Current considerations on a Loa loa simian reservoir in the Congo

Human filariasis due to Loa loa is confined to the tropical rain-forest of Africa. In the same area, simian populations are infected with parasites which have no clear-cut morphological differences with human Loa and exhibit a nocturnal microfilarial periodicity (Duke and Wijers, 1958). Chrysops lungi and C. centurionis are probably responsible for their transmission among monkeys (Duke, 1955). Although it appears that human and simian strains of Loa have evolved in two separate host-vector complexes (Duke and Wijers, 1958), the possibility of a transmission of simian Loa to man cannot be ruled out. Fain et al. (1974) did observe in Zaïrian patients from the Mayumbe area Loa microfilariae with a nocturnal periodicity. On-the other hand, simian loiasis observations showing a marked diurnal periodicity were reported by Wanson and Rodhain (1953) and Fain (1978). It is most unlikely that C. Iangi and C. centurionis, which are strictly zoophilic, would be responsible for the transmission from monkey to man. Anthropophilic Chrysops species (C. silacea and C. dimidiata) are the only potential vectors (Duke, 1955). In our study area in the Congo, the Chaillu mountains, 5189 anthropophilic flies have been examined in one year. Of these, 3848 were C. silacea (74.2%) and 1341 were C. ditnidiata (25.8%). Host preferences of anthropophilic Chrysops were investigated with a view to identifying possibilities of transmission of Lou between simian and human hosts. To this end a total of 408 blood-meal samples from C. silacea (262 flies) and C. dimidiata (146 flies) were analysed according to a procedure described by Staak et al. (1981). 88.9% and 90.4% of the samples from C. silacea and C. dimidiata, respectively, originated from man, and the rest from hippopotamus, rodents, wild pigs, wild ruminants and monitor lizards. No bloodmeal from monkey was identified. Despite their propensity for coming into contact with monkeys at canopy level (Duke, 1955), our results show that C. silacea and C. dimidiata feed mainly on man and cannot be responsible for the transmission of parasites between simian and human hosts in that part of the Congo.