Mode of Infection 593 



of the infected houses, the fleas of all kinds quickly attack them 

 with resulting infection, but if the guinea-pigs are kept in flea- 

 proof cages, or if the cages are surrounded by "Tangle-foot," or 

 "sticky fly-paper," the fleas, not being able to spring over the 

 barrier, are caught on the sticky surfaces and do not reach the guinea- 

 pigs, which then remain uninfected. What is true of the guinea- 

 pigs is undoubtedly true of the rats; the disease is transmitted from 

 rat to rat by fleas. When the rats die, the fleas being hungry, 

 jump upon any convenient warm-blooded animal to satisfy their 

 appetites, and when human beings become their victims, infection 

 may foUow the bites. It is now clearly demonstrated that though 

 Pulex irritans, the human flea, prefers to bite human beings, and 

 Xenopsylla cheopis, the rat flea, prefers to bite rats, under stress of 

 necessity preferences are set aside and miscellaneous feeding prac- 

 tised by these and probably aU other fleas. 



A peculiar circumstance attending flea infection has been dis- 

 covered by Bacot and Martin* who find that when Xenopsylla cheo- 

 pis and Ceratophyllus fasciatus are fed upon septicemic plague blood, 

 the respective fleas suffer from a temporary obstruction at the en- 

 trance of the stomach, caused by a massive growth of the plague 

 bacilli. This cultiure appears to start in the intercellular recesses of 

 the proventriciflus and grows so abundantly as to choke this organ 

 and extend into the esophagus. Fleas in this condition are not 

 prevented from sucking blood, as the pump is in the pharynx, but 

 they only succeed in distending an aheady contaminated esophagus, 

 and on the cessation of the pumping act, some of the blood is forced 

 back into the wound. Such fleas are persistent in their endeavors to 

 feed and this renders them particularly dangerous. 



Bacotf found that infected fleas remained infectious when starved 

 for forty-seven days, and that when they were subsequently per- 

 mitted to feed upon mice, another period of twenty days might 

 supervene before the mice became infected. 



The cutaneous and subcutaneous iiioculation in man is followed 

 by lymphatic invasion with bubo formation. Beyond this lymphatic 

 barrier but few bacilH get so that in the greater number of cases 

 with buboes there is little blood infection. However, should the 

 bacilli be highly virulent or the patient exceptionally susceptible, 

 the septicemic form of the disease may supervene, and the case 

 progress to a rapidly fatal termination. 



Intravenous and Intraperitoneal Inoculations produce rapidly fatal 

 septicemic forms of plague. 



KlemJ found that intraperitoneal injection of the bacillus into 

 guinea-pigs was of diagnostic value, producing a thick, cloudy, 

 peritoneal exudate rich in leukocytes and containing characteristic 



* "The Journal of Hygiene," Plague Supplement, m, 1914, p. 423. 

 t "Journal of Hygiene," Plague Supplement, No. iv, Jan., 1915, P- 77o. 

 X "Centralbl. f. Bakt. u. Parsitenk.," xxi, No. 24, July 10, 1897, p. 849. 

 38 



