Factors affecting estimation of tsetse challenge and the expression of trypanotolerance

Trypanotolerance is a relative rather than an absolute trait. Thus, trypanotolerant breeds of livestock can be severely affected by trypanosome infections in high challenge situations or when under stress. Similarly, it has been reported that whilst small ruminants, sheep and goats can be kept under low tsetse challenge with little apparent disease, under a high challenge they will suffer a high degree of mortality. This has been reported by MacLennan (1970) and has also been observed in experimental studies in sheep at the African Trypanotolerant Livestock Network (ATLN) site at Boundiali in Côte d’Ivoire (Hecker et al., 1993). In order to compare trypanotolerance and productivity between breeds in field situations, it is essential to have some estimate of the tsetse challenge or trypanosomiasis risk under which the livestock are kept. Thus, Murray et al. (1981) considered it vital that a quantitative evaluation of tsetse challenge should be made in such studies, however inadequate the available methods for assessing it, in order to critically compare results from different localities. For the purposes of our studies in the ATLN, tsetse challenge has been defined as the product of the relative density of tsetse, their trypanosome infection rate and the proportion of feeds which they have taken from domestic livestock. This provides an index of the numbers of infected tsetse feeding on cattle. Using available data on intervals between feeds this can be converted into a figure for the number of infected bites an animal receives from tsetse in a given period of time (Snow and Tarimo, 1983). Data from a number of sites of the ATLN have provided estimates of tsetse challenge, which have been related to trypanosome prevalence in trypanotolerant and trypanosusceptible cattle at these sites (Leak et al., 1990). Figure 1 shows the relationship obtained from these data, indicating the trypanosome prevalence which might be predicted at any given level of tsetse challenge for trypanotolerant and trypanosusceptible breeds. As would be expected, the curves predict trypanotolerant cattle to have a lower trypanosome prevalence than trypanosusceptible cattle at a given level of tsetse challenge. The rate at which trypanotolerant cattle appear to acquire infections is lower at the same level of challenge than that for trypanosusceptible breeds, and, at sites where the ATLN has carried out its studies, trypanosusceptible cattle were not found in areas with a tsetse challenge as high as that found in some areas with