Part 14

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 │ Number │ Per │ Average │Rainfall│Character of │examined.│infected.│cent.│temperature.│ in │ days. │ │ │ │ │inches. │ ─────────┼─────────┼─────────┼─────┼────────────┼────────┼───────────── 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. ─────────┼─────────┼─────────┼─────┼────────────┼────────┼───────────── │ │ │ │ │ │{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 rôle 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]

Footnote AF:

Genus Citellus, Oken; subgenus Otospermophilus. “California Mammals,” Frank Stephens.

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 woman (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 to 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.

Footnote AG:

“California Mammals,” Frank Stephens.

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.

RODENT EXTERMINATION.

By Passed Asst. Surg. WILLIAM COLBY RUCKER,

_United States Public Health and Marine-Hospital Service_.

It should be remembered that rodents are extremely wily creatures and that any campaign against them is a contest between the wit of man on the one hand and acute animal instinct on the other. The rat, by his constant association with man, has become extremely wary, and is frightened by anything in the least out of the ordinary. They will eat the bread on which poison is spread so carefully that they will leave behind the poison and take practically all the bread; they will open traps by pressing down the pan, and they have been known to repeat this operation several times within an hour, entering the trap, eating the bait, and then liberating themselves. At other times they will enter the trap and stand on the pan with their hind legs, eat the cheese, then carefully turn around and back out. This, of course, is not possible with snap traps, but they have been known to spring them by causing pieces of wood to fall upon them, after which the bait would be eaten. Rats are found wherever food exists in abundance or where they can find suitable breeding and nesting places.

Rodent extermination is a problem, with difficulties arising from the animal’s highly developed regard for self-preservation. In main, the rat requires two conditions for life. He needs plentiful food and places suitable for nesting and breeding. Eliminate either of these elements and you drive away your rats. Yet the problem remains far more difficult than shown in the simple terms of the above equation. The fabulous speed at which rats multiply will baffle all but the most determined and efficient efforts to exterminate them. Under normal conditions each female bears 3 litters a year and each litter produces 10 young. Under conditions ideally favorable, it has been computed that 1 pair of rats will in five years, providing all can live so long, increase to 940,369,969,152. Such a result is, of course, impossible in nature, for it means that every rat born of the original pair survive five years; that every litter of 10 contains 5 males and 5 females; and that the ideally favorable conditions persist. On the other hand, rodent existence is an unending struggle in which an enormous percentage succumbs; the ratio of half males and half females does not hold; and ordinary conditions of life are hardly even favorable. Nevertheless, the above proves emphatically that no rat eradication can be effective unless the breeding is curtailed. Any campaign against rodents must aim (_a_) to slaughter the greatest possible number of those already living and (_b_) to prevent the possibility of further breeding.

The existing rats are best attacked by trapping, by poisoning, and by their natural enemies. Traps and poisons alone have been found insufficient to keep pace with the rat’s speed of multiplication. The surest of the rat’s enemies are his natural ones, and once they have been loosed upon him his chance of escape is reduced. The cat, dog, skunk, and other rodent foes, given a fair chance, quickly drive out rats. But these animals do not eradicate the pest. The rats will probably migrate to some other shelter, returning when their natural enemies have quieted down. Absolute extermination is reached only when conditions make the continuation of species impossible for the rat.

The size and frequency of rodent litters decreases proportionately with every cutting off of food supplies. Separate the rat from his pabulum and he will not breed so freely nor so often as when he is well fed. Destroy rat habitations and make it impossible for them to find new nesting places, and breeding will virtually cease, since the unsheltered progeny can no longer survive, and since the starving parent rats are driven to cannibalism in the struggle for existence.

Campaigns against rodents must cover five directions: (1) Trapping, (2) poisoning, (3) exposing them to natural enemies, (4) cutting off food supply, and (5) destroying existing nests at the same time that the making of new ones is prevented.

Parenthetically, it may be noted that while these principles apply equally to the extermination of rats in cities and in country districts, their application must vary according to the place.

TRAPPING.

The kind of traps to be used varies with the rodent to be captured and the locality which it infests.

CAGE TRAPS.

The large 19-inch French cage trap gives good results where rats are plentiful. It should be made of stiff, heavy wire and well reenforced, as a large, strong rat will force his head between the wires in a weak trap and thus escape. Before setting, the lever on the trap should be tested to see that it works properly. The trap should be placed on a hard surface, with the rear end a little higher than the entrance, so that the trap will close promptly. When setting the trap in the open it should be fastened to a board on which about an inch of soft dirt has been spread. Place the trap where the rat usually goes for food or in a runway and disturb the surroundings as little as possible. It is sometimes well to place the trap near where there is dripping water, as the rats come there to drink. If the trap is set in hay or straw or wood it should be covered (with the exception of the entrance) with this material. When this is not possible it should be covered with a piece of sacking or placed in a dark corner or beneath the floors. When setting the traps in the sewer a dry place should be chosen.

The rat is more or less of an epicure, therefore the bait should be changed at frequent intervals. Also he should be given food which he is not in the habit of getting. For example: In a meat market vegetables are the best bait, while in a location where vegetables are plentiful fresh liver and fish heads, or a little grain, are best. The following may be suggested as good bait to be used: Fish, fish heads, raw meat, cheese, smoked fish, fresh liver, cooked corn beef, fried bacon, pine nuts, apples, carrots, and corn. When trapping in chicken yards a small chick or duckling is remarkably good. When a large number of rats are caught in one trap, search for the female and leave her alive in the trap, as she may call in the young or the males. The bait should be fastened to the inner side of the top of the trap with a piece of fine wire so that the first rat in can not force the bait underneath the pan and thus prevent the entrance of other rats. A few grains of barley should be scattered near the entrance of the trap and a small piece of cheese or meat fastened to the pan with a piece of wire. It is often well to touch the pan with a feather which has been dipped in oil of anise or oil of rhodium. Before leaving the trap it should be smoked with a piece of burning newspaper to kill the smell of the human hands or the rats which have been in it. Do not handle the trap after burning it out. When trapping in a neighborhood where rats are known to exist the traps should not be moved for three or four days unless they have rats in them, as it is well for the rats to become accustomed to seeing them and thus careless about entering. It is not wise to kill rats where they are caught, as the squealing may frighten the other rats away.

SNAP TRAPS.

Snap or spring traps are best for use in houses and stores, with the exception of fish and meat markets. Snap traps are best for use in runways, beams, and shelves. It is sometimes well to disguise the trap by covering its floor with a little sawdust or dirt. They should be first tested to see that they work properly and that the staples are secure. New traps should be smoked or stained to render them an inconspicuous color.

The bait should consist of some firm material, such as fried bacon or tough meat, and should be tied on so that the rat will be obliged to pull on it and thus spring the trap. The trap should be placed in a corner or close to the wall on a flat, hard surface, so that the rat can not spring it with his tail or by walking on it.

BARREL TRAPS.

In warehouses and granaries large numbers of rats may frequently be trapped by using a barrel or garbage can having a metal top which is carefully balanced. Large pieces of strong cheese are placed in the middle of the cover and a plank laid from the floor to the edge of the barrel. The rat runs up the plank onto the smooth metallic lid which tips and the rat is precipitated into the barrel.

In cities trapping is one of the most effective of the three methods to slaughter rodents. The rat highways are easily discovered and in them traps capture great numbers of the unwary. In the country one can not so readily determine the rat highway. This difficulty diminishes the effectiveness of trapping. To make up for what is thus lost shooting has been resorted to with good results. In Honolulu, where a vigorous campaign against rodents is being waged, a very large proportion of the captured rats (_Mus rattus_ and _M. alexandrinus_) have been shot from trees. In Contra Costa County, Cal., where ground squirrels are being exterminated, it has been found that rodents possess an instinctive suspicion of traps and that during the summer months shooting is not only the most practical but also about the only effective means of attacking them. Shotguns are the weapons to use. A rifle requires the hunter to be a better shot than is ordinarily obtainable for such work, and, furthermore, the danger from its longer range and from ricocheting bullets menaces cattle and farm hands who may be working in the vicinity. As to the shot and the powder charge for shells hunters differ. It is a different problem for every shotgun, depending upon the gun’s caliber and choke. The principle is to put the greatest number of the largest shot the gun will carry into the rodent body. Thus in 10 and 12 gauge guns shoot No. 8 shot and in 16-gauge guns shoot No. 9 shot, but this varies with each individual gun. The use of soft lead or chilled shot seems a matter for personal preference. The charge must be as much as the gun will carry. In the country smokeless powder becomes a necessity during summer months since black powder is liable to ignite the dry grass and stubble.

POISONING.

PLASTER FLOUR.

Plaster flour is prepared by mixing one part dry plaster of Paris with two parts flour or meal. When this is taken in sufficient quantity by the rodent it produces death by the formation of enteroliths, death occurring in from four to eight days. This is a poison of uncertain value and is recommended chiefly on account of its cheapness and small danger to children and domestic animals. It can not be used in wet weather, and judging from the small number of rats found dead with plaster casts in the alimentary canal it is not believed to be very efficacious.

PHOSPHORUS PASTE.