Onchocerca volvulus has a complex life cycle involving a definitive and intermediate host. The only natural definitive host is humans. The intermediate host is a blackfly in the genus Simulium. Common blackfly hosts include Similium damnosum and S. neavei in Africa and S. ochraceum, S. metallicum, S. callidum, and S. exiguum in the Americas. Adult worms typically live from 8-15 years in human hosts, and microfilariae can live up to 2 years. Adults can be found grouped together in various regions of the human body, with groups containing about half as many males as females. The males move from group to group, inseminating the females. Fertilized eggs develop into live microfilariae in 3-12 weeks, and the females release the microfilariae into the host body. A single female worm may release 1300-1900 microfilariae per day for 9-11 years. The microfilariae migrate away from the adult worms, through blood vessels, lymphatics, skin and connective tissue, and generally settle in the skin.
Since the mandibles of the blackfly are not useful for deep piercing, a blackfly feeding on an infected human host will ingest blood and tissue fluid containing microfilariae. Once ingested, the microfilariae are attracted to the fly's thoracic flight muscles, and travel there. In 6-12 days, the microfilariae then develop into juvenile stage 1 (J1), molt into juvenile stage 2 (J2), and molt into juvenile stage 3 (J3), which is the infective, filariform stage. J3 worms then migrate to the labium of the blackfly host, and can infect new human hosts when the female fly next feeds on blood. Upon gaining entry into a human host, the J3 worms develop into adults in 1-3 months. After initial infection, microfilariae from the adults will appear in 10-20 months.