XIX. On the natural occurrence of chronic plague in rats

Abstract

(1) The characteristic feature of chronic rat plague as described in the foregoing account is the presence of circumscribed abscesses containing plague bacilli in rats caught alive, the animals usually showing no other lesions nor signs of ill-health. No bacilli were seen on microscopical examination of the heart-blood and of the spleen tissue in any of the rats. The bacilli in the great majority of the cases were virulent.(2) We have grouped the 45 rats conforming to this description which were found during the year's investigation in the Punjab into two classes, one group including those in which the lesions were situated in the abdominal viscera, and the other group including those in which the abscesses were found in regions occupied by peripheral lymphatic glands.(3) Lesions of the viscera were found principally in the spleen and in the mesentery, while the submaxillary group are most frequently affected among the lymphatic glands.(4) The peripheral type was observed chiefly during the decline of the epizootic, while the visceral type predominated in the off-season.(5) In Bombay, only one chronic plague rat was met with out of 17,000 plague-infected rats. In Kasel, 9% of all the rats which were proved plague infected had the chronic disease, while in Dhand the proportion was as high as 28%. With our present knowledge we can advance no adequate explanation of these facts.(6) We have no direct evidence that chronic plague, as it occurs in the Punjab villages, possesses any significance in the seasonal recurrence amongst the rats of the infection in an acute form, nor is any evidence available which excludes this possibility.