Chemoprophylaxis against bovine trypanosomiasis has been in widespread use in Africa for more than 30 years. However, there have been few attempts to assess its effectiveness in terms of animal performance achieved. Studies undertaken to date have usually considered only viability and growth and have tended to be on a small scale and of short duration. However, records kept at Mkwaja Ranch, Tanzania, by virtue of their completeness and volume, represented a unique and previously untapped source of information for an evaluation of the long-term effectiveness of chemoprophylaxis.
Thus, ILCA and ILRAD in collaboration with Amboni Ltd. (the owners of Mkwaja Ranch) and May and Baker Ltd., carried out an analysis of matching animal health, animal productivity and trypanocidal treatment data based on more than 20000 calving records over the 10-year period from 1973 to 1982.
The level of tsetse challenge at Mkwaja Ranch is such that cattle cannot survive unless protected by trypanocidal drugs. Since 1964 a chemoprophylactic regime based on the use of Samorin (May and Baker Ltd., England) has been used. From 1973 to June 1980, all animals from weaning onwards were maintained under Samorin prophylaxis on a herd basis; Samorin was used at 0.5 mg/kg. Trypanosome infection was monitored by thick blood smears. One month after the last herd prophylaxis, 30 to 40 animals per herd, (herds averaged 225 to 300 animals) were tested every 1 or 2 weeks depending on a subjective assessment of the level of challenge. When approximately 20% of the sample tested was positive for trypanosomes the entire herd was treated with Samorin. In addition, individual animals that appeared sick when entering or leaving night paddocks were tested. If positive for trypanosomes and the next Samorin treatment of the herd was not yet due, that animal would be treated with Berenil (Hoechst, West Germany) at 3.5 mg/kg. All pre-weaning calves were treated at monthly intervals with Berenil. In June 1980, the criteria for herd treatment and the drug regime employed were changed. Beginning 2 months after the last prophylaxis, as soon as routine examination revealed the first positive case, all animals in a herd were treated with Berenil and then 1 week later with Samorin at 1.0 mg/kg. However, in late 1981 and 1982, only Samorin was used as Berenil was not available. The only other control measure tested was that of sterile male release of Glossina morsitans morsitans in a trial carried out during 1978.
Age at first calving was 47 ± 0.1 months and calving interval 15.9 ± 0.1 months. Average weaning weight at 8 months was 134 ± 0.2 kg and calf pre-weaning mortality was 8.0 ± 0.2%. The mean weight of cows, weighed approximately 10 months after their previous calf had been weaned, was 286 ± 0.1 kg while the average annual cow mortality was 5.8%. COW productivity was 101.9 ± 0.5 kg of weaner calf per cow per year.
In order to achieve this level of productivity, an average of 4.6 Samorin and 0.7 Berenil treatments were required per animal per year. The number of treatments varied from year to year and was greater in the south of the ranch where the tsetse challenge was higher. However, age and season of calving had no effect on the number of treatments required. Despite such extensive use of trypanocidal drugs there was no indication from productivity levels of the development of drug resistance. In the same way, there was no evidence that the multiple inoculations of Samorin each animal received over the years had affected its productivity.
Infectious diseases (anaplasmosis and salmonellosis) and predators (mainly lion) were the most important causes of death. Trypanosomiasis was diagnosed in only 1% of calves necropsied, and 3% of adults, indicating that the trypanocidal drug strategy was highly effective.
The reduction in the density of the principal vector, G. m. morsitans Westwood, in part of the ranch during 1978, achieved by a combined programme of insecticide application and sterile male release, had little effect on the numbers of positive blood smears or drug treatments required, but had a small beneficial effect on productivity.
Comparison of the pre-weaning growth of calves born at two locations within Mkwaja and sired by either ranch-bred bulls or selected Kenya Boran bulls through artificial insemination, allowed genotype-environment interaction effects to be examined. The use of the Kenyan sires was beneficial in the location where the overall level of performance was high, but of no advantage in the harsher location where the overall level of performance was low. This suggests that the superiority of the Kenya Boran could not be expressed in the harsher environment.
Thus, in an area where cattle if left untreated rapidly succumb to trypanosomiasis, the strategic use of the prophylactic drug Samorin every 80 days on average, allowed the cattle to survive and be productive, without the development of any drug resistance or other side effects from the use of the drug. Productivity indices built up from the important performance traits allowed not only an evaluation of the productivity at Mkwaja Ranch but permitted comparison with other livestock situations in Africa. Herd productivity was approximately 80% of that of Kenya Boran reared on trypanosomiasis-free ranches considered among the best in Africa. A similar comparison with trypanotolerant N'Dama reared in West and central Africa showed the herd productivity of the Mkwaja Boran to be 35% superior.
In conclusion, the study of the records at Mkwaja Ranch covering a 10-year period shows that with good management and an efficient trypanosomiasis monitoring programme, chemoprophylaxis is highly effective in maintaining beef cattle in areas of high tsetse challenge. The fact that this result is based on one of the largest data sets ever analysed offers immediate hope for increased exploitation of tsetse-infested areas by encouraging more widespread rational use of chemoprophylaxis as an integral part of management. These findings should also provide encouragement to pharmaceutical companies and international agencies to develop new and improved trypanocidal drugs.
