Chrysops silacea and C. dimidiata: fly densities and infection rates with Loa loa in the Chailu mountains, Congo Republic

Seasonal activity of the loiasis vectors Chrysops dimidata Wulp and Chrysops silacea Austen (Diptera: Tabanidae) was studied during 1987-89 in villages and surrounding forest of the Chaillu Mountains, Congo. Chrysops were captured mainly in the hot rainy season (November-May) and densities of both species were higher in the forest than in villages. C. silacea predominated at all sites and C. dimidiata was rarely found in villages. In the forest, between 07.00 and 18.00 hours, 12 times more C. silacea and 3 times more C. dimidiata were collected with hand-nets (range 15-81 per man-day) than were caught in landing/biting collections on human bait. Chrysops man-biting rates were higher in villages closer to the forest, but variations in loiasis prevalence among villagers (microfilaraemia rates 16-37%) were not proportional to the intensity of contact between people and vectors in the villages, including that the majority of loiasis transmission probably occurs when people go into the forest.