450 FAMILY: TRYPANOSOMID^ 



In some animals, particularly the goat, only transitory infections are 

 produced, the animals quickly ridding themselves of their trypanosomes. 

 The incubation period and the subsequent course of the infection varies 

 with the dose or number of trypanosomes injected, and also with the type 

 of injection, the intraperitoneal route leading to more certain and rapid 

 infection than the subcutaneous. Naturally, any conditions which lower 

 the vitality of the experimental animals at the same time lower their 

 resistance, and lead to a more intense infection. 



4. IMMUNITY. — As noted above, some animals, though acquiring 

 an infection from inoculation, recover after a lapse of time. This is 

 particularly true of the goat, sheep, and ox. Such recovered animals 

 are found to be resistant to reinoculation with the same trypanosome, 

 but are still susceptible to another. Laveran and Mesnil have employed 

 this method extensively in differentiating trypanosomes. Thus, a goat 

 rendered immune to T. brucei (T. rhodesiense), as proved by reinoculations, 

 was susceptible to T. gatnbiense. Though this method will undoubtedly 

 distinguish trypanosomes of distinct species, it is possible that mere races 

 of one and the same species may give similar results. Furthermore, 

 animals which have recovered from an infection with one species may 

 sometimes be reinfected with the same species. Thus, Noller (19136) 

 has shown that frogs may be infected twice or even three times with the 

 trypanosome, which occurs naturally in these animals (T. rotatorium). 

 Such superimposed infections demonstrate that in many cases the 

 immunity acquired against any particular trypanosome is very inconstant, 

 and that great caution has to be exercised in making deductions from such 

 cross-infection experiments. Martin and Darre (1912) gave an account 

 of a trypanosome which had been acquired accidentally by Lanfranchi 

 when working with T. evansi in the laboratory. The strain of trypano- 

 some recovered from his blood, and also that with which he supposed he 

 had infected himself, were investigated by Mesnil and Blanchard (1914). 

 Lanfranchi stated it was T. evansi, but Mesnil and Blanchard, by use of 

 immunity tests, concluded that the two strains were different. They 

 were, however, unable to identify the human strain with any known 

 trypanosome, and decided to refer to it as " Trypanosoma Lanfranchi.''' 

 Such a result is a direct indication of the unreliability of the immunity 

 test. In his work on T. melophagium of sheep referred to below, Hoare 

 (1923) has shown that so long as sheep are infested with keds, trypano- 

 somes are present in the blood. If the keds are removed, the infection 

 disappears in two or three months, only to reappear again when exposure 

 to keds again takes place. In this case it would seem that any batch of 

 trypanosomes introduced by a ked are able to multiply in the sheep and 

 survive for a limited period. If their disappearance is due to an acquired 



