TRYPANOSOMA KHODESIENSE 8 1 



When the converse set of experiments is tried, namely, immuniz- 

 ing an animal against T. rhodesiense, and then inoculating with 

 T. gambiense, the difficulty immediately arises that it is impossible to 

 immunize an animal against T. rhodesiense, owing to its virulence. 

 But a partial and transitory immunity to T. rhodesiense can be obtained 

 by treating the infected animal with drugs, such as arsenophenylglycin. 

 The results, so far as they go, seem to show that an animal immunized 

 against T. rhodesiense is immune not only to T. rhodesiense, but also 

 to T. gambiense, a fact which, according to Mesnil and Leger, does 

 not invalidate the specificity of T. rhodesiense, but tends to show that 

 the two trypanosomes are closely related. 



(5) Mode of Transmission and Reservoir. Kinghorn has shown that 

 T. rhodesiense is transmitted by Glossina morsitans in which it under- 

 goes development. Kinghorn and Yorke 1 found that about 16 per 

 cent, of the wild game examined in Northern Rhodesia was naturally 

 infected with T. rhodesiense. The wild game examined included water- 

 buck, hartebeest, mpala, bushbuck and warthogs. One native dog near 

 the Nyasaland border was found infected, but not domestic stock. 

 Taute doubts whether T. rhodesiense really occurs in wild game. 

 Approximately 3*5 per cent, of the tsetse flies fed on infected animals 

 may become permanently infected with T. rhodesiense, and capable 

 of infecting clean animals. Furthermore, a tsetse fly when once 

 infective probably remains infective for the rest of its life. 



Kinghorn and Yorke, however, have shown that climatic condi- 

 tions, namely, those of temperature, also affect the infectivity of the 

 tsetse fly, as the ratio of flies capable of transmitting T. rhodesiense to 

 those incapable of transmitting the virus is i : 534 in hot valley 

 districts (e.g., Nawalia, Luangwa Valley, temperature 75 to 85 F.), 

 while on elevated plateaux (e.g., Ngoa, on the Congo-Zambesi water- 

 shed, temperature 60 to 70 F.) the ratio falls to i : 1312. 



Mechanical transmission by the tsetse fly does not occur, if 

 a period of twenty-four hours has elapsed since the infecting meal. 



Developmental Cycle in the Fly. The period which elapses between 

 the infecting feed of the flies and the date on which they become 

 infective varies from eleven to twenty-five days in the Luangwa 

 Valley, according to Kinghorn and Yorke. Attempts carried out at 

 laboratory temperature on the Congo-Zambesi plateau, during the 

 cold season, to transmit T. rhodesiense by means of G. morsitans 

 were always unsuccessful. The developmental cycle of the trypano- 

 some in the fly is influenced by the temperature to which the flies 

 are subjected (as stated above). The first portion of the develop- 

 mental cycle proceeds at the lower temperatures (60 to 70 F.), but 

 higher temperatures are necessary for the completion of the develop- 



1 Annals Trap. Med. and Parasitol. , vii, p. 183. 



