Whether or not drug-resistant trypanosomes are less pathogenic than susceptible ones remains a controversial issue. Several authors (Berger, Carter and Fairlamb, 1995; Mutugi, Boid and Luckins, 1995; Silayo and Marandu, 1989; Stephen, 1962; Whiteside, 1962) have observed a loss of virulence and/or a loss of fitness in drug-resistant trypanosomes. Transmission by tsetse flies, however, does not appear to affect the drug sensitivity of trypanosomes and drug-resistant strains remain resistant after passage through tsetse flies. This was shown by several workers in the past and has been confirmed more recently (Moloo and Kutuza, 1990; Peregrine, Gray and Moloo, 1997). Recent studies at the International Livestock Research Institute (ILRI), however, using four populations of Trypanosoma congolense, ranging from extremely sensitive to strongly resistant to ISMM, found no differences in virulence between them (ILRI, 1996). Only the most resistant one showed a reduced viability, i.e. it took longer to establish parasitaemia than the other three. The loss of fitness in other drug-resistant parasites is a well known phenomenon and is probably also present in trypanosomes. Well-designed experiments in trypanosome naive definitive hosts using significant numbers of resistant and sensitive isolates should provide valuable data on this controversial but important topic.
To date, few studies have accurately assessed the impact of drug-resistant trypanosomes on livestock productivity, although it is generally assumed that uncontrolled infections will have a severe impact on both survival and productivity. A useful recent study to assess the impact of drug-resistant trypanosomes on the productivity of the local cattle was carried out in the Ghibe valley, Ethiopia, where a high prevalence of multiple drug resistance was reported (Codjia et al., 1993). Rowlands et al. (1994a; 1994b) followed more than 300 East African zebu calves from birth to three years of age, together with their dams, in this region (between 1986 and 1992). During most of the period, animals that were parasitaemic and had a packed cell volume (PCV) below 26 percent, or animals with clinical signs of trypanosomiasis were treated with diminazene aceturate at 3.5 mg/kg, although resistance against this drug was known to occur. Some effects of trypanosome infection on the growth rate of parasitaemic calves were observed, but these were temporary. The authors concluded that regular trypanocidal therapy might have helped to maintain health and productivity of the young cattle. Although calf mortality was rather high, growth rates compared favourably with those in other village-managed systems in Africa. Similarly, reasonable levels of reproduction in terms of calving interval and age at first calving were maintained under regular trypanocidal therapy in the cows that were monitored over the same period (1986 to 1992) (Rowlands et al., 1994b). There was, however, an impact of trypanosome infections on the incidence of abortion. Over 8 percent of pregnancies resulted in abortion or stillbirths and there was a significant increase in the rate of abortion associated with cases of parasitaemia detected during the last trimester of pregnancy.
A benefit-cost analysis was carried out by Itty et al. (1995) to evaluate the financial and economic returns generated by cattle raised in this area (over 500 cattle belonging to nine herds from 1986 to 1989). The latter authors showed that, despite the high level of trypanosomiasis risk, the high prevalence of drug-resistant T. congolense and a moderately high average number of diminazene treatments per year (2.2 and 3.2 for animals aged 12 to 24 and > 24 months, respectively) cattle production was able to generate attractive economic returns for herdowners. The financial analysis (ten-year projections) showed a net benefit-investment ratio varying from 1.1 to 2.4 for the nine herds and an internal rate of return between 12 and 30 percent. This case study shows that profitable cattle production is possible in a problem area with high prevalence of drug-resistant T. congolense. Similar studies should be carried out in other regions with different host genotypes and under different management conditions.