Onchocerca volvulus is a nematode (roundworm) that infects humans. Its vector is Simulium blackflies. The parasite is prevalent in tropical Africa and approximately 20 million people are infected. It is also found in South America and Mexico. 75% of coffee pickers in Mexico are infected with Onchocerca volvulus. The Africans who are infected are able to get the parasite out through the boyle that it forms on the skin by winding it slowly around a matchstick. People from Asia or Europe who contract the parasite have boyles formed on internal surfaces and can require surgery to remove them. The parasite causes river blindness and also damages the skin, turning it leathery. These parasites can live up to 15 years.
A Simulium blackfly bites a human and ingests a microfilaria (a pre-larval stage). This stage penetrates the stomach wall of the fly, enters the thoracic muscles, and developes into the 1st stage larva. It eventually develops into the 3rd stage larva and moves to the head and proboscis of the fly. When the blackfly bites another human, the 3rd stage larva enters the skin and travels through the subcutaneous tissue. There, they develop into males or females and mate to produce more microfilariae. The microfilariae travel up to the skin where they are ready to be taken up by the bite of another blackfly.
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