TRYPANOSOMA BRUCEI 551 



Morphology, — As already remarked, the human strain of T. brucei 

 is morphologically indistinguishable from that derived from animals, 

 so that Fig. 225, illustrating the latter, will apply equally well to the 

 human strain. In the inoculated small animals the posterior nuclear 

 forms appear. The number of these, however, varies considerably. 

 Thus, in three strains isolated from men and investigated by the writer 

 and Hanschell (1913) the percentages in rats examined on different days 

 varied between and 9-3, and 7-2, and 13 and 40. These observations 

 were made in a series of rats, in which 1,000 trypanosomes were counted 

 every three days. After long passage through rats, the number of posterior 

 nuclear forms may diminish considerably till they become difficult to find. 



Susceptibility of Animals.— In its effect on animals, T. rhodesiense does 

 not differ in any way from T. brucei. It is readily inoculable from 

 man to laboratory animals. If a rat is inoculated with even a few drops 

 of blood from a human case, it quickly acquires a large infection. The 

 trypanosome is virulent to rats from the start, and in this respect differs 

 from T. gambiense. A rat inoculated directly from a man with T. gam- 

 biense acquires a very chronic type of infection, during which trypanosomes 

 are rarely numerous in the blood. It is only after many passages through 

 rats that T. gambieyise attains anything like the virulence the human 

 strain of T. brucei has at its first passage into laboratory animals. 



Transmission. — The human strain of T. brucei was proved to be 

 conveyed by G. morsitans by Kinghorn and Yorke (1912) in Northern 

 Rhodesia. These observers found that the percentage of wild flies infected 

 varied with the altitude. In a valley (2,100 feet, temperature 75° to 83°) 

 1 in 534 flies was found naturally infected, whereas on the plateau (4,400 

 feet), with a mean temperature 15° to 20° lower than in the valley, only 

 1 in 1,260 was found infected. By actual feeding experiments in the 

 valley, about 3 per cent, of flies became infective. Flies fed and kept at 

 the lower temperature did not become infective, though it was shown that 

 after a period of sixty days at the lower temperature the flies became 

 infective when the temperature was raised. The low temperature is thus 

 compatible with the early stages of development, but the final stage 

 requires a higher one. 



Bruce and his co-workers (1913, 19146) in Nyasaland found that 

 G. morsitans was the principal agent, but G. brevipalpis was also incrim- 

 inated. The course of development in the fly is identical with that of 

 T. gambiense and the animal strains of T. brucei. 



Reservoir. — As regards the reservoir hosts, Bruce and his co-workers 

 found that in Nyasaland a large proportion of the wild game harboured 

 the trypanosome, and Kinghorn and Yorke (1912) found the same state 

 of affairs in Northern Rhodesia They, like Bruce, found a natural 



