Plague / Its Cause and the Manner of its Extension, Its Menace, Its Control and Suppression, Its Diagnosis and Treatment

by Thomas Wright Jackson

When infection takes place through the digestive tract, or in other words, by the ingestion of bacilli, either the flesh of plague-infected animals or fowls, or food superficially contaminated with plague bacilli by rats, cockroaches or other carriers, serves as the medium.

Speaking practically, the possibility of infection through ingestion is nearly negligible. Indeed, the conclusion of Simpson in regard to this possibility has been disputed and denied. However, the recent occurrence of plague in a cat in Manila, in my own experience, observed with me and carefully worked out by Dr. Otto Schöbl, points strongly to the possibility of ingestion plague, the cat in this case apparently having acquired plague from eating rats dead from plague.

A full account of this case appears in the bacteriologic observations of Dr. Schöbl and in my recital of the history of the Manila epidemic.

Types of Plague.—Plague in man may be of several types and these are designated by names descriptive of the symptoms or of the regions of the body most affected. Thus we have bubonic, septicæmic and pneumonic types. As both mild and virulent cases occur, we also use terms descriptive of the severity and course of the cases. Thus we describe certain cases as ambulant, abortive, larval and fulminant. In the rat the evidences of plague are less striking in life than they are at the post-mortem table. Indeed plague-stricken rats, either naturally or artificially (experimentally) infected, often show very slight evidences of disease before death. Chronic plague in rats and a relative immunity to inoculation in certain wild rats are fairly well recognized phenomena.

Flea Conveyance of Plague Bacilli.—Both male and female fleas convey plague, but the exact method of carrying the plague bacilli from diseased rats to man, while fairly well determined, is of such recent decision as to leave room for further experimentation. At present it is believed that the flea deposits plague bacilli, at the time of biting, upon the skin, by ejecting the contents of its rectum and by regurgitation of its stomach contents. At least the flea is known to perform these acts at the time of biting, and the rubbing or scratching of the flea bite with the hand may easily introduce the bacilli into the skin at this spot.[1]

[1] Acknowledgment is hereby made to the Contributors to "The Rat and Its Relation to Public Health" by various authors, prepared by direction of the Surgeon-General, P. H. and M. H. S., for numerous facts utilized in the preparation of this article. The particular contributors whose valuable chapters have been drawn upon for information are D. E. Lantz, C. W. McCoy, D. H. Currie, Carrol Fox, Rupert Blue, W. C. Rucker, R. H. Creel, M. J. Rosenau, V. C. Heiser, W. C. Hobdy, and J. W. Kerr.

The possibility that the flea introduces the plague bacilli upon his mandibles, or the skin-piercing armament with which he is provided, is also to be considered. However, the following facts support the first proposition. It has been experimentally shown that the average capacity of a flea's stomach is about one-half of a cubic millimetre and that thousands of plague bacilli may be ingested by the flea during the biting of a plague-diseased rat; that the plague bacilli multiply enormously and for many days in the flea's stomach and that the bacilli are found only in the insect's digestive tract; that plague bacilli are regurgitated from the stomach and are voided from the rectum with the digested blood.

It has also been proved that almost all varieties of rat fleas, under favorable circumstances, will bite man and that the most common human flea (Pulex irritans) is frequently found upon rats, the flea, generally speaking, being much less particular in his choice of hosts and in his permanence of residence than most insects and ectoparasites in general.

Of the rat fleas, Pulex pallidus (Lœmopsylla cheopis) is common under various names in India, the Philippines, Australia, Italy, Brazil and in tropical countries generally. It bites both rat and man. Ceratophyllus fasciatus, the common rat flea of Great Britain and the United States, also bites both rat and man. In North America and elsewhere certain other fleas of the genus Ceratophyllus have been found upon ground squirrels, cats, rats, sparrows and in chicken yards.

Dog fleas and cat fleas (genus Ctenocephalus) also infest rats, and fleas of other genera are found upon mice, rats and ground squirrels rather indiscriminately.