Onchocerciasis
Key facts
- Onchocerciasis or “river blindness is caused by the parasitic worm Onchocerca volvulus.
- Symptoms include severe itching, disfiguring skin conditions and visual impairment, including permanent blindness.
- People become infected through the bites of infected blackflies (Simulium spp.).
- More than 99% of infected people infected live in sub-Saharan Africa; the disease also exists in some foci in Latin America and Yemen.
- Community-directed treatment with ivermectin is the core strategy to eliminate onchocerciasis in Africa; in the Americas the strategy is biannual large-scale treatment with ivermectin.
Onchocerciasis – or “river blindness” – is a parasitic disease caused by the filarial worm Onchocerca volvulus. It is transmitted through the bites of infected blackflies (Simulium spp.) that breed in fast-flowing rivers and streams, mostly in 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.
Signs and symptoms
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, especially when they die. Infected people may show symptoms such as severe itching and various skin lesions. In most cases, nodules develop under the skin. Some infected people develop eye lesions which can lead to visual impairment and permanent blindness.
Geographical distribution
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
- the Central African Republic
- Chad
- Côte d’Ivoire
- the 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
- the United Republic of Tanzania
It has also been introduced in Yemen.
Onchocerciasis is also found in 13 foci scattered in six countries in Latin America:
- Brazil
- Colombia
- Ecuador
- Guatemala
- Mexico
- Venezuela (Bolivarian Republic of).
Following successful large-scale treatment of populations in affected areas with ivermectin, and with the support of international partners, transmission of the disease had been interrupted in 10 foci by the end of 2011. Elimination efforts are now focused on the Yanomami people living in Brazil and Venezuela.
Prevention, control and elimination programmes
There is no vaccine or medication to prevent infection with O. volvulus.
Between 1974 and 2002, onchocerciasis was brought under control in West Africa through the work of the Onchocerciasis Control Programme (OCP), using mainly the spray of insecticides against blackfly larvae (vector control) by helicopters and airplanes. This was supplemented by large-scale distribution of ivermectin since 1989.
The OCP relieved 40 million people from infection, prevented blindness in 600 000 people, and ensured that 18 million children were born free from the threat of the disease and blindness. In addition, 25 million hectares of abandoned arable land were reclaimed for settlement and agricultural production, capable of feeding 17 million people annually.
In 1995, the African Programme for Onchocerciasis Control (APOC) was launched with the objective of controlling onchocerciasis in the remaining endemic countries in Africa. Its main strategy has been the establishment of self-sustaining community-directed treatment with ivermectin, and, where appropriate, vector control with environmentally-safe methods. In 2010, nearly 76 million ivermectin treatments were distributed in 16 APOC countries where the strategy of community-directed treatment with ivermectin (CDTI) was being implemented. At least 15 million additional people need to be reached in the next few years as the programme has now shifted from control to elimination.
The Onchocerciasis Elimination Program of the Americas (OEPA) began in 1992 with the objective of eliminating ocular morbidity and transmission throughout the Americas by 2012 through biannual large-scale treatment with ivermectin. All 13 foci in this region achieved coverage of more than 85% in 2006, and transmission was interrupted in 10 out 13 by the end of 2011.
Treatment
WHO recommends treating onchocerciasis with ivermectin at least once yearly for about 10 to 15 years. Where O. volvulus co-exists with Loa loa, another parasitic filarial worm that is endemic in Cameroon, the Central African Republic, the Congo, the Democratic Republic of the Congo, Nigeria and South Sudan, it is recommended to follow the Mectizan Expert Committee (MEC)/APOC recommendations for the management of severe adverse events that may occur.
WHO response
WHO is the executing agency of APOC. The WHO Regional Office for Africa supervises APOC’s management, while WHO headquarters provides administrative, technical and operational research support. Through the OEPA partnership, WHO collaborates with endemic countries and international partners.
WHO is currently facilitating the launch of an elimination programme in Yemen in collaboration with the Ministry of Health, the World Bank and other international partners.