Measuring the capacity of blackflies as vectors of onchocerciasis: Simulium damnosum s.l. in southwest Sudan.
To predict the long-term epidemiological consequences of onchocerciasis vector control an entomological index is needed which is independent of parasitological measurements, but which changes in direct proportion with the parasite inoculation rate. Several indices, based on the vectorial capacity and incorporating Simulium biting and survival rates, were compared. They were calculated from collections of S. damnosum s.l. at human bait at 5 sites within a 12-km radius on the Bussere and Jur Rivers, south-western Sudan, throughout a 20-year pre-control and a 1-year control period. Those indices that included both biting and survival rates performed no better than the biting rate alone in accounting for variation in inoculation rate observed between each site and over the 3 years. Since there was little site-to-site or year-to-year variation in the proportion of flies infected with or infective for Onchocerca spp., estimates of Simulium survival rate were redundant components of the vectorial capacity in this study. Methods for estimating Simulium survival rate need to be tested when there is much more variation in fly infection rate. It would also be useful to investigate possible seasonal fluctuations in survival. These results suggest that the annual biting rate is a reliable guide to the epidemiological success of localized onchocerciasis vector control in this area.