Human serum resistance of metacyclic forms of Trypanosoma brucei brucei, T. brucei rhodesiense and T. brucei gambiense.

Resistance against the lytic action of human serum has been tested among metacyclic and bloodstream forms of Trypanosoma brucei brucei, T.b. rhodesiense and T.b. gambiense stocks and clones. The resistance was determined by applying an in vitro human serum resistance test. Whereas the majority of T.b. gambiense metacyclic forms exhibited stable human serum resistance, T.b. rhodesiense metacyclics showed inconsistent resistance within a minority of parasites, which tended to diminish completely with prolonged passages in rodents. Infection of tsetse flies with in vivo or in vitro selected human serum resistant forms did not significantly increase the proportion of resistant parasites among extruded metacyclic forms. In a T.b. rhodesiense stock which never showed human serum resistance in the metacyclic forms, human serum resistance reappeared after a 2-day cultivation period in the presence of a mammalian serum. These results reflect important phenotypic dynamics and may lead to a better understanding of the epidemiology of African human sleeping sickness.