Page:Tropical Diseases.djvu/370

328 eruption of small whitish granulations resembling in appearance miliary tubercles. All the organs, and even any serous fluid that may be present in peritoneum or pleura, will be found to contain plague bacilli. In the blood, besides those free in the liquor sanguinis, bacilli are to be found in the mononuclear, though not, it is said, in the polynuclear leucocytes. Feeding experiments.— Rats or mice fed on cultures of plague bacilli, or on fragments of the liver or spleen of animals dead of plague, acquire the disease, and generally die with the characteristic symptoms and lesions. Similarly, as has been shown by Simpson, pigs, calves, sheep, monkeys, hens, pigeons, turkeys, geese, and ducks are affected with plague when fed on plague material. The type of plague induced by feeding is the septicæmic. The disease thus induced may be either of an acute or of a chronic nature. When the latter, it may be ill defined and not easily recognized. Thus, pigs may show no marked signs of illness until a month after feeding on infective material, and then only a few days or hours before death. Susceptibility to plague of the animals of the farmyard, and the chronicity and ill- defined nature of the disease which not infrequently occurs among these animals, as well as in rats, are likely to be important factors in continuing the disease in those endemic centres where people, cattle, pigs, and poultry, in addition to the ordinary domestic vermin, are housed under the same roof, and even in the same room.

Rôle of the rat in plague.— Although small and circumscribed epidemics of plague may occur without the intervention of the rat (Hossack), there can be no doubt that in most epidemics of the bubonic form this ubiquitous rodent plays an important part, both in the introduction and in the spread of the infection. The species principally concerned are Mus decumanus and Mus norvegicus (the grey rat) and Mus rattus (the black rat). The bandicoot and musk rat are of little importance in these respects, although susceptible to the infection. In Bombay the epizootic appears first in the Mus