Horn the tlea may Transmit its Infection to the Healthy 

 Animal. 



Several methods of transmission are possible, such as (a) 

 the animal eating the infected fleas ; (b) the mechanical 

 conveyance of the bacilli by the pricker ; (c) the regurgita- 

 tion of the stomach contents down the pricker ; (d) the 

 deposition of the fasces on the skin, the bacilli being 

 subsequently rubbed in by scratching. 



This last method is the only one which has been proved 

 capable of bringing about infection. A well-fed flea deposits 

 .a considerable amount of fasces in a surprisingly short time, 

 and it was proved by both the Commission for the Investiga- 

 tion of Plague in India (Report 1907, p. 418) and Yerjbitski 

 (1904) that wounds made by the pricker might afford a 

 sufficient avenue for the entrance of bacilli when liquid 

 containing them was gently applied to recent bites. 



Whilst, however, experiments have shown that infection 

 may be brought about in this way, the possibility of infec- 

 tion by a regurgitation from the stomach preliminary to 

 sucking blood cannot be excluded. 



Proof that Plague is Carried from Animal to Animal by Fleas. 



Simond (1898) infected one rat from another by placing 

 them in a bottle together with 20 fleas. The second, un- 

 infected, rat was enclosed in an iron box with a grating, so 

 that the two animals could not come into contact. 



Gauthier and Raybaud (1902-1903) repeated Simond's 

 experiments under better controlled experimental conditions. 

 They employed a cage, divided in the middle by two wire grills 

 2 cm. apart, placed in a glass jar. In one compartment was 

 placed an inoculated white rat on which had been deposited 

 a dozen fleas captured upon rats from ships in the harbour at 

 Marseilles. When the inoculated animal died a healthy rat 

 was placed in the second compartment, and after some 

 hours had elapsed, during which the fleas transferred them- 

 selves from the dead to the living animal, the cadaver was 

 removed. Gauthier and Raybaud succeeded five times in 

 conveying the infection from one rat to another ; the number 

 of negative experiments is not stated. An examination of 

 the stomachs of the fleas found upon the septicEemic animals 

 showed the presence of B. pestis. The fleas used in their 

 early experiments were not identified. Tidswell (1903) made 

 further attempts to convey plague from rat to rat by the 

 agency of fleas, but was unable to do this. 



Verjbitski (1904) carried out an extensive series of experi- 

 ments on the flea-transmission of plague in St. Petersburg. 

 He showed that the plague bacilli could be recovered from 

 *,he stomach contents of fleas six days after they had fed on 



