Onchocerciasis (River Blindness)

Key Facts

  • Onchocerciasis (äŋ-kō-ˌsər-ˈkī-ə-ˌsēz), commonly known as “river blindness”, is caused by the parasitic worm Onchocerca volvulus (filarial nematode).
  • The parasite is spread and transmitted to humans by the repeated bites of infected blackflies that breed in rapidly flowing rivers.
  • Onchocerciasis is most prevalent in sub-Saharan Africa, where >99% of the world’s cases are found; the remaining 1% of cases occur on the border between Brazil and Venezuela
  • The disease is endemic in areas where blackflies thrive, particularly in communities located near rivers and streams where the flies breed.
  • Symptoms include subcutaneous nodules, intense pruritus, dermatitis, adenopathy, atrophy, disfiguring skin conditions, and eye lesions that may lead to permanent blindness.
  • Diagnosis is by finding microfilariae in skin samples, the cornea, or the anterior chamber of the eye; identifying adult worms in subcutaneous nodules; or using polymerase chain reaction (PCR) to detect parasite DNA.
  • The disease primarily affects rural populations in sub-Saharan Africa, and Yemen, with smaller endemic areas foci found in parts of Latin America.
  • Population-based treatment with ivermectin (also known as mass drug administration or MDA) is the current core strategy to eliminate onchocerciasis, with a minimum requirement of 80% therapeutic coverage. Ivermectin is donated by Merck & Co., Inc. (MSD outside of the United States) under the brand name of Mectizan®.

Overview

Onchocerciasis (äŋ-kō-ˌsər-ˈkī-ə-ˌsēz), more commonly known as river blindness, is a neglected tropical disease (NTD) caused by the parasitic worm Onchocerca volvulus. Onchocerciasis is primarily a disease of the skin and eye. Skin symptoms typically develop years before any vision problems. onchocerciasis is the second leading infectious cause of blindness worldwide, after trachoma, another NTD. The disease is transmitted to humans through the bite of an infected female blackfly (Simulium spp.), which breeds in fast-flowing rivers and streams. The blackfly vector ingests microfilariae (immature worms) when it bites an infected person. Inside the fly, the micro filariae develop into infective larvae that are then transmitted to another human during subsequent bites. Once inside the human host, the larvae mature into adult worms, forming nodules under the skin. As they continue to mate and produce microfilariae, communities must be treated for a minimum of 10 to15 years to eliminate transmission, corresponding to the lifespan of the adult O. volvulus.

Scope of the Problem

Onchocerciasis is most prevalent in sub-Saharan Africa and Yemen (small foci), where >99% of the world’s cases are found; the remaining 1% of cases occur on the border between Brazil and Venezuela. The disease is endemic in areas where blackflies thrive, particularly in communities located near rivers and streams where the flies breed. The geographical association with rivers gives the disease its common name, “river blindness”.

In 2023 at least 249.5 million people required preventive treatment for onchocerciasis. An estimated 21 million people are infected with onchocerciasis worldwide. 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.

Five countries have been verified by WHO as free of onchocerciasis after successfully implementing elimination activities for decades: four in the region of the Americas: Colombia (2013), Ecuador (2014), Mexico (2015) and Guatemala (2016), and one in Africa: Niger (2025)

In 2022, Senegal has stopped treatment and is now under post-treatment surveillance. Equatorial Guinea, Ethiopia, Mali, Nigeria, Sudan, Tanzania, Togo, Uganda, and Venezuela (Bolivarian Republic of) have stopped MDA in at least one focus.

Globally, 1.8 million people live in areas that no longer require MDA for onchocerciasis. These milestones provide proof of concept that progress against neglected tropical diseases (NTDs) is possible across the entire African continent.

Pathophysiology, Clinical Signs and Symptoms

The lifecycle of O. volvulus within the human body is complex and prolonged. Adult worms live in their subcutaneous nodules for up to 15 years. During this time, the female worms produce millions of microfilariae that migrate through the skin, eyes and other tissues and are responsible for the symptoms associated with onchocerciasis. As they move through the skin, they cause intense itching, rashes and depigmentation, leading to a condition known as “leopard skin”. Chronic inflammation from the presence of microfilariae can also cause skin thickening and atrophy, resulting in lizard skin-like changes.

The subcutaneous (or deeper) nodules that contain adult worms may be visible or palpable but are otherwise asymptomatic. They are composed of inflammatory cells and fibrotic tissue in various proportions. Old nodules may caseate or calcify. Patients may also have enlargement of inguinal, femoral, or other lymph nodes. Localized swelling of the genitalia and inguinal hernias can develop. Onchocercal dermatitis is caused by the microfilarial stage of the parasite. Intense pruritus may be the only symptom in lightly infected people.

Skin lesions usually consist of a nondescript maculopapular rash with secondary excoriations, scaling ulcerations and lichenification, and mild to moderate lymphadenopathy. Other skin abnormalities can include premature wrinkling, atrophy, patchy hypopigmentation, and loss of elasticity. In severe cases, patients may develop folds of atrophic skin in the lower abdomen and upper medial thighs (“hanging groin”).

The most devastating consequence of onchocerciasis is, however, on the eyes. When microfilariae migrate to the eyes, they can trigger an immune response that leads to inflammation and damage to ocular tissues. Over time, this can result in vision impairment and eventually blindness. Ocular involvement ranges from mild visual impairment to complete blindness. Lesions of the anterior portion of the eye include (i) Punctate keratitis; (ii) Sclerosing keratitis; and (iii) Anterior uveitis or iridocyclitis. Chorioretinitis, optic neuritis, and optic atrophy may also occur.

Socioeconomic Impact and Stigma

The socioeconomic impact of onchocerciasis is profound. Infected individuals often suffer from severe itching, disfiguring skin conditions and vision loss, which can significantly impair their ability to work and perform daily activities. This, in turn, leads to loss of income and increased dependence on others, perpetuating cycles of poverty in affected communities. The stigma associated with the visible manifestations of the disease contributes to social exclusion. Early exposure to onchocerciasis in childhood has also been associated with epilepsy, known as onchocerciasis-associated epilepsy.

Prevention, Control and Elimination Programs

Efforts to eliminate onchocerciasis have been under way for several decades. Between 1974 and 2002, onchocerciasis was brought under control in West Africa through the work of Onchocerciasis Control Program (OCP) primarily by spraying of insecticides against blackfly larvae (vector control) using helicopters and airplanes. This was later supplemented by large-scale distribution of ivermectin since 1989.

The African Program for Onchocerciasis Control (APOC) was launched in 1995 with the objective of controlling onchocerciasis in the remaining endemic countries in Africa and closed at the end of 2015 after beginning the transition to onchocerciasis elimination. Its main strategy was the establishment of sustainable community-directed treatment with ivermectin and vector control with environmentally-safe methods where appropriate.

Building on the successes of OCP and APOC, the Expanded Special Project for Elimination of Neglected Tropical Diseases (ESPEN 2016-present) was launched by the WHO Regional Office for Africa to provide national NTD programs with technical and fundraising support to help accelerate elimination of river blindness in African countries.

National Onchocerciasis Elimination Committees (NOECs) have been established in 25 countries in Africa to develop and implement new strategies. The Global Onchocerciasis Network for Elimination (GONE) was launched in January 2023 by WHO, its Member States and partners whose goal is to support countries to accelerate progress towards the achievement of the road map targets for onchocerciasis elimination.

The Onchocerciasis Elimination Program for the Americas (OEPA 1992–present) was launched in 1992 with the goal of eliminating morbidity and interrupting transmission of river blindness in six endemic countries in the Americas: Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela. OEPA is a partnership consisting of the six endemic countries, the Pan American Health Organization (PAHO), the private sector (MSD), donor countries, and nongovernmental development organizations (NGDOs).

Treatment

The mainstay is mass drug administration (MDA) of ivermectin, an antiparasitic medication that effectively kills microfilariae and prevents progression of the disease. Ivermectin primarily targets the microfilariae (immature worms) in the skin, killing them and preventing the release of new microfilariae by adult female worms. This is known as the embryostatic effect. While ivermectin does not kill adult worms, it effectively reduces the microfilarial load, which helps manage symptoms and reduce transmission until it is suppressed, preventing uptake of larvae by flies and subsequent incident cases. If ivermectin MDA is discontinued prematurely, transmission will resume, as adult worms may still be alive. Ivermectin is also known to cause some damage to the adult worm, including damage to its reproductive capacity. Transmission is interrupted when all adult worms are dead or can no longer reproduce.

Ivermectin has been distributed in affected communities through large-scale public health campaigns, often supported by international organizations and pharmaceutical donations. Since 1987, ivermectin has been provided free of charge for use in humans by Merck & Co., Inc. (MSD outside of the United States) through the Mectizan donation program (MDP). The MDP works together with ministries of health and non-governmental development organizations (NGOs), such as the World Health Organization (WHO), to provide free ivermectin to those who need it in endemic areas. Ivermectin is given as a single oral dose of 150 mcg/kg, repeated at 6- to 12-month intervals. WHO recommends treating onchocerciasis with ivermectin at least once yearly for 10 to 15 years.

In addition to MDA, vector control strategies, such as spraying of insecticides and clearing of vegetation to reduce blackfly populations have also been used in some areas. These interventions have significantly reduced the prevalence of onchocerciasis in many regions, resulting in elimination in some countries and a significant reduction in transmission in others. Challenges remain, however, particularly in remote and conflict- affected areas where access to health services is limited and in Loa loa co-endemic  areas where a different strategy is required.

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 central Africa (including Cameroon, the Central African Republic, Congo, the Democratic Republic of the Congo, Nigeria and South Sudan). Treatment of individuals with high levels of Loa loa in the blood can sometimes result in severe adverse events. Before treatment with ivermectin, patients should be assessed for coinfection with Loa loa, if they have been in areas of central Africa where both parasites are transmitted.

Elimination of Onchocerciasis: Ongoing Challenges

WHO has set ambitious targets for the transmission of onchocerciasis. Continued efforts, including sustained MDA, enhanced surveillance and research into new tools and strategies, will be crucial to reach this goal. Onchocerciasis is one of the diseases targeted for elimination of transmission in the WHO road map on neglected tropical diseases (NTDs) 2021-2030. The aim of the 2030 targets is to stop MDA with ivermectin in at least one focus area in 34 countries, in more than 50% of the population in at least 16 countries, and in the entire endemic population in at least 12 countries.

Elimination of onchocerciasis, while achievable, faces several significant challenges, which depend on the region and the circumstances of affected communities. Sustained financial support, including domestic resources is crucial for ongoing MDA, surveillance and vector control activities, as funding gaps can lead to interruption of these activities, allowing the disease to become reestablished in previously controlled areas. A shortage of trained health-care workers and volunteers to conduct MDA and surveillance is a common issue, particularly in remote and rural areas. Elimination requires robust surveillance to detect any resurgence of the disease and monitor the impact of interventions.

In many endemic areas, however, surveillance systems are weak and cannot track progress and identify areas where transmission persists. Even after transmission has been interrupted or verification achieved, long-term surveillance is required to ensure that the disease does not re-emerge. Maintaining these efforts over time can be challenging, as attention and funding may shift to other priorities. Addressing the challenges requires a multi-faceted approach, including strengthened health systems, community engagement, sustained funding and continued research to find new tools and strategies for controlling and eliminating the disease.

Available diagnostics do not distinguish infected individuals with active infection (live adult worms) from those who were infected in the past but no longer harbor any living adult worms or microfilariae. This represents a major challenge to deciding when it is appropriate to stop MDA, and strong surveillance is necessary during the few years after a decision to stop MDA has been made to assess whether elimination of transmission has been achieved. New diagnostics are required that could reliably detect the presence of reproducing adult worms.

At least 249.5 million people in 28 countries require interventions to eliminate onchocerciasis. This total does not include people living in areas in which transmission status is unknown. By 2024, MDA had been stopped in 221 implementation units (IU) that had been previously affected by active transmission, with 18.8 million people requiring preventive chemotherapy (PC) for onchocerciasis, which have either been completed or are under post-treatment surveillance (PTS). Most are in Africa. In 2023, a total of 172.2 million people were treated for onchocerciasis, representing 69.0% global coverage.

Regional Highlights and Milestones

  • In January 2025, WHO announced that Niger met the criteria for onchocerciasis elimination, making it the first country in Africa and the fifth country globally to interrupt transmission of the parasite Onchocerca volvulus. Niger joins four other countries that have been verified by WHO for eliminating onchocerciasis, all in the Region of the Americas: Colombia (2013), Ecuador (2014), Guatemala (2016) and Mexico (2015).
  • Onchocerciasis is the second NTD eliminated in Niger: the country was certified free of dracunculiasis (Guinea-worm disease) transmission in 2013. 
  • Globally, 54 countries have eliminated at least one NTD, including 21 countries in Africa.
  • Four countries in Africa endemic for onchocerciasis (Benin, Chad, Gabon and Guinea-Bissau) did not conduct MDA in 2023. All the other countries in Africa that required preventive chemotherapy delivered MDA in 2023, resulting in 68.9% regional coverage.
  • Treatment for lymphatic filariasis and onchocerciasis was not provided in any implementation units in Chad due to lack of funding.
  • In Guinea-Bissau, ivermectin was not distributed in 2023 while awaiting the results of a transmission assessment survey.
  • Nineteen implementation units in the Democratic Republic of the Congo in which the onchocerciasis status was unknown but were previously classified as hypoendemic, received MDA with  ivermectin,  for  a  treated population  of 3,682,082.
  • In Senegal, the transmission of O. volvulus by the blackfly Simulium damnosum s.l. is generally considered to have been interrupted from data obtained during the entomological assessments in 2019 and epidemiological results in 2022. The country is under post-treatment surveillance from January 2023 to January 2025.
  • Two foci of active transmission remain in the Region of the Americas, Brazil and Venezuela, with a total population of 38,045 people. Of these, 24,214 received MDA, with 10,418 in Brazil and 13,796 in Venezuela and a regional coverage of 63.6%. Two rounds of MDA were generally implemented in both countries, although some communities in Venezuela (Bolivarian Republic of) received four rounds.
  • Sudan and Yemen are endemic for onchocerciasis and a total of 1.7 million people require treatment to interrupt transmission.
  • Reaching the Last Mile Fund (RLMF), launched in 2018 by His Highness Sheikh Mohamed bin Zayed  Al Nahyan, President of the United Arab Emirates, in partnership with the Gates Foundation, The Helmsley Charitable Trust and The ELMA Philanthropies. The US$ 100 million multi-donor fund addresses onchocerciasis elimination in 7 countries – Ethiopia, Chad, Mali, Niger, Senegal, Sudan and Yemen – and elimination of lymphatic filariasis where it is co-endemic. The first phase of the RLMF included delivery of 96 million treatments, >2000 hydrocoele surgeries and investment in regional laboratories. The RLMF also supported Niger and Senegal in reaching significant milestones (see above).
  • Building on these successes, the RLMF in 2024 increased its funding target from US$ 100 million to an additional US$ 500 million to support a new target: to accelerate elimination of onchocerciasis and lymphatic filariasis in 39 African countries and Yemen by 2030.

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