Onchocerca volvulus is one of the eight parasitic nematode (roundworm) species that account for most cases of filariasis in humans. This form of filariasis is known as onchocerciasis, or river blindness. Onchocerca volvulus is one of the three of these eight species responsible for most of the morbidity attributable to filariasis (the other two being Wuchereria bancrofti and Brugia malayi, which cause lymphatic filariasis). Onchocerca volvulus occurs mainly in Africa, with additional foci in Latin America and the Middle East. (Centers for Disease Control Parasites and Health website)
During a blood meal, an infected Simulium blackfly introduces Onchocerca volvulus third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound. In subcutaneous tissues, the larvae develop into adult filariae (a slow process than can require up to 18 months), which commonly reside in nodules in subcutaneous connective tissues. Adults can live in the nodules for around 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 and can invade the eyes. A blackfly ingests the microfilariae during a blood meal. After ingestion, the microfilariae migrate from the blackfly's midgut through the hemocoel to the thoracic muscles. Over the next one to two weeks, the microfilariae develop into first-stage larvae and subsequently into third-stage infective larvae. The third-stage infective larvae migrate to the blackfly's proboscis and can infect another human when the fly takes a blood meal. (Centers for Disease Control Parasites and Health website)
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