412 FLEAS 



1906 that the rat flea was the principal means of transmission of 

 the bubonic form of the disease. 



As far as is known the plague bacilli live only in the digestive 

 tract of the fleas and do liot infect either the saliva or the body 

 cavity. From this fact it is evident that the germs are inoculated 

 into the wound made by the flea, either with the excrement which 

 is commonly voided while sucking blood or with regurgitated 

 blood. It has been pointed out that a rat flea's stomach will 

 hold about one-half a cubic centimeter of blood and could there- 

 fore take 5000 germs with a single feed from an infected animal. 

 These often multiply to such an extent as to form a solid mass of 

 organisms, blocking the digestive tract of the insect (Fig. 182). 

 It has been stated that when the stomach and 

 intestine of a flea are plugged with plague 

 germs the normal action of the valves of the 

 digestive tract is lost, and the pumping move- 

 ments of the pharynx result in regurgitating 

 infected material into the wound instead of 

 sucking fresh blood from it. Fleas were found 

 by the British Plague Commission to remain 

 infective for 15 days during the height of an 

 FIG 182 Di es e pidemic, though during the non-epidemic 

 tive tract of flea season no individual remained infective for 



plugged with solid more tnan seven (jayg In j ava tne 



growth (in black) of J 



plague bacilli. (After rat fleas have been found to remain infective 

 Manson.) f or 33 days and Bacot has found the European 



rat flea, Ceratophyllus fasciatus, to remain infective, when kept 

 away from a host, for 47 days. 



The Indian rat flea, Xenopsylla cheopis (Fig. 177), is the species 

 most intimately associated with plague transmission. This spe- 

 cies has been introduced with rats on ships into all parts of the 

 tropics, and into seaports in many temperate countries, especially 

 such ports as San Francisco, which trade extensively with the 

 Orient. This species, however, is by no means the only one 

 which can serve in the transmission of plague. It is probable 

 that any species which will attack both man and other sus- 

 ceptible animals, such as rats and ground squirrels, may transmit 

 the infection. Thus in India the human flea, Pulex irritans, 

 and the European rat flea, Ceratophyllus fasciatus, have been 

 proved experimentally to be plague carriers, and in California 



