RODENTS IN RELATION TO THE TRANSMISSION OF BUBONIC PLAGUE.
Reading edition
By Surgeon Rupert Blue,
United States Public Health and Marine-Hospital Service.
Man has associated the rat with bubonic plague since the dawn of history. The monuments and coins of the earliest times yield abundant evidence of this association. Æsculapius, the god of the healing art, is represented by the Greeks with a rat at his feet. An early scriptural reference may be found in the first Book of Samuel in the fifth and sixth chapters. The historian records therein the occurrence of a fatal epidemic of “emerods” in the land of the Philistines coincident with an invasion of “mice.”
The inhabitants of southern China in recent times have learned to look upon the finding of sick and dead rats in their homes as a harbinger of evil, in fact, as a forerunner of that dread scourge—“wan-yick,” or plague. In the villages and cities of the Kwantung and Kwangsi provinces, as recorded by medical missionaries, epizootic plague almost invariably precedes an outbreak among human beings. So well is this fact known to the common people that many seek safety in flight, feeling assured that in a short time “yang-tzu” or “wan-yick” will claim a harvest of victims among those who remain.
Doctor Mahē, sanitary officer for the port of Constantinople, in 1889, called attention to the fact that epidemics of plague were always announced by a great mortality among rats and mice. In 1894 Yersin reported the fatal epizootic among rats then prevailing in Canton and Hongkong coincident with the outbreak of plague among the Chinese. Recent researches have confirmed these observations and a great deal has been added to the literature of plague, especially in relation to its mode of transmission. Indeed, it should be said that wherever the disease has prevailed in recent years the relation of rats to its spread has been observed, and that since the discovery of the specific bacillus by Yersin and Kitasato, in 1894, bacteriological investigations have shown that there is no difference morphologically or culturally between the bacilli of human and rat plague. 146Moreover, the gross and microscopic lesions in the lymph nodes are practically the same, and the B. pestis recovered in both fulfills the postulates of Koch.
Nothing was definitely known, however, of the mode of transmission of the disease from rat to rat or from rat to man until the completion of the experimental work of the Indian Plague Commission. Simond, Ogata, Thompson, and Koch each expressed the belief that the infection was transferred by the rat flea. Nuttall (1897) and Simond (1898) demonstrated the presence of B. pestis in the bodies of bugs (Cimex) and fleas which had been taken from plague-sick rats, and the latter observer, in the same year, succeeded in transmitting the disease from rat to rat without contact.
The work of the Indian Plague Commission was undertaken (1905) with a view to establishing the exact relationship between epizootics among rats and epidemics among men, and included both field and laboratory observations. The experiments of Gauthier and Raybaud (1903) and of Simond were repeated on a larger scale and greatly improved in that all rats and fleas used were first identified as to species. The findings of the commission may be briefly summarized as follows: That fleas and bugs taken from plague-sick rats contain B. pestis, and that some of them remain alive in the bodies of the insects from five to sixteen days; that plague is conveyed by the bites of fleas which have previously fed on the blood of animals suffering with the disease; that rat fleas bite man; that under experimental conditions the infection is not transferred from rat to rat in the absence of fleas.
A careful study of the findings of the workers in India justifies the assumption that plague is a disease of the rodent primarily and accidentally, and secondarily a disease of man. An analysis of the epidemiological facts collected in San Francisco leads to the same conclusion. As a result our practice with regard to suppressive measures and quarantine procedure has undergone a radical change in the last decade. If the infection is flea-borne from rat to man in the majority of cases, then the extermination of the rat should be the first principle upon which to base a campaign. In the former contribution on the subject (1907) I stated that “if we destroy the host there is no longer danger of infecting the parasite.” This basic principle has been recognized and successfully applied in two campaigns against plague in San Francisco. First in the outbreak in Chinatown in 1903–4, and again in the larger epidemic of 1907.
The outbreak of 1907 began May 27, a little over a year after the great fire and earthquake, but no cases were discovered between that time and mid-August when the disease began to appear in various parts of the city. The source of infection was, in all probability, a recrudescence from a focus which was not destroyed in the campaign 147of 1903–4. There occurred 160 cases with 77 deaths, the last case appearing January 30, 1908. The following table shows the incidence of human plague:
Year. Cases. Deaths.
1907.
May 1 1
August 13 6
September 56 25
October 34 25
November 41 12
December 13 7
1908.
January 2 1
Total 160 77
EPIDEMIOLOGICAL OBSERVATIONS IN SAN FRANCISCO.
Abundant epidemiological data associating the ratAE with plague have been collected in San Francisco. For the purpose of illustration a detailed reference to a few cases will be made. Two small boys (October, 1907) while playing in an unused cellar found the body of a dead rat. The corpse was buried with unusual funeral honors. In forty-eight hours both were ill with bubonic plague. A laborer finding a sick rat on the wharf picked it up with the naked hand and threw it into the bay. He was seized three days later with plague. Doctor C. and family lived in a second-story flat over a grocery store in the residence section. Being annoyed for some days by a foul odor the doctor caused the wainscoting around the plumbing to be removed. One or two rat cadavers were found in the hollow wall. In two or three days the two members of the family who used the room sickened, one dying on the fifth day of cervical bubonic plague. It is probable that infected rat fleas were set free by the removal of the wainscoting.
Dead rats were frequently found in or near houses where plague had occurred. Immediately upon the discovery of a case of plague trained men were sent into the neighborhood and a thorough search made for rats. This work consisted in the removal of defective wooden floors and walls of insanitary buildings and other harboring places. Extensive rat catacombs were frequently found in these operations. In the yard of a house in which 4 cases had occurred 20 cadavers were found under the board covering. In the walls of a Chinese restaurant 87 dead rats were uncovered.
148Very little can be said of the relation of mice (M. musculus) to the epidemic. While many thousands were trapped, only a few hundred were examined microscopically and in these no infection was found. They are nonmigratory in habit and for this reason are not considered of much importance from an epizoological standpoint.
Transmission from man to man was observed in but a small percentage of cases, 3 per cent to be exact. In these the probability of transference by fleas (P. irritans) or by bugs (Cimex) must be admitted. When more than one case occurred in a house a common source of infection was indicated, such cases occurring simultaneously or within from forty-eight to seventy-two hours after the first. Deratization was the measure mainly relied upon. After an infected house was rat proofed, and the harboring places in the block destroyed, no further cases occurred.
The course of epizootic plague was not interrupted at any time by climatic conditions, there being as many cases in proportion to the rat population in the winter of 1908 as there were at the height of the epidemic. The last case of human plague occurred January 30, 1908, but the infection remained active among rats for eight months longer, or until October 21, 1908. (See following table.)
Month. Number examined. Number infected. Per cent. Average temperature. Rainfall in inches. Character of days.
1907. °F. {Clear, 13.
September 1,002 27 2.69 60.6 0.11 {Part cloudy, 15.
{Cloudy, 2.
{Clear, 10.
October 2,679 23 .86 60.6 1.36 {Part cloudy, 10.
{Cloudy, 11.
{Clear, 14.
November 3,954 36 .88 57.8 .04 {Part cloudy, 13.
{Cloudy, 3.
{Clear, 6.
December 4,308 48 1.11 52.4 3.66 {Part cloudy, 11.
{Cloudy, 14.
1908. {Clear, 5.
January 6,622 70 1.05 50.8 4.88 {Part cloudy, 11.
{Cloudy, 15.
{Clear, 11.
February 11,700 45 .38 51.0 5.39 {Part cloudy, 12.
{Cloudy, 6.
{Clear, 20.
March 19,263 52 .26 54.8 .90 {Part cloudy, 10.
{Cloudy, 1.
{Clear, 17.
April 15,524 34 .21 56.3 .22 {Part cloudy, 10.
{Cloudy, 3.
{Clear, 17.
May 11,311 20 .13 55.4 .76 {Part cloudy, 12.
{Cloudy, 2.
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{Clear, 16.
June 13,624 4 0.02 55.3 0.01 {Part cloudy, 9.
{Cloudy, 5.
{Clear, 11.
July 11,204 2 .017 57.4 .02 {Part cloudy, 17.
{Cloudy, 3.
{Clear, 11.
August 10,988 0 .0 57.3 .01 {Part cloudy, 10.
{Cloudy, 10.
{Clear, 16.
September 15,902 0 .0 59.3 .29 {Part cloudy, 9.
{Cloudy, 5.
{Clear, 16.
October 10,178 2 .019 58.8 .061 {Part cloudy, 7.
{Cloudy, 8.
The rats examined for September, 1907, were very largely collected from the badly infected districts; the remaining months give a truer picture of the extent of the epizootic in the entire rat population.
THEORIES AS TO THE CAUSE OF SEASONAL PREVALENCE.
The marked seasonal prevalence of plague in man in San Francisco may be given as additional proof of the association of the rat with its spread. In the cold, rainy season, from December to April, the epidemic ceases while the epizootic is apparently not influenced. The anomaly is accounted for when we remember that the rat and its parasites are very susceptible to cold and rain. It is then that the animal seeks a warm, comfortable place from which it does not venture until driven thence by dire necessity. In other words, the association of the rat with man is not so intimate in winter, while the reverse is true of the relation of rat with rat. The rains, while interrupting the overground migrations and domiciliary visits of rats, drive them to overcrowded burrows and harboring places. Another factor should be mentioned in this connection. Human fleas (P. irritans), and probably rat fleas also, are markedly reduced in numbers at that season of the year. We must conclude, therefore, that the seasonal prevalence of plague in man is due to the effect of climatic conditions upon the habits of rats and the life history of the insect carriers of the bacilli.
An examination of the foregoing should convince everyone that all former theories as to the prolonged viability of B. pestis in contaminated soil or in polluted streams, and of the periodical spread of the infection therefrom, are no longer tenable. It may also be stated that insanitary conditions, except in so far as they furnish food and shelter to rats and other vermin, play no important 150rôle in the continuance of plague. This general revision has also eliminated overcrowding as an important factor. In the absence pneumonic cases, and of suctorial insects, this bête noire of the sanitarian may be disregarded.
THE OCCURRENCE OF PLAGUE IN THE MARMOT OF ASIA AND THE GROUND SQUIRREL OF CALIFORNIA.
Rudenko (1900) first pointed out the possibility of contagion by the “Tarbagan,” a species of the arctomyinæ found in Siberia. He observed a connection in 1894 between this rodent and an outbreak of plague in a Cossack family of Soktuewsk. According to Beliatsky and Zabolotny, each having been an observer in the same field, the natives of Siberia and Mongolia often acquire plague in this manner. Le Dantec and other writers have called attention to the probable susceptibility of the marmot (Arctomys bobac), a hibernating rodent of India and China. The marmot of Thibet, in the opinion of this writer, is the natural animal host and purveyor of the virus. The literature of the subject presents no bacteriological evidence, however, of such a relationship, and plague in the arctomyinæ of Asia is merely an hypothesis. There is positive evidence though of the susceptibility of the tree squirrel (Sciurinæ) to plague infection. Dr. Alice Corthorn (1898) reported the finding of a plague-infected squirrel in one of the outbreaks in the Bombay Presidency.
PLAGUE INFECTION IN GROUND SQUIRRELS.[AF]
The demonstration of natural plague in the California ground squirrel (Otospermophilus beecheyi) is perhaps the most important observation of the antiplague work of the service in 1908. The existence of a plague epizootic in Contra Costa County was suspected as early as the summer of 1903, and efforts were made at that time to collect sick and dead rodents for bacteriological examination. In August (1903) two fatal cases of human infection occurred in widely separated sections of the county. The investigation which followed failed to connect either with a previous case of human plague, but showed an association with ground squirrels. These deaths occurred during a fatal epizootic among ground squirrels and suggested a connection which unfortunately was not confirmed.
None of the circumstances were forgotten, however, and in the second campaign, begun in September, 1907, in San Francisco, inspectors were detailed to examine all persons dying in the area under suspicion. No plague was reported that autumn and winter. Fatal cases occurred and were reported by the inspectors in July, 1908, as follows: A boy (J. F.) died July 15, near Concord, and a young 151woman (M. P.) died July 28, on a ranch 10 miles from Martinez. The two were not associated. An investigation was ordered at once and a force of trappers was hurried to the scene with instructions to collect squirrels from the ranches in the vicinity. The first plague-infected squirrel was found August 5 on the ranch where the boy had died July 15. Of 425 squirrels collected from August 1 to October 12, 4 showed the gross and microscopic lesions of natural plague.
A lad (F. M.) sickened August 5, 1908, in Los Angeles, Cal., after being bitten by a sick ground squirrel. A polyadenitis, which afterwards proved to be plague, developed in a few days. A dead squirrel was found nearby and pathological specimens taken from it were sent to the United States Plague Laboratory in San Francisco. McCoy recovered B. pestis from the tissue of the animal. This was the only case of plague reported in Los Angeles. In order to complete the list of those who contracted plague in the country, two other cases should be mentioned. F. S., a pregnant woman, died of bubosepticæmic plague near Concord, Cal., February 29, 1904. The B. pestis was recovered in pure culture from the axillary glands. In April, 1906, a school boy of east Oakland developed a multiple plague adenitis. Investigation showed that he had shot and handled ground squirrels in the country four or five days before his illness.
THE NATURAL HABITAT OF PLAGUE.
The location of the natural habitat of plague has concerned sanitarians for many years. Not a few have settled upon India as the endemic center, while others associate China with the epidemics which have devastated Europe from remote times. Le Dantec, a recent writer, suggests the “lofty mountains” between India, Thibet, and China as the exact location, and selects the rodent (marmot) of that region as the natural enzootic host.
A panzootic leaves in its wake enzootics of plague in various countries which persist until the rodents upon which they thrive are either exterminated or rendered immune. At varying intervals epidemics spring from them and finally cease with the exhaustion or destruction of the enzootic foci. Plague disappears in time from these temporary abodes and retires to its original habitat in India or China.
Of serious import in this connection is the fact that all the conditions necessary for the establishment of a permanent focus of plague exist on the Pacific coast of the United States. The broad valleys and lofty mountains of this region are rich in the arctomyinæ, there being no less than 12 species in California alone. In the high Sierras the marmot (Marmota flaviventer),[AG] a species of the natural enzootic host of Le Dantec, is found in great numbers. The ground squirrel infests the valleys and foothills in an unbroken chain from Oregon 152to the Mexican border. Once planted in this ideal soil, infection may never be uprooted or its growth and extension controlled. Small outbreaks will occur here and there, and periodical visitations of greater magnitude may be expected in cities where a combination of epidemiological factors is permitted.
The facts as set forth in this paper have caused grave apprehension in the minds of those who have been at all conversant with the conditions in the transbay counties since 1903. At that time the writer recognized the probability of the establishment of a permanent focus of plague in that locality, and subsequent discoveries have proven the correctness of the assumption. This changes the aspect of the problem from that of a local infection to one of national importance. Once established in such a rural community, plague is dislodged with difficulty and only after a campaign covering a considerable length of time. Being a national problem it can be best solved by the Federal Government.
REFERENCES.
The Croonian Lectures on Plague, W. J. Simpson; Journal of Hygiene, Volume VI, No. 4; Volume VII, No. 6; Volume VIII, No. 2; Plague among the Ground Squirrels of California, W. B. Wherry, Journal Infectious Diseases, Volume V, No. 5; California Mammals, Frank Stephens.
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