258 PUBLIC HEALTH BACTERIOLOGY 



the course of which corresponds exactly to that induced 

 in animals by subcutaneous injection, as described above. 

 An outbreak of bubonic plague is always preceded by an 

 increased death-rate among rats, and that from a disease 

 now known to be due to the B. pestis. The rat flea becomes 

 infected by sucking the blood of the rat, and infects other 

 rats, or it may be human beings. It is now believed that 

 the flea does not inject the bacillus when biting, as no 

 bacilli have been found in its biting apparatus. It is 

 therefore surmised that the mode of infection is by the 

 inoculation of the biting wound by the rubbing in of the 

 excreta and vomit of the flea, both of which are highly 

 charged with B. pestis. The proof that B. pestis multiplies 

 in the stomach of the flea is held to follow from the fact 

 that abundant bacilli may be found in it up to twelve days 

 or longer. It has also been observed that in India plague 

 does not maintain itself in epidemic form after the mean 

 temperature has risen above 80 to 85 F. (26-6 to 29-4 

 C.). This has been found to be associated with a rapid 

 disappearance of the bacilli from the alimentary canal of 

 infected fleas during the prevalence of this higher tempera- 

 ture. Transmission by inoculation must be extended to 

 the infection of attendants on the sick and others, by 

 the rubbing in of soiled linen, etc., and also from earth 

 soiled by the excreta, vomit, and sputum of rats dead or 

 dying of bubonic or pneumonic plague. Such transmission 

 is urged by some as a likely one in the case of barefoot 

 peoples, living in dwellings with earth floors and infested 

 with rats. The transmission from rat to rat is believed 

 to be by the flea, in which case the buboes are mainly 

 cervical ; and by ingestion of rats dead of plague by 

 other rats, when the buboes are mesenteric. 



The subject of transmission in this manner has been 

 mainly studied in India. In Bombay there are two kinds 

 of rats : (i) Mus decumanus, and (2) Mus rattus. Both 

 kinds are infested by the same flea, the Pulex cheopis. The 

 Mus decumanus is the large brown rat, and is the same 

 species as that present in Suffolk, England, to-day. The 

 Mus rattus is the black rat, and is identical with the English 

 black rat. These two species of rats differ fundamentally 

 in their habits. The Mus decumanus is a timid rat, which 



