Walsh & Hoyt: Onchocerca volvulus (Onchocerciasis)

Onchocerca volvulus is a white, threadlike (filarial) worm, similar to Loa loa. Female worms are much longer than male warms. Each female produces large numbers of unsheathed larvae (microfilaria). These organisms migrate through the skin and connective tissues. The life cycle is continued when they are ingested by female blackflies of the Simulium species that usually breed near fast-flowing rivers or streams. Like the larvae of Loa loa, the ingested larvae of O. volvulus reach the gut of the fly, penetrate the gut wall, and eventually migrate tothe thoracic musculature and fatty tissues where they metamorphose into third-stage infective larvae over 58 days. The infective larvae subsequently migrate to the proboscis of the fly and are deposited into humans during a blood meal. Once deposited, they enter the skin through the wound made by the insect, molt to fourth-stage larvae, and eventually develop into sexually mature adult worms.