Cattle migration and stocking densities in relation to tsetse-trypanosomiasis challenge in The Gambia.

The local migration of village N'Dama cattle between two study sites, Niamina East and Bansang, 40 km apart in the inland region of The Gambia, West Africa, is described. The consequences of seasonal variations in local stocking densities on the epidemiology of African animal trypanosomiasis are reported. Tsetse abundance at each study site was monitored throughout the study period from trap catches, and cattle censuses at each site were carried out on a monthly basis. Detailed ecological, productivity and health data, including dietary intake and trypanosomiasis prevalence, were collected from selected study herds resident at the two sites and from a third group of (four) herds that migrated annually between the two areas to spend the late dry season period in Niamina East. It was shown that the migration strategy allowed migrants to maintain a high level of green grazing in the diet throughout the year. Cattle were moved to the area of highest tsetse density in the region to obtain this grazing, but it appeared that individual risk of trypanosome infection was diminished by a dilution effect created by locally high livestock densities. Trypanosomiasis prevalences in resident cattle at Niamina East were best correlated with the tsetse catch/trap/day 2 months previously, once this index of tsetse abundance had been corrected to allow for changes in relative stocking density.