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. 



(6) 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 

 (Oiospermophilus beecheyi), a condition readily arising from the 

 frequencj' of association of rats with the squirrels in the neighbor- 

 hood of towns, and from the fact that the two species of fleas foxmd 

 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, (= Lcemopsylla 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 canis, the human flea, 

 Pulex irritans, the rat fleas, Ceratophyllus fasciatus and Ctenopsyllus 

 musculi have all been shown to meet the conditions. 



