vii INFECTION OF ANIMALS WITH PLAGUE 147 



been indicted by Ogata, Simmonds, Tideswell, Ashburton 

 Thomson, and others ; while Hankin has gone a step 

 further, considering the flea a real intermediary host of 

 plague in much the same fashion as the mosquito is host 

 to malaria. 



As to this latter hypothesis, that of an intermediary 

 insect host of plague, a number of considerations deserve 

 notice which militate greatly against its acceptance even 

 provisionally. But with reference to the flea as a mere 

 agent in spread of plague, Captain Liston in a paper read 

 before the Bombay Natural History Society, as reported 

 in the Lancet of February 25, 1905, is inclined to 

 attribute to the flea, proper to a particular species of 

 rat, an important role not only in the transmission of 

 plague from rats to other animals, but also from the rat 

 to the human subject. Captain Liston's suggestion that 

 plague is not readily transmitted from the wild rat to 

 man is quite in accord with my observation as to the 

 lesser susceptibility of the sewer rat to plague. It 

 harmonises also with the results of my own observations 

 (as described in my reports to the Medical Officer of the 

 Local Government Board for 1902-1903 and 1903-1904) 

 as to the lesser virulence of B. pestis, type 2, or the " rat 

 type," and the probably greater virulence of B. pestis of 

 the domestic rat, which, living in India more in relation 

 with human dwellings, is therefore more likely than the 

 wild rat to be subject to the extra virulent or " human 

 type" of B. pestis. As to Captain Liston's view that 

 plague is commonly transmitted from the domestic rat 

 to man by means of the flea proper to that species of rat, 

 more evidence is wanting. Transmission of plague in 

 this way, though theoretically quite possible, does not seem 



