126 



Guinea pigs were allowed to run in houses where cases of human 

 and of rat plague were known to have occurred and where many- 

 fleas were present. These rodents served as good traps for the fleas 

 and 29 per cent of them contracted plague. 



Most of the experiments of the Indian Plague Commission were 

 done with the Indian rat flea, the Lmmopsylla cheopis, but they also 

 performed 27 experiments with the cat flea, Ctenocephalus felis, with 

 negative results; 35 experiments with the human flea, Pulex irritans, 

 3 of which were successful; and 2 experiments with the CeratopliyUus 

 fasciatus, the common rat flea of Europe and North America, both 

 of which were successful. 



In San Francisco a few experiments under purely experimental 

 conditions have been carried on by McCoy to determine the abihty 

 of the squirrel flea, the CeratopTiyllus acutus, to transmit plague. 

 Fleas that had been previously fed on the blood of a septicajmic 

 plague-infected squirrel were then allowed to feed from test tubes on 

 healthy guinea pigs. While the feces of some of these fleas up to 

 four days, when inoculated into guinea pigs, were proven to be infect- 

 ive, none of those guinea pigs on which the fleas were allowed to- 

 feed contracted plague. It might be said, however, that in no case 

 were they seen to eject feces while feeding, the significance of which 

 will be apparent later. 



THE BACILLUS IN THE FLEA. 



The Indian Plague Commission found that the average capacity of 

 the rat flea's stomach {Leomopsylla cheopis) was 0.5 cubic milUmeter, 

 and that it might receive as many as 5,000 germs while imbibing blood 

 from a plague rat. They further found that the bacillus would mul- 

 tiply in the stomach of a flea and that the percentage of fleas with 

 bacilli in the stomach varied with the season of the year. In the 

 epidemic season the percentage was greatest for the first four days, 

 and on one occasion the stomach was found filled with Bacillus pestis 

 on the twentieth day. In the nonepidemic season no plague bacilli 

 were found in the stomach after the seventh day. They also found 

 that in the epidemic season fleas might remain infective up to fifteen 

 days, while in the nonepidemic season but seven days, and in the 

 latter case the percentage of infection in animals was much less than 

 in the epidemic season. They showed that while one flea was occa- 

 sionally able to carry the infection this was not usual. It was found 

 that both the males and the females were capable of transmitting 

 the disease. 



After a number of dissections they were unable to demonstrate the 

 presence of bacilli anywhere but in the stomach and rectum. At no 

 time was anything found in the body cavity or salivary glands and 



