496 PLAGUE 



in a hut in which guinea-pigs had died of plague, and which contained 

 infected fleas, two cages were placed, each containing a monkey. One 

 cage was surrounded by a zone of sticky material broader than the jump 

 of a flea, another was left without this protection. The monkey in the 

 former cage remained unaffected, but the other monkey contracted 

 plague. 



r - Other experiments showed that when plague bacilli were 

 placed on the floors of houses, they died off in a comparatively 

 short period of time. After forty-eight hours it was not found 

 possible to reproduce plague by inoculation with material from 

 floors "which had been grossly contaminated with cultures of the 

 bacillus. Afterwards, however, animals placed in such a house 

 might become infected by means of fleas. In all these ex- 

 periments the common rat-flea of India — Pulex cheopis (Roths- 

 child) — was used, but it has been shown that this flea also 

 infests and bites the human subject. Recent observations show 

 that not only is plague transferable by means of fleas, but that 

 this is practically the only method obtaining in natural condi- 

 tions, with the exception that rats may become infected by eating 

 the carcases of other animals containing large numbers of 

 plague bacilli. It is improbable from the experiments made 

 that bubonic plague is transmitted by direct contact even when 

 of a close nature ; in fact, it has been shown that plague-infected 

 guinea-pigs may suckle their young without the latter acquiring 

 the disease. The general results show that in tlie bubonic type 

 direct infection by dust and other material through small lesions 

 of the shin plays a comparatively trifling part in the spread 

 of the disease, fleas apparently being in nearly all cases the 

 earners of infection. 



The later work of the Committee supplied information of 

 the highest value with regard to the epidemiology of the 

 disease ; it showed, in short, that plague in its epidemic form 

 is dependent on the epizootic among rats, and with regard 

 to this some further facts may be given. Plague in Bombay 

 occurs in two chief species of rats, the mus rattus, the black 

 house-rat, and mus decumanus, the grey rat of the sewers. 

 The former, owing to its presence in dwelling-houses, is chiefly 

 responsible for the transmission of the disease to man ; while the 

 latter, on account of the large number of fleas which infest 

 it, is of special importance in maintaining the disease from 

 season to season. The year may be divided into two portions 

 — an epizootic season, from December to May inclusive, and 

 a non-epizootic, from June to November. During the latter 

 period there are few cases of plague in rats on account of fleas 



