TRYPANOSOMA GAMBIENSE 533 



(1903) had shown in ZuluUmd that the trypanosomes which develop in 

 the stomach of the tsetse flies are not infective to animals, an observation 

 which was confirmed by Koch (1905), Gray and Tulloch (1905), Minchin, 

 Gray, and Tulloch (1906), and Bouet (1907). Minchin (1908) remarks 

 that Manson made the suggestion that this lack of infectivity was due to 

 the trypanosomes being in a developmental stage, and was in favour of a 

 developmental cycle in the fly. Other observers, notably Cazalbou (1906), 

 Dutton, Todd, and Hannington (1907), Bouet (1907), Eoubaud (1907), 

 Ross, P. H. (1908), and others, made contributions to the subject without, 

 however, solving the problem. They all effected transmission of trypano- 

 somes by means of tsetse flies fed first on infected animals and shortly 

 after on healthy ones. As wild flies were used, it is highly probable that 

 some of the flies were already infected when caught. Ross, P. H. (1907), 

 succeeded in infecting a monkey with what he regarded as T. gambiense 

 by means of wild G. pallidipes, and in the following year (1908) a monkey 

 with the same trypanosome by G. fusca feeding alternately on an infected 

 and the healthy animal. There is, however, considerable doubt as to 

 the species of trypanosome used in these experiments. Kleine (1909a), 

 working in German East Africa with T. brucei, discovered that laboratory 

 bred flies do not become infective till after the expiry of about twenty 

 days from their infecting feed. This important observation proved con- 

 clusively that a definite cycle of development took place in the fly, and 

 explained the failure of the earlier observers, who did not extend their 

 experiments over a sufficiently long period after feeding the flies on infected 

 animals. Kleine's experiments were conducted with T. brucei and G. 

 palpalis, but his results were quickly confirmed by Bruce et al. (1909, 

 1910a, 191 1(^), working in Uganda with T. gambiense and G. palpahs. 

 The important fact of the necessary incubation period in tsetse flies 

 having been established, it was soon demonstrated by Bruce et al. (191 Irf) 

 that T. gambiense went through a cycle of development terminating in 

 infection of the salivary glands, where infective metacyclic trypanosomes 

 appeared (Fig. 223). Bruce et al. (1911c) showed that after ingestion by 

 the fly, T. gambiense in the stomach remained infective to inoculated 

 animals for two days, after which the infectivity was lost. The forms 

 which appear in the salivary glands eventually become capable of infecting 

 animals, the period of non-infectivity of the fly forms corresponding with 

 that during which the fly is unable to transmit the infection by its bite. 



Though G. palpalis is the host of T. gambiense, not every fly which 

 feeds on an infected animal becomes infective. The percentage of flies 

 in which the developmental cycle completes itself varies, but it is well 

 under 10 per cent. As regards the transmission of T. gatnbiense by other 

 species of tsetse flies there is some experimental evidence. Rodhain, 



