172 Arthropods as Direct Inoculators of Disease Germs 
(a) Many fleas were caught in the tangle-foot, a certain pro¬ 
portion of which were found on dissection to contain in their stomachs 
abundant bacilli microscopically identical with plague bacilli. Out 
of eighty-five human fleas dissected only one contained these bacilli, 
while out of seventy-seven rat fleas twenty-three were found thus 
infected. 
' ( b ) The animals surrounded with tangle-foot in no instance 
developed plague, while several (24 per cent) of the non-protected 
animals died of the disease. 
Thus, the experimental evidence that fleas transmit the plague 
from rat to rat, from rats to guinea pigs, and from rats to monkeys 
is indisputable. There is lacking direct experimental proof of its 
transfer from rodents to man but the whole chain of indirect evi¬ 
dence is so complete that there can be no doubt that such a transfer 
does occur so commonly that in the case of bubonic plague it must 
be regarded as the normal method. 
Rats are not the only animals naturally attacked by the plague 
but as already suggested, it occurs in various other rodents. In 
California the disease has spread from rats to ground squirrels 
(Otospermophilus beecheyi), a condition readily arising from the 
frequency of association of rats with the squirrels in the neighbor¬ 
hood of towns, and from the fact that the two species of fleas found 
on them are also found on rats. While the danger of the disease 
being conveyed from squirrels to man is comparatively slight, the 
menace in the situation is that the squirrels may become a more or 
less permanent reservoir of the disease and infect rats, which may 
come into more frequent contact with man. 
The tarbagan (Arctomys bobac ), is a rodent found in North Man¬ 
churia, which is much prized for its fur. It is claimed that this ani¬ 
mal is extremely susceptible to the plague and there is evidence to 
indicate that it was the primary source of the great outbreak of 
pneumonic plague which occurred in Manchuria and North China 
during the winter of 1910-11. 
Of fleas, any species which attacks both rodents and man may be 
an agent in the transmission of the plague. We have seen that in 
India the species most commonly implicated is the rat flea, Xenopsylla 
cheopis, (= Leemopsylla or Pulex cheopis) (fig. 89). This species has 
also been found commonly on rats in San Francisco. The cat flea, 
Ctenocephalus felis, the dog flea, Ctenocephalus cards, the human flea, 
Pulex irritans, the rat fleas, Ceratophyllus fasciatus and Ctenopsyllus 
musculi have all been shown to meet the conditions. 
