ON THE INTERPRETATION OF AGE-PREVALENCE CURVES FOR TRYPANOSOME INFECTIONS OF TSETSE-FLIES

Epidemiological models are used to analyse 8 published data sets repor ting age-prevalence curves for trypanosome infections of the tsetse fl y Glossina pallidipes. A model assuming a fixed maturation period and a rate of infection which is independent of fly age is adequate for Tr ypanosoma vivax-type infections, explaining 98% of observed variance i n prevalence by site and age, allowing that the rate of infection may be site dependent. This model is not adequate for T. congolense-type i nfections and the fit can be improved by allowing (i) the rates of inf ection to decline with age (although non-teneral flies remain suscepti ble), (ii) a fraction of resistant flies, which may vary between sites , (iii) increased mortality of infected flies and (iv) variation in th e maturation period. Models with these features can explain up to 97% of observed variance. Parameter estimates from published experimental data suggest that all may contribute in practice but that (i) and/or ( ii) are likely to be the most important.