498 PLAGUE 



of the disease ; it has shown, 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 

 being scanty ; especially is this so in the case of mus rattus. 

 In fact, in certain villages where this species alone is present, 

 the disease may actually die out at the end of the epizootic 

 season, and accordingly when plague reappears in these places 

 this is due to a fresh importation a fact of great practical 

 importance. A fresh epizootic first affects chiefly mus decumanus, 

 and a little later spreads to mus rattus, while a little later still 

 the disease attacks the human subject in the epidemic form ; 

 in each case fleas form the vehicle of transmission, and an 

 interval of from ten to fourteen days intervenes between the 

 outbreak of the epizootic and that of the epidemic. The 

 proportion of cases of plague in mus decumanus is much higher 

 than in mus rattus, for the reason mentioned. It has been 

 further shown that the bacilli flourish in the stomach of the 

 flea and are passed in a virulent condition in the fseces, that a 

 large proportion of the fleas removed from plague-infected rats 

 contain plague bacilli, and that the fleas may remain infective 

 for a considerable number of days, sometimes for a fortnight. 

 The subsidence of plague when the mean temperature rises 

 above a certain level (about 80 C.) is probably in part, at least, 

 due to the fact that the bacilli disappear much more rapidly 

 from the alimentary tract of fleas at the higher temperatures ; in 

 accordance with this, experimental transmission of the disease to 

 animals by means of fleas is more frequently successful at 

 lower temperatures. The repeated contamination of flea-bites by 

 means of the excrement of fleas seems to be the most likely means 

 of infection of the human subject. 



In primary plague pneumonia, from a consideration of the 

 anatomical changes and the clinical facts, the disease may be 

 said to be produced by the direct passage of the bacilli into the 

 respiratory passages by inhalation. And accordingly a case of 



