What is onchocerciasis?

A blackfly (Simulium-damnosum) under microscope (x 100). a parasite comes out of one of its antena

The filarial worms (O. volvulus) are transmitted from person to person 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. A single female worm can release up to 1000 microfilariae larvae per day into the surrounding tissue. 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.

Photomicrograph of Onchocerca volvulus (adult worms)

Adult O. volvulus worms have the ability to live for fifteen years in the human body. Male and female worms are encapsulated in the subcutaneous fibrous tissue known as nodules.

Further reading