vii] FLEAS AND PLAGUE 91 



alimentary canal. On rare occasions it is found in 

 the gullet when fleas have been killed immediately 

 after feeding on septicsemic blood. But no plague 

 bacillus has been found in the body-cavity or in the 

 salivary glands. 



In the stomach of the flea, plague bacilli have 

 been found in vast numbers twelve and even twenty 

 days after the insect has imbibed septiceemic blood. 

 It is naturally of great practical importance to know 

 how long fleas taken from plague-infected rats remain 

 infective : that is to say, are capable of transmitting 

 the infection to healthy animals. Two series of careful 

 experiments, made during the epidemic plague season 

 in India, have shown that fleas could remain infective 

 for as long as fifteen days. In a third series of ex- 

 periments, made during the non-epidemic season, it 

 was found that the fleas remained infective for only 

 seven days. 



It has been ascertained that both the male and 

 the female oriental rat (X. cheopis) flea can transmit 

 plague. 



We come now to one of the most interesting 

 questions of all : namely, the method by which the 

 rat-flea transmits plague to a healthy animal. 



A variety of suggestions have been made, several 

 of which can be shortly dismissed. It was thought, 

 at one time, that infection might be conveyed by the 

 animal eating the infected fleas. But it is very 



