Onchocerciasis – or “river blindness” – is a parasitic disease caused by the filarial worm Onchocerca volvulus transmitted by repeated bites of infected blackflies (Simulium spp.). These blackflies breed along fast-flowing rivers and streams, close to remote villages located near fertile land where people rely on agriculture.

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

Onchocerciasis is an eye and skin disease. Symptoms are caused by the microfilariae, which move around the human body in the subcutaneous tissue and induce intense inflammatory responses when they die. Infected people may show symptoms such as severe itching and various skin changes. Some infected people develop eye lesions which can lead to visual impairment and permanent blindness. In most cases, nodules under the skin form around the adult worms.

Onchocerciasis occurs mainly in tropical areas. More than 99% of infected people live in 31 countries in sub-Saharan Africa: Angola, Benin, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Republic of Congo, Côte d’Ivoire, Democratic Republic of the Congo, Equatorial Guinea, Ethiopia, Gabon, Ghana, Guinea, Guinea-Bissau, Kenya, Liberia, Malawi, Mali, Mozambique, Niger, Nigeria, Rwanda, Senegal, Sierra Leone, South Sudan, Sudan, Togo, Uganda, United Republic of Tanzania.

Onchocerciasis is also transmitted in Brazil, Venezuela (Bolivarian Republic of) and Yemen.

WHO recommends treating onchocerciasis with ivermectin at least once yearly for 10 to 15 years. Where O. volvulus co-exists with Loa loa, treatment strategies have to be adjusted. Loa loa is a parasitic filarial worm that is endemic in Cameroon, the Central African Republic, Congo, the Democratic Republic of the Congo, Nigeria and South Sudan. Treatment of individuals with high levels of L. loa in the blood can sometimes result in severe adverse events. In affected countries, it is recommended to follow the Mectizan Expert Committee (MEC)/APOC recommendations for the management of severe adverse events.