588 The Blood Flagellates 



seems to be more effective against T. rhodesiense, while the reverse ap- 

 parently is true for tryparsemide and atoxyl. 



An important complication of chemotherapy has been the tendency 

 of trypanosomes to develop a resistance to arsenicals or antimonials 

 (Chapter IX). However, the terms, "arsenic-fastness" and "antimony-fast- 

 ness," sometimes applied to such phenomena, may not be entirely ac- 

 curate. Resistance may be developed against substituted phenyl groups 

 rather than against arsenic or antimony as such. 



Control of sleeping sickness. Man apparently is the main source of 

 infection with T. gambiense, and in many areas, the human reservoir is 

 more than adequate for the maintenance of sleeping sickness. In various 

 parts of British West Africa, for instance, a general incidence of 1-6 per 

 cent has been observed and some villages show a much higher percentage 

 of infection. Distinct reduction in the incidence of human infection has 

 been obtained in West Africa by surveys and treatment of populations. 

 In addition, pentamidine has shown some promise in mass prophylaxis, 

 but there are serious practical difficulties in carrying out such programs 

 on a wide scale. 



Lower animals also may serve as a source of human infection. Domestic 

 and wild animals apparently become infected with T. gambiense occa- 

 sionally and may act as reservoirs, but there is little evidence that wild 

 mammals are especially important in this respect. However, it appears 

 that various types of game, particularly antelopes, may serve as reservoirs 

 for T. rhodesiense, and cattle (72) also are known to become infected. 

 Recent progress in chemotherapy and prophylaxis, on a practical scale, 

 seems to promise not only elimination of the reservoir problem in 

 domestic animals, but also the general control of trypanosomiasis in 

 cattle and other domesticated herbivores. 



Direct attack on tsetse flies helps to control sleeping sickness and also 

 to reduce the incidence of nagana, a serious disease which is caused by 

 T. brucei and has prevented the maintenance of domestic animals in 

 densely infested areas. Measures effective in controlling the flies vary 

 with the haunts and habits of the different species.^ Four species are 

 known to be important in transmission of sleeping sickness. The Gambian 

 variety is spread by Glossina palpalis, and in some areas at least, by 

 G. tachinoides. Rhodesian sleeping sickness is transmitted by G. morsitans 

 and G. swynnertoni. Glossina palpalis is seldom found far from water, 

 unless carried off while feeding on some animal (33), and G. tachinoides 

 has similar habits. Consequently, these species can be controlled in some 

 areas by clearing out bushes and low trees along rivers in the vicinity 

 of villages and river crossings. Such a cleared strip, extending for a half 

 mile or more on each side of a bridge or ford, offers fairly good pro- 



^ The literature of the past forty years, dealing with ecological relationships, feeding 

 habits and breeding habits of tsetse flies, has been reviewed by Jackson (33a). 



