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Casual Agents

Onchocerciasis is caused by nematodes (roundworms) that inhabit 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 The number 1. In subcutaneous tissues the larvae The number 2 develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues The number 3. Adults can live in the nodules for approximately 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 9 years. The microfilariae, measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that may reach 2 years. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues The number 4. A blackfly ingests the microfilariae during a blood meal The number 5. After ingestion, the microfilariae migrate from the blackfly's midgut through the hemocoel to the thoracic muscles The number 6. There the microfilariae develop into first-stage larvae The number 7 and subsequently into third-stage infective larvae The number 8. The third-stage infective larvae migrate to the blackfly's proboscis The number 9 and can infect another human when the fly takes a blood meal The number 1.

Life cycle image and information courtesy of DPDx.