Life cycle of Onchocerca volvulus

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 (1). In subcutaneous tissues, the larvae (2) develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues (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 micron in diameter, while males measure 19 to 42 mm by 130 to 210 micron. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately nine years. The microfilariae, measuring 220 to 360 micron by 5 to 9 micron and unsheathed, have a life span that may reach two 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 (4). A blackfly ingests the microfilariae during a blood meal (5). After ingestion, the microfilariae migrate from the blackfly's midgut through the hemocoel to the thoracic muscles (6). There, the microfilariae develop into first-stage larvae (7) and subsequently into third-stage infective larvae (8). The third-stage infective larvae migrate to the blackfly's proboscis (9) and can infect another human when the fly takes a blood meal (1).