It has become increasingly noticeable that common features of many forest and forest/ savanna foci of West African human trypanosomiasis (WAT) are peri-domestic tsetse populations closely associated with domestic pigs. Recent evidence from various sources almost completely proves that, in at least some WAT foci, domestic pigs play an active role as reservoirs of Trypanosoma brucei gambiense. On the basis of this assumption, various aspects of Glossina palpalis and G. tachinoides local distribution and ecology, in relation to domestic pig and human populations, are discussed.

In those WAT foci where domestic pigs are kept, tsetse are widely distributed in forest islands, forest/plantations, and riverine forests, but major concentrations are in villages containing pigs. Close pig/fly relationships at the village level, involving peri-domestic feeding, breeding and resting behaviour on the part of flies, could be important factors in maintaining T.b. gambiense transmission in pig populations.

Endemic transmission of WAT to man could occur, either in villages where low pig densities dictate that a significant proportion of the fly population must also feed on man, or away from the village as a result of contact with infected flies which have strayed from a village transmission cycle.

The development of epidemic conditions could be explained, on the basis of changes in pig density and resultant changes in fly feeding behaviour. The sudden reduction of a pig population could result in an increased number of infected flies feeding on man, until such time as the fly population became stabilized on a reduced pig population (or on other hosts if pigs disappeared totally). During the period of increased feeding of flies on man, T.b. gambiense transmission could be intense.

Minor flare-up of WAT could occur if small numbers of infected pigs were introduced into a village area which had no previous history of pig husbandry. If a loose man/fly association persisted in such a village, endemic WAT could become a characteristic feature of that village.

Because of the fluidity of pig distribution and density, infected pigs could be instrumental in dispersing T.b. gambiense from village to village within a WAT focus and contribute to the persistent character of the focus. Pig movements involving much greater distances could also be important to the transfer of T.b. gambiense strains from one focus to another.