Proc. Zool. Soc. 1888, pp. 517, 534.
When pressed by hunger rats become cannibals and destroy their weaker
fellows. However, when ordinary food is abundant, cannibalism among rats
is rare.
PLAGUE INFECTION IN RATS.
By GEORGE W. MCCOY,
_Passed Assistant Surgeon United States Public Health and
Marine-Hospital Service._
The rat is a known or a suspected factor in the transmission of several
diseases, yet at present, and perhaps for many years to come, the most
immediate and pressing question that concerns us is its relation to the
origin and spread of plague among human beings. For this reason a
discussion of the reaction of these animals to natural and to artificial
infection with _B. pestis_ becomes of prime importance.
Not only are rats believed to be more or less directly responsible for
cases of human plague in a community, but in addition, they are believed
to be the most frequent medium through which plague is carried from one
locality to another, for these animals are good travelers, can live on a
very meager ration, and can do without water for a long time if food is
available. We have found that on a diet of dry grain alone a rat may
live for over a month.
In this connection it may not be amiss to call attention to the
importance of the rat as an agent in conveying plague infection to other
rodents and especially to ground squirrels. There is every reason for
believing that the infection among the squirrels in California was
derived originally from rats. Wherry[16] states that more rats than
ground squirrels have been trapped in the squirrel burrows in the
vicinity of Berkeley, Cal. This shows how easy it might be for rats to
infect squirrels and vice versa.
The clinical manifestations of plague in rats are of little importance.
It is generally said that the plague-infected rat staggers about with a
drunken gait, loses fear of its natural enemies, and is readily
captured. Our experience with artificially infected rats indicates that
the animals show no marked manifestations of illness until shortly
before death when they become quiet, crouch in the corner of the cage,
and try to hide.
It is rather surprising to observe that comparatively few plague rats
are found dead. In the San Francisco campaign, while no accurate figures
are obtainable, certainly not more than 20 per cent of the infected
rodents were found dead, the remainder being trapped. This is probably
due to the fact that the disease is one of several days’ duration, from
two to six most frequently, and during this period there are more
chances of catching the sick rodent in a trap than there are of finding
the body after death, unless the immediate surroundings are known to
harbor infected animals and an especially careful search is made for
cadavers in the places, often difficult of access, where rats have their
burrows and nests.
As plague is a disease that gives rise to such characteristic gross
pathological lesions in man and in laboratory animals, it is but
reasonable to expect that equally distinctive lesions would be found in
the rat, and this we find to be the case.
Skschivan[1], Kister and Schumacher[2], and other writers have observed
and recorded the gross lesions of plague in rats. It remained, however,
for the Indian Plague Commission[3], which had the opportunity of
examining an enormous number of plague rats in Bombay and elsewhere in
India, to crystallize our knowledge of this subject and to point out its
field of usefulness.
As to the comparative value of microscopical and macroscopical methods
of diagnosis, the Indian Plague Commission[3] states that: “The results
of tests carried out for the purpose of comparison make it manifest that
the naked eye is markedly superior to the microscopical method as an aid
in diagnosis, and as the result of our experience we are prepared to
make a diagnosis of plague on the strength of the macroscopical
appearances alone, even though the other results of cutaneous
inoculation and culture are negative and the animal shows signs of
putrefaction.”
Our experience with rat plague, though limited, leads us to the same
conclusion as that arrived at by the Indian Commission in regard to the
value of the gross lesions of plague in making the diagnosis. To one who
is acquainted with them, these lesions are as characteristic as those of
any infectious disease in man. It is quite true that occasionally
atypical cases are encountered where the majority of the gross lesions
are wanting, and in such cases it becomes necessary to resort to the
inoculation of animals or to cultural investigations in order to make a
diagnosis. Such cases are, however, if anything, rarer than are atypical
post-mortem findings in pneumonia or in typhoid fever in man.
MODE OF EXAMINATION.
A brief description of the actual manner of examining rats for plague
infection will be given here.
The rats are immersed in any convenient solution for the purpose of
killing fleas and other ectoparasites that might be capable of carrying
infection from a plague-infected rat.
The following plan of handling rats has been found satisfactory in the
federal laboratory at San Francisco. The rats are nailed to a shingle by
an attendant. Another attendant reads off the address on the tag
attached to the rat, puts a check number on the shingle, and records the
address from which the rat was taken and the check number on the card
shown on page 48. This card is arranged so as to give the data as to the
address from which the rat came, its size, sex, and species. After being
checked the rats are dissected and finally, after examination by the
medical officer, they are removed from the shingle; any plague-infected
rats are burned as soon as the necessary investigation has been made.
The dissection is made by reflecting the skin from the whole front of
the body and neck so as to expose the cervical, axillary, and inguinal
regions. The thoracic and abdominal cavities are then opened with
scissors.
In the inspection, careful search for buboes must be made in the regions
of the various peripheral lymph glands. The abdominal and thoracic
organs must be subjected to a careful scrutiny. It is needless to say
that this work should be done in a rat-proof, well-lighted building that
is provided with water, gas, and sewer connections. The utmost care
should be taken to avoid any undue risk of infection. The wearing of
rubber gloves is not necessary. Everyone who has to handle infected
animals must be sufficiently alive to the danger of infection.
In the extensive work conducted by the Indian Plague Commission[3],
attendants were protected with Haffkine’s prophylactic. This is
undoubtedly a wise precaution and should be taken if possible.
For a worktable on which to dissect the rats we use in San Francisco a
table which slopes gently from the sides and ends toward the center,
where a drain pipe is attached which leads to a vessel containing a
disinfectant. The table is covered with sheet lead.
The layman of average intelligence readily learns to recognize the gross
lesions of rat plague and it is wise to train the laboratory attendants
to do this. Every rat should, however, be subjected to a careful
scrutiny by the physician responsible for the work. The great majority
of rats may be put aside after a cursory examination as entirely beyond
suspicion of infection. Probably 8 or 10 per cent of them will require a
very careful examination for the gross lesions of plague. A card which
we have found very useful for keeping records of suspected and infected
animals is shown on page 34. Probably all of the species of the genus
_Mus_ are susceptible to plague infection. I shall, however, confine
myself to a consideration of plague in the rats found the world over
(_M. norvegicus_, _M. rattus_, _M. alexandrinus_).
In Bombay[18] it has been found that the epizootic among _Mus
norvegicus_ appears first and is probably responsible for the diffusion
of plague among _Mus rattus_. It precedes the infection among _Mus
rattus_ by about ten days, and the opinion is expressed by the Indian
Plague Commission that the usual course of the infection is from the
_Mus norvegicus_ to the _Mus rattus_, and as the latter rodent is a
house dweller in India it is the most frequent source of human
infection.
In San Francisco the _Mus rattus_ population is comparatively small,
contributing perhaps 2 per cent of the total rat population of the city;
but in the section of the city where the large warehouses are found,
especially those where oriental goods are stored, about 15 per cent of
the rats taken are _Mus rattus_. So far as concerns plague infection
about 5 per cent of the rat cases were in _Mus rattus_. It may be of
interest to note that the last infection found among rats in San
Francisco was among the _Mus rattus_ in a large warehouse near the water
front. Two plague-infected rats were found in this building, one October
21, 1908, and the other October 23, 1908. A large number of mummified
carcasses, all _Mus rattus_, were found in the building, and it seems
not unlikely that a somewhat extensive epizootic had occurred among
them. No previous case of rat plague had been found in the city for
eighty-five days, though about 25,000 rats had been examined during that
period, and none have been found in the six months since, although over
30,000 rats have been examined. Our records show that of 84 infected
rats, 79 were _Mus norvegicus_, and the remainder were _Mus rattus_.
Some of the latter may have been _Mus alexandrinus_, as the two species
(_Mus rattus_ and _Mus alexandrinus_) were not clearly differentiated in
the earlier examinations.
THE GROSS LESIONS OF NATURAL RAT PLAGUE—ACUTE PLAGUE.
SUBCUTANEOUS INJECTION.
This is the sign which usually first attracts attention. White[4], in
discussing plague in rats, states that “the most noticeable post-mortem
appearance of the plague rat is the engorgement of the subcutaneous
blood vessels, together with a diffuse pink color of the subcutaneous
muscles, which have a peculiar dry, waxy translucency.” It has been our
experience frequently to have an attendant who is dissecting rats remark
that he had found an infected rat after the first incision was made in
reflecting the skin. The injection is dark red, and upon close
inspection one sees that the small vessels are uniformly distended with
blood. It is usually distributed over the whole surface of the body, but
on two occasions we have seen it confined to the side of the body on
which the primary bubo was found. A bright pink injection is a rather
common finding among rats in San Francisco. It is not likely to be
mistaken for the injection of plague infection. Subcutaneous œdema,
confined to the vicinity of the bubo, is occasionally encountered.
In our experience in San Francisco an injection identical in appearance
with that found in plague infection was found only twice, and in each
case there was associated with it a small discharging subcutaneous
abscess. There were no other lesions in either case and the pus from
these abscesses failed to produce plague in guinea pigs.
In a series of 61 consecutive plague rats in San Francisco, injection
was present fifty-two times, it was confined to the region of the bubo
twice, it was unilateral twice, and was general in distribution
forty-eight times. It was slight thirteen times, moderate fifteen times,
marked sixteen times, intense eight times.
THE BUBO.
This is the most reliable single sign of plague infection, and when
present in typical form is enough on which to base a diagnosis which
rarely proves erroneous.
The gland involved is usually surrounded by a more marked injection than
is present elsewhere, and an infiltration which at times is hemorrhagic.
This surrounding hemorrhage which was common in the plague rats
described by the Indian Plague Commission was met with very rarely in
San Francisco. The gland proper is usually caseous. The contents may be
shelled out very readily, though prior to section the gland feels very
firm. In the cases seen at the federal laboratory in San Francisco, the
contents of the buboes were recorded as being hemorrhagic four times and
as caseous twenty-nine times. Pest-like bacilli were noted as present in
18 cases, in 6 of which the “coccoid” form predominated. They were
recorded as absent five times.
Indolent enlargement of the lymph glands is very commonly encountered in
rats that are not infected with plague. Among old rats probably 15 per
cent will show this. Such glands, however, are tough, elastic, and not
surrounded by infiltration. They are not likely to be mistaken for the
plague buboes. In the leprosy-like disease of rats, the glands may reach
an enormous size.
Observers differ as to the location of the primary bubo. Skschivan[1]
states definitely the location of five primary buboes in plague rats
seen in Odessa in 1901. Two were in the axilla, two in the inguinal
region, and one in the neck. Kitasato[5] says: “To judge from the
experience of the past it can be suggested that in examining rats
particular attention should be paid to their submaxillary and cervical
glands and to the spleen. These organs in most cases show the evidence
of infection, if there be any.” From this it would appear that he
regarded the neck glands as the most frequent seat of the bubo. It may
be remarked here that his experience was derived from plague rats seen
in Asia.
We find a marked difference between the experience in San Francisco and
that in Bombay. This is demonstrated in the following table, which shows
the location in percentage of single buboes in each situation:
───────────────────────────────────────┬───────┬───────┬───────┬───────
│ Neck.│Axilla.│ Groin.│Pelvis.
───────────────────────────────────────┼───────┼───────┼───────┼───────
│ _Per│ _Per│ _Per│ _Per
│ cent._│ cent._│ cent._│ cent._
Indian Plague Commission, Bombay—2,923 │ 75│ 15│ 6│ 4
rats[3] │ │ │ │
Wherry, Walker, and Howell, San │ 12│ 12│ 75│
Francisco[6]—8 rats │ │ │ │
Federal laboratory, San Francisco—32 │ │ 22│ 72│ 6
rats │ │ │ │
───────────────────────────────────────┴───────┴───────┴───────┴───────
The American figures are too small to be of much significance, but one
is struck with the fact that in Bombay three-fourths of the buboes are
in the neck, while in San Francisco three-fourths of all found are in
the inguinal region. We have records of only three multiple buboes found
in rats in San Francisco, and in no case was either of the buboes in the
neck; while in Bombay, to quote from the report[3], “Of the rats with
multiple buboes 54.5 per cent had a bubo in the neck.” Striking as these
figures are, we have collected further evidence that the inguinal region
is the commonest location of the bubo in plague rats in this vicinity.
Passed Asst. Surg. J. D. Long, Public Health and Marine-Hospital
Service, who has had an extensive experience with rat plague in Oakland,
Cal., tells me that the majority of the buboes were found in the groin,
very few in the neck. Acting Assistant Surgeon Wherry, Public Health and
Marine-Hospital Service, informs me that in a series of plague rats
examined after the report made in association with Walker and Howell[6],
the cervical bubo was very rarely encountered.
Particular care was taken to look for cervical buboes, as it seemed
rather inconsistent to find the other lesions so fully in accord with
those found in India, yet to have the location of the bubo to differ so
radically. We have not encountered a mesenteric bubo in our work in San
Francisco. The Indian Plague Commission found none in over 5,000
naturally infected plague rats. As mesenteric buboes are very commonly
encountered in plague infection brought about by feeding, they conclude
that the absence of these buboes in naturally infected rats is strong
evidence that the infection does not enter by the alimentary canal.
THE GRANULAR LIVER.
Two lesions of the liver are encountered in plague rats. The one most
frequently observed is spoken of by the Indian Plague Commission as
“fatty” change, though it is explained that this term refers to the
naked eye appearance as, microscopically, the lesion is found to be due
to a necrosis of the liver tissue. When this change is present the organ
is found to be rather yellowish in color and is studded with an enormous
number of yellowish white granules which are about the size of a pin
head. This lesion, which was very common in the San Francisco cases, is
very readily recognized.
The other lesion is a marking of the organ with grayish white spots;
“they are typically of the size of a pin’s point, and give the surface
of the organ a stippled appearance as if dusted over with gray
pepper”[3] (p. 331). This appearance, which is less frequently
encountered than is the preceding one, is more difficult to recognize;
indeed the most careful scrutiny is necessary to avoid overlooking it.