Part 3
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.
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.