Infection transmitted by the bite of infected blackflies causing severe itching and eye lesions as the adult worm produces larvae and leading to visual impairment and permanent blindness.

The filarial worms (O. volvulus) are transmitted by the repeated bites of infected black flies (Simulium spp.) . These black flies breed in fast-flowing rivers and streams, mostly in remote villages located near fertile land where people rely on agriculture.

In human, female adult worms produce embryonic larvae (microfilariae) that migrate to the skin, eyes and other organs. When a female black fly bites an infected person during a blood meal, it also ingests microfilariae which develop further into the infectious L3 stage and are then transmitted to the next human host during subsequent bites.