Localized onchocerciasis vector control in the Bahr el Ghazal Region of South-Western Sudan. I. Precontrol.

Over a two year period, onchocerciasis vector biting and transmission rates were recorded from 11 sites within a 40 km radius of Wau, in the Bahr El Ghazal Region of South-Western Sudan. These data provide the baseline on which the efficacy of localized vector control in the third year of the study, has been assessed (Baker and Abdelnur 1986). During 1018 collection days, 59,660 Simulium damnosum s.l. vectors were attracted to man, 41,548 of which were dissected. 2.8% of the dissected flies were infective with head L3 Onchocerca with a mean of 2.4 head L3s per fly and a maximum of 19. Larval collections revealed that only Simulium sirbanum and S. damnosum s.str. were breeding in the study area. The results were analyzed to show the hourly, seasonal and annual patterns of biting and transmission. These patterns were found to be remarkably similar in both years even though vector biting and transmission was 24-28% lower in the second year. The sites could be divided into two groups. Those with Annual Transmission Potentials (ATPs) over 1000 lay beside fast flowing, well wooded stretches of river and those with ATPs below 500 lay near rivers flowing slowly in a wide grassy plain. Blinding onchocerciasis had been recorded only from the first group.