TRYPANOSOMA LEWISI 461) 



rats by transferring to them fleas {Ceratojjhylhis fasciatus) taken from 

 infected animals (Fig. 198). Swingle (1911) also conveyed infection by 

 means of fleas (C lucifer and Pulex hrasiliensis), but the exact mechanism 

 of infection was first definitely established by the work of Noller (1912(7), 

 the writer (19136), and Minchin and Thomson (1915), though Swellen- 

 grebel and Strickland (1910) had previously proved that infection was not 

 conveyed by the flea in the act of biting, and had described the course of 

 development in the flea which terminated in the production of the small 

 metacyclic trypanosomes in the rectum. It is now known that infection 

 takes place by uninfected rats eating the dejecta of fleas, or the fleas 

 themselves, which have previously fed on infected rats (Fig. 199). Fleas 

 do not become infective till after the lapse of about six days from the time 

 of their feed on infected blood, during which interval a definite cycle of 

 development takes place in the intestine. Yamasaki (1924) claims that 

 the dog flea is able to infect by its bite as a result of regurgitation of 

 trypanosomes which occur in the stomach and proventriculus, and that 

 this method is as effective as the fsecal method of transmission. 



Minchin and Thomson's method of experiment was to introduce clean 

 rats into a cage of fleas which had previously had an opportunity of feeding 

 on an infected rat. After remaining in the cage for about three days the 

 rats were removed and exposed to chloroform vapour for a short time to 

 immobilize the fleas upon them. The fleas were removed from the rats 

 and returned to the cage. The course of the infection in the rats was then 

 studied. The development in the fleas was traced by exposing clean 

 fleas to infection from an infected rat, and examining them after various 

 intervals. 



Cycle in the Flea. — As already remarked, the main outlines of the 

 developmental cycle in the flea culminating in the production of meta- 

 cyclic trypanosomes in the rectum was first described by Swellengrebel 

 and Strickland (1910), and Swingle (1911), while the mechanism of infection 

 was established by Noller (1912(7) and the writer (19136). Minchin and 

 Thomson (1911) discovered the intracellular stage in the stomach of the 

 flea, an observation confirmed by Noller (1912(7). Minchin and Thomson 

 (1915) published a detailed account of the complete developmental cycle 

 in the flea, and the experiments which led them to accept the view that 

 infection of the rat was brought about by its ingesting the excreta of the 

 flea. Further experiments on the mechanism of transmission with the 

 dog flea, in which he claims that infection may be brought about by the 

 bite, have been conducted by Yamasaki (1924). 



The trypanosomes, which are of the type seen in the late phase of an 

 infection in the rat, are taken into the stomach of the flea, where during 

 the first six hours they undergo a change, which, however, is chiefly a 



