Rdle of Fleas in the Transmission of Plague 171 



Very recently, Bacot and Martin (1914) have paid especial 

 attention to the question of the mechanism of the transmission of 

 the plague bacilli by fleas. They believe that plague infested fleas 

 regurgitate blood through the mouth, and that under conditions 

 precluding the possibility of infection by dejecta, the disease may be 

 thus transmitted. The evidence does not seem sufficient to establish 

 that this is the chief method of transmission. 



Conclusive experimental proof that fleas transmit the disease is 

 further available from a number of sources. The most extensive 

 series of experiments is that of the English Plague Commission in 

 India, which reported in 1906 that: 



On thirty occasions a healthy rat contracted plague in sequence 

 of living in the neighborhood of a plague infected rat under cir- 

 cumstances which prevented the healthy rat coming in contact with 

 either the body or excreta of the diseased animal. 



In twenty-one experiments out of thirty-eight, healthy rats living 

 in flea-proof cages contracted plague when exposed to rat fleas 

 (Xenopsylla cheopis) , collected from rats dead or dying of septicaemic 

 plague. 



Close contact of plague-infected with healthy animals, if fleas 

 are excluded, does not give rise to an epizootic among the latter. 

 As the huts were never cleaned out, close contact included contact 

 with feces and urine of infected animals, and contact with, and eat- 

 ing of food contaminated with feces and urine of infected animals, 

 as well as pus from open plague ulcers. Close contact of young, 

 even when suckled by plague-infected mothers, did not give the 

 disease to the former. 



If fleas are present, then the epizootic, once started, spreads from 

 animal to animal, the rate of progress being in direct proportion to 

 the number of fleas. 



Aerial infection was excluded. Thus guinea-pigs suspended in a 

 cage two feet above the ground did not contract the disease, while 

 in the same hut those animals allowed to run about and those placed 

 two inches above the floor became infected. It had previously 

 been found that a rat flea could not hop farther than about five 

 inches. 



Guinea pigs and monkeys were placed in plague houses in pairs, 

 both protected from soil contact infection and both equally exposed 

 to aerial infection, but one surrounded with a layer of tangle-foot 

 paper and the other surrounded with a layer of sand. The follow- 

 ing observations were made: 



