A great deal of published experimental evidence shows that parasite-induced changes in host choice, and biting and survival rates of bloodsucking insects are an important source of bias in estimates of Vectorial Capacity. I argue here that, because of the difficulties of handling these and other biases in practice, indices (based on the Vectorial Capacity) which purport to measure transmission rate are best used in a comparative rather than an absolute way. They should attempt to measure, not transmission rate, but changes in transmission rate. In this case, the less restrictive assumption that parasites are responsible for constant bias (rather than no bias) leads to relatively small errors. The results encourage a more optimistic view of the performance of classical entomological indices such as the parous rate. They should also help to redirect the principal epidemiological question about vector–parasite interactions: future studies should be concerned with the impact of parasites, not directly on Vectorial Capacity, but on the comparative measures which are actually used to evaluate transmission rate.

Trypanosoma congolense, T. brucei and T. simiae isolated from wild-caught Glossina pallidipes were fed to laboratory-reared G. morsitans centralis and G. m. morsitans to determine the effect of host blood at the time of the infective feed on infection rates. Bloodstream forms of trypanosomes were membrane-fed to flies either neat, or mixed with blood from cows, goats, pigs, buffalo, eland, waterbuck and oryx. The use of different bloods for the infective feed resulted in differences in infection rates that were repeatable for both tsetse subspecies and most parasite stocks. Goat, and to a lesser extent, pig blood facilitated infection, producing high infection rates at low parasitaemias. Blood from cows and the wildlife species produced low infection rates, with eland blood producing the lowest. Addition of D(+)-glucosamine (an inhibitor of tsetse midgut lectin) increased infection rates in most cases. These results indicate the presence of species-specific factors in blood that affect trypanosome survival in tsetse. In certain hosts, factors actually appear to promote infection. The nature of these factors and how they might interact with midgut lectins and proteases are discussed.