Estimates of the time delay between the infective bloodmeal and maturation (incubation or maturation time) for 4 trypanosome stocks (2 Trypanozoon and 2 Trypanosoma congolense) show that maturation time in tsetse is not a parasite species-specific constant. The mean incubation time of a Trypanosoma brucei rhodesiense stock (EATRO 2340 – 18 days) was not significantly different from one T. congolense stock (SIKUDA88 – 15·5 days) but was significantly greater than another (1/148 FLY9 – 12·5 days). There was no significant difference in incubation times between male and female Glossina morsitans morsitans for any of the stocks but in both of the Trypanozoon stocks the proportion of female flies producing mature infections was significantly less than in males. However, estimates of gene frequency, assuming a model in which maturation is controlled by an X-linked recessive allele, gave inconsistent results indicating that maturation cannot be controlled by a single sex-linked gene. Maturation was shown to be a tsetse sex-dependent phenomenon in Trypanozoon but not in T. congolense infections. Incubation time was quite variable even for a single trypanosome stock (e.g. standard deviation of 5 days for one Trypanozoon stock); we discuss how this variability can affect disease transmission, and the interpretation of age-prevalence data.

The epidemiology of trypanosome infections of Glossina pallidipes was studied at a riverine site in the Zambezi Valley, Zimbabwe for a period of 13 months. Over 9000 flies were captured using a single trap. These flies were dissected, screened for trypanosome infection, sexed, and aged using both wing fray and (for females) ovarian category indices. Midgut infections were identified to species using recently developed DNA probes. The overall prevalence of mature infections was 5·5%, comprising 3·1% Trypanosoma vivax-type and 2·4% T. congolense-type (which included very low prevalences of T. brucei, T. simiae and another Nannomonas species). The prevalence of infection increased with age. For T. vivax-type infections in flies aged by ovarian category this relationship could be described by a simple ‘catalytic’ model assuming a constant per capita rate of infection. For T. congolense-type infections this model tended to over-estimate prevalence in older age classes, implying that the rate of infection decreases with age, and/or that infected flies have higher mortality rates, and/or that a significant fraction of the population is resistant to infection. Prevalences of infection also varied between months. This variation was more marked for T. vivax-type infections and was negatively correlated with both temperature and rainfall. The shape of the age-prevalence relationship, however, did not vary significantly between months. These observations are not fully explained by variation in the age-structure of the tsetse population and are consistent with temporal variation in the rate of infection (rather than in the trypanosome developmental period or in effects of infection on fly mortality). Possible causes of this variation are discussed.