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Onchocerca volvulus life cycle.
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1. During a blood meal, an infected blackfly transmits filarial larvae onto the skin of the human host, where the larvae enter the bite wound.
2. The larvae migrate to subcutaneous tissues.
3. There, the larvae develop into adult filariae, which commonly reside in subcutaneous nodules for up to about 15 yr.
4. Female worms produce unsheathed microfilariae, which are typically present in the skin and the lymphatics of connective tissues but are occasionally present in peripheral blood, urine, and sputum.
5. A blackfly ingests the microfilariae during a blood meal.
6. After ingestion, the microfilariae penetrate the blackfly's midgut and migrate to the thoracic muscles.
7. There, the microfilariae go through 3 stages (L1-L3) of larval development.
8. Larvae migrate to the blackfly's proboscis and can infect another human when the fly takes a blood meal.
Image from the Centers of Disease Control and Prevention Image Library.