Microepidemiology of onchocerciasis in Guatemala: dispersal and survival of Simulium ochraceum.

Abstract Wild Simulium ochraceum females, both blood engorged and non-blood engorged, were collected from human volunteers infected with Onchocerca volvulus, marked with fluorescent dyes, and released from the same locality as they were collected during February and March 1989. A small hyperendemic village located within 0.5 km of streams supporting large populations of S. ochraceum larvae served as the site for both collection and release of adult females. Fifteen sites for the recapture of flies were located within this same village, within two other villages located 1.0 and 3.7 km from it, and at other places spaced approximately 0.25–3.5 km within a coffee agroecosystem. Flies from both groups were recaptured at distances ranging to 3.5 km from the point of release. Non-blood-engorged flies, however, exhibited a greater tendency to disperse away from the release site. Of the total number of blood-engorged flies recaptured, 51.9% were collected at the release point, 25.7% at 1.0 km, and 1.6% at 3.3–3.5 km. The corresponding percentages for non-blood-engorged flies were 26.9%, 40.4%, and 4.4%, respectively. No flies from either group were recaptured at the most distant site, a large village that was 3.7 km away. Marked flies from both groups were recaptured 12–14 days after release, which was sufficient time for the development of infective O. volvulus larvae. A survival rate (4.7%) of marked, blood-engorged flies over the second and third gonotrophic cycles was estimated from the slope of the regression line of the log number of flies recaptured. This compares with the 15.8% survival rate estimated from the ratio of infective stage O. volvulus larvae to early first stage larvae in wild populations of S. ochraceum. This difference suggests that survival rates estimated from mark, release, recapture studies using fluorescent dyes are lower than actual rates by a considerable margin, probably due to excess mortality from marking and handling the insects.