Life-cycle of Onchocerca volvulus
The parasites that cause onchocerciasis are transmitted from human to human through the bites of blackfly vectors.
The human host
Adult Onchocerca volvulus worms can live for fifteen years in the human body. The male and female worms entwine in nodules in the subcutaneous tissue of the skin. After mating, the female worm releases around 1000 microfilariae larvae a day into the surrounding tissue.
Microfilariae live for 1–2 years, moving around the body in the subcutaneous tissue. When they die, they cause an inflammatory response that leads to skin rashes, lesions, intense itching and skin depigmentation. Microfilariae also migrate to the eye, where they cause inflammation and other complications that can lead to blindness.
The blackfly vector
Onchocerciasis is transmitted through the bites of Simulium blackflies. Blackflies breed in fast-flowing rivers and streams, increasing the risk of infection to people living nearby.
When a female Simulium blackfly bites an infected person during a blood meal, microfilariae are transferred to from the person the fly. Over the course of one to three weeks, the microfilariae develop inside the blackfly to form infective larvae. These are then passed on to other people when the blackfly takes another blood meal. In the human host, the larvae migrate in the subcutaneous tissue, form nodules and slowly mature into adult worms, completing the cycle.