Casual Agents

Onchocerciasis is the disease caused by the nematode (roundworm) Onchocerca volvulus when it inhabits subcutaneous tissues.

Life Cycle

Onchocerciasis lifecycle

During a blood meal, an infected blackfly (genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound image . In subcutaneous tissues the larvae image develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues image . Adults can live in the nodules for approximately 10–15 years. Some nodules may contain numerous male and female worms. Females measure 33 to 50 cm in length and 270 to 400 μm in diameter, while males measure 19 to 42 mm by 130 to 210 μm. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 10–15 years. The microfilariae, measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span of approximately 12–15 months. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues image . A blackfly ingests the microfilariae during a blood meal image . After ingestion, the microfilariae migrate from the blackfly’s midgut through the hemocoel into the thoracic muscles image . There the microfilariae develop into first-stage larvae image and subsequently into third-stage infective larvae image . The third-stage infective larvae migrate to the blackfly’s proboscis image and can infect another human when the fly takes a blood meal image .

Life cycle image and information courtesy of DPDx.



Page last reviewed: September 9, 2019