550 FAMILY: TRYPANOSOMIDiE 



reversion of the trypanosome to its antelope type. As pointed out above 

 (p. 539), the possibility of the occurrence of posterior nuclear forms 

 in small laboratory animals was not excluded in Duke's investigations 

 of 1912. 



It is well known that G. morsitans, the chief carrier of T. hrucei, lives 

 a considerable distance from water, while G. palpalis, the vector of 

 T. gambiense, is found along the water-courses or near the lake shores. 

 The former is sometimes spoken of as the " dry fly," and the latter as 

 the " wet one." It may be supposed that when T. hrucei first gained 

 entrance to human beings, who naturally live near to water, it was 

 G. palpalis which necessarily passed the infection from man to man. 

 Antelope, on the other hand, spend the daytime in districts far from 

 water, where G. morsitans is found, and travel long distances at night 

 to drink. In this manner it may be supposed that the human strain 

 gradually became adapted to G. palpalis, while the animal strain remained 

 in association with G. morsitans. If t.his be the case, there would be a 

 double chance of the trypanosome becoming modified morphologically. 

 Duke (1923) described an epidemic of trypanosomiasis east of Mwanza 

 in the Tanganyika Territory (lat. 5° to 2° 3' S., long. 33° 30' to 34°), 

 which throws light on the question under discussion. The trypanosome 

 causing the disease was of the T. hrucei type when inoculated to small 

 animals. Duke believes the human infection arose in 1919 during a 

 famine and an influenza outbreak which reduced the resistance of the 

 already ankylostome-ridden population, and made them susceptible to 

 infection with T. hrucei, which occurred in the game. He does not think 

 the outbreak was due to imported infection. The vector in this locality 

 is the recently discovered G. swynnertoni. Duke believes that the epidemic 

 was due to mechanical transmission from man to man, in spite of the fact 

 that wild flies, infected from either game or man, were discovered in this 

 locality. The cyclically infected flies, including those infected from 

 man, he regards as responsible for the spread of infection amongst the 

 game, man not being susceptible even when the flies had a salivary gland 

 infection derived from man. According to this hypothesis, a fly with a 

 salivary gland infection acquired from a feed on an infected human being 

 would not necessarily be capable of infecting another human being, though 

 it would certainly infect game. In the writer's opinion very substantial 

 evidence is required before this view can be accepted. 



It has still to be mentioned that Laveran and Mesnil (1912) separate 

 T. rhodesiense as a distinct species from T. gamhiense and T. hrucei, as 

 a result of cross-immunity and serological tests carried out chiefly by 

 Mesnil and Ringenbach (1911a, 19126), Laveran (191 1«, 1912, 1912a), 

 and Laveran and Nattan-Larrier (1912, 19126). 



