Seasonal occurrence of Simulium ochraceum, the principal vector of Onchocerca volvulus in the southeastern endemic area of Guatemala.

Investigation of the seasonal occurrence of Simulium ochraceum, the principal vector of the Guatemalan strain of Onchocerca volvulus, were made from August 1978 until January 1980 in the upper reaches of the two main rivers of endemic onchocerciasis in Escuintla, Guatemala. The results showed that in the late dry season S. ochraceum larvae were restricted to permanent streams at intermediate altitudes. During the rainy season, though, the preimaginal sites of this black fly extended to the upper reaches of the numerous small intermittent streams due to partial subterranean flows. Seasonal occurrence of S. ochraceum in these temporary streams was observed throughout the wet season. However, this was most pronounced during the early months of the dry season (October-December) as long as these streamlets continued to flow. The adult population along the headwaters of these two rivers showed a high peak from October-December. This represented a pattern distinct from that of other places where the preimaginal sites of S. ochraceum occurred mostly in larger perennially running streams. The implications which the present findings have for any future larvicidal control program on the onchocerciasis-endemic region of Guatemala are discussed.