Role 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 tlje 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: 
