EPIDEMIOLOGY OF TRYPANOSOME INFECTIONS OF THE TSETSE-FLY GLOSSINA-PALLIDIPES IN THE ZAMBEZI VALLEY

The epidemiology of trypanosome infections of Glossina pallidipes was studied at a riverine site in the Zambezi Valley, Zimbabwe for a perio d of 13 months. Over 9000 flies were captured using a single trap. The se flies were dissected, screened for trypanosome infection, sexed, an d aged using both wing fray and (for females) ovarian category indices . Midgut infections were identified to species using recently develope d DNA probes. The overall prevalence of mature infections was 5.5%, co mprising 3.1% Trypanosoma vivax-type and 2-4% T. congolense-type (whic h included very low prevalences of T. brucei, T. simiae and another Na nnomonas species). The prevalence of infection increased with age. For T. vivax-type infections in flies aged by ovarian category this relat ionship could be described by a simple 'catalytic' model assuming a co nstant per capita rate of infection. For T. congolense-type infections this model tended to over-estimate prevalence in older age classes, i mplying that the rate of infection decreases with age, and/or that inf ected flies have higher mortality rates, and/or that a significant fra ction of the population is resistant to infection. Prevalences of infe ction also varied between months. This variation was more marked for T . vivax-type infections and was negatively correlated with both temper ature and rainfall. The shape of the age-prevalence relationship, howe ver, did not vary significantly between months. These observations are not fully explained by variation in the age-structure of the tsetse p opulation and are consistent with temporal variation in the rate of in fection (rather than in the trypanosome developmental period or in eff ects of infection on fly mortality). Possible causes of this variation are discussed.