HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



to infect lower animals with trichomonads from man 

 have not succeeded beyond a reasonable doubt. Escomel 

 (1913) claims to have infected the dog, cat, rabbit and 

 guinea-pig. Lynch (1915a) the rabbit, Boyd (19 19) the 

 rat, Kessel (1924) the monkey, and Escomel (1925) the 

 frog. In every one of these cases there is the possibility 

 that the infection obtained was already present before 

 the experimental animal was inoculated. Kessel (1923) 

 found trichomonads in a rat to which he had fed cysts 

 of Endamoeha histolytica and recovered later motile flag- 

 ellates "smaller than Tritrichomonas of rat, apparently 

 Trichomonas hominis," but this case is also very doubt- 

 ful. Pringault (1920) was unable to infect the cat, rab- 

 bit, guinea-pig and rat; Hogue (1922) likewise failed 

 to infect cats and rabbits that were free from tricho- 

 monads. Kessel (1926b) on the other hand was suc- 

 cessful in infecting 10 of 14 kittens with T. hominis from 

 man either per os or per rectum ; and the diarrheic symp- 

 toms that developed in the positive animals were accepted 

 by him as due to the presence of the trichomonads. The 

 host-parasite specificity of the human intestinal Tricho- 

 monas seems, however, quite rigid. 



4. GIARDIA LAMBLIA 



(i) Epidemiology of transmission. Infection by 

 trophosoites. Giardia lamblia (Figs. 13a, 13b) enters 

 the human digestive tract by way of the mouth either 

 in the trophozoite or cyst stage. Although infections 

 are probably usually brought about by the ingestion of 

 cysts, recent experiments (Hegner, 1926c) indicate that 

 trophozoites may also be infective. Active motile speci- 



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