Onchocerciasis (river blindness): symptoms, treatment and prevention

The World Health Organization (WHO) describes onchocerciasis as a parasitic disease caused by the filarial worm Onchocerca volvulus, transmitted by repeated bites of infected blackflies of the genus Simulium. These flies breed along fast-flowing rivers and streams, close to remote villages near fertile land, which gives the disease its common name of river blindness.

It is an eye and skin disease. The WHO explains that symptoms are caused by the microfilariae, the immature worms, which move around the body in the subcutaneous tissue and cause intense inflammatory responses when they die. The sources used here are WHO documents because no UK government or NHS patient page on this disease was found; UK readers with travel-related concerns should speak to a travel health or tropical medicine service.

The WHO describes the life cycle. A blackfly ingests microfilariae when it bites an infected person. Inside the fly the microfilariae develop into infective larvae, which are transmitted to another person during later bites. In the human host the larvae mature into adult worms, forming nodules under the skin, and the adult worms mate and produce more microfilariae.

Because adult worms can live for a long time, the WHO says communities must be treated for a minimum of 10 to 15 years to eliminate transmission, corresponding to the lifespan of the adult worm.

Infected people may show severe itching and various skin changes, and may develop eye lesions which can lead to visual impairment and permanent blindness. In most cases, nodules form under the skin around the adult worms. The WHO also notes that early exposure to O. volvulus infection is associated with epilepsy in children.

The WHO says the Global Burden of Disease Study estimated in 2017 that 14.6 million of the infected people already had skin disease and 1.15 million had vision loss. These figures illustrate why the infection is described as a major cause of preventable disability in affected regions.

• Severe itching.

• Skin changes, which may be disfiguring.

• Nodules under the skin around adult worms.

• Eye lesions leading to visual impairment and, in some people, permanent blindness.

The WHO says the disease primarily affects rural populations in sub-Saharan Africa and Yemen, with smaller endemic areas in parts of Latin America. More than 99% of infected people live in Africa and Yemen, and the remaining 1% live on the border between Brazil and Venezuela.

It reports that in 2024 at least 252.3 million people required preventive treatment against onchocerciasis, and that 26 countries reported using treatment, reaching 171.6 million people. For UK travellers, the sources read do not discuss risk to short-stay visitors, so ask a travel clinic about the destination and the duration of stay.

The WHO says population-based treatment with ivermectin, also known as mass drug administration, is the core strategy to eliminate onchocerciasis, with a minimum requirement of 80% therapeutic coverage, and that ivermectin is donated by Merck under the brand name Mectizan. It recommends treating with ivermectin at least once yearly for 10 to 15 years.

It adds a caution: where O. volvulus co-exists with Loa loa, another filarial worm endemic in parts of central Africa, treatment of people with high levels of Loa loa in the blood can sometimes cause severe adverse events, so strategies may need to be adjusted. Individual treatment decisions in the UK would be made by an infectious diseases or tropical medicine specialist.

The WHO describes decades of programmes, from the Onchocerciasis Control Programme in west Africa (1974 to 2002), which used insecticide spraying against blackfly larvae and later ivermectin, to the African Programme for Onchocerciasis Control, launched in 1995 with community-directed treatment, and the more recent Global Onchocerciasis Network for Elimination (GONE) launched in January 2023.

Five countries have been verified by WHO as free of onchocerciasis: Colombia (2013), Ecuador (2014), Mexico (2015), Guatemala (2016) and Niger (2025). The WHO describes these milestones as proof that progress against neglected tropical diseases is possible across the African continent.

The sources read do not give UK-specific diagnostic or treatment pathways. In practice a GP can refer someone with a relevant travel history to an infectious diseases or tropical medicine service.

Private travel clinics offer pre-travel advice. If you have lived in an endemic area and have itching, skin nodules or eye symptoms, tell your GP about your travel history so that an appropriate specialist referral can be made.