PATHS AND MODE OF INFECTION 497 



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 faeces, 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° F.) 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. C. J. Martin has shown that infection 

 occurs by regurgitation of infected blood from the stomach of 

 the flea during the act of biting, the proventriculus being some- 

 times blocked by a mass of plague bacilli. The possibility of 

 infection by contamination of the skin by the excrement of fleas 

 containing the bacilli, 'however, cannot be excluded. 



As regards the dying out of epidemics, some interesting facts 

 have been brought forward by Liston. He and his co-workers 

 have shown that rats taken from different towns vary greatly in 

 their susceptibility to inoculation with plague bacilli, and that 

 immunity is most marked in the rats from the towns which have 

 suffered most severely from plague. This relative immunity 

 appears to be due to the survival of the more resistant animals, 

 and holds also with regard to their young. The diminution of 

 plague amongst rats, and thus the subsidence of an epidemic, 

 accordingly depends on the killing off of the more susceptible 

 animals. 



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 



3 2 



