538 FAMILY: TRYPANOSOMIDiE 



cycle in the gut (Figs. 217 and 223). As the glands are not infective 

 when injected into animals till the final trypanosome stages appear, the 

 latter are the actual infective metacyclic forms. During the whole of this 

 development no sexual process was observed. In spite of failure to 

 observe it, Robertson considers that there is a good deal of circumstantial 

 evidence that a conjugation or some equivalent process takes place. The 

 passage through the fly seems to have some biological significance in 

 playing an " essential role in maintaining the integrity of the species, 

 quite apart from its being a convenient method of transmission." If a 

 sexual process occurs, it will probably be at that stage " which is absolutely 

 essential to the production of a trypanosome viable in the blood of the 

 vertebrate — namely, the crithidial phase in the salivary gland." 



Reservoir Hosts. — Owing to the increase of sleeping sickness along the 

 shores and on the islands of Lake Victoria Nyanza, the prophylactic 

 measure of removing the native population was adopted. Though this 

 was carried out, five years later Duke (1915) found that two fly-boys who 

 had been bitten by G. palpalis on the lake shore or islands contracted 

 sleeping sickness, an inadvertent experiment which proved that reservoirs 

 of T. gambiense still existed in the locality. Bruce et al. (191 le) failed to 

 find T. gambiense in animals examined on the lake shore. Duke (1912a, 

 1912c), however, was able to demonstrate that the sitatunga, Tragelaphus 

 spekei, harboured a trypanosome which he regarded as T. gmnbiense. It 

 was concluded that this antelope was acting as a reservoir for the virus 

 in 1915. But whether the flies acquired their infection only from this 

 source must be doubtful, for more recent observation seems to indicate 

 that the islands had not been kept so free from human beings as was at 

 first supposed. It was proved by Gray and Tulloch (1907) that the dogs 

 of LTganda in endemic areas of sleeping sickness might harbour what was 

 apparently T. gambiense, an observation which was also made, according 

 to Koch, Beck, and Kleine (1909), by Van Someren. Bruce and his co- 

 workers (1910c) showed that cattle might act as a reservoir for T. gam- 

 biense. Healthy animals could be experimentally infected by G. palpalis, 

 and in the fly area they found a naturally infected cow. In a similar 

 manner, antelope (water buck, bush buck, reed buck) could be infected 

 with T. gambiense, while bred flies fed on these animals became infected. 

 No antelope, however, were found naturally infected, though trypanosomes 

 were found in a monkey {Cercopithecus pygerythrus centralis) from the 

 lake shore. Fraser and Duke (1912) showed that antelope may remain 

 in perfect health for over a year after experimental infection with T. gam- 

 biense, and that G. palpalis could be infected from them 315 days after 

 inoculation. Blood from an antelope 327 days after inoculation was still 

 capable of producing infection in rats. Kleine and Taute (1911) succeeded 



