Trypanozoon: infectivity to humans is linked to reduced transmissibility in tsetse. I. Comparison of human serum-resistant and human serum-sensitive field isolates.

The transmissibility of recent isolates of human serum-sensitive (HSS) and human serum-resistant (HSR) Trypanozoon was compared by transmission of 37 stocks through an inbred line of Glossina m. morsitans. As in previous studies maturation was found to be dependent on fly sex with males producing significantly greater proportions of salivary gland infections than females. HSS stocks were, however, 1.8 times more likely to mature to mammalian infective form than HSR stocks in male tsetse and 2.7 times more likely to mature than HSR stocks in female tsetse. Infectivity to man has apparently evolved at the expense of transmissibility in tsetse. The likelihood of sexual processes occurring in Trypanosoma b. rhodesiense in wild flies is discussed.