1. An infected blackfly bites a person and deposits Onchocerca larvae in the skin. The larvae then enter the bite wound.
2. The larvae move into the tissues under the skin (subcutaneous tissues) and form lumps (nodules).
3. The larvae develop into adult worms in the nodules. Adult females may live up to about 15 years in these nodules.
4. After mating, mature female worms produce eggs, which develop into immature forms of the worm called microfilariae. A worm may produce 1,000 microfilariae each day. The microfilariae typically live in the skin and the lymph vessels but are occasionally present in the bloodstream, urine, and sputum.
5. The infection spreads when a blackfly bites an infected person and is infected with the microfilariae.
6. After the microfilariae are ingested, they travel to the middle part of the fly's gut (midgut), then to muscles in its midsection (thoracic muscles).
7–8. There, the microfilariae develop into larvae.
9. Larvae travel to the fly's mouth parts (proboscis) and can be transmitted to other people when the fly bites them.
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.