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.
Clinical 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 when they die. Infected people may show symptoms such as severe itching and various skin changes. Infected people may also develop eye lesions which can lead to visual impairment and permanent blindness. In most cases, nodules under the skin form around the adult worms.
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, 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 the Yanomami area of Brazil and Venezuela (Bolivarian Republic of) as well as in Yemen.
Prevention, control and elimination programmes
Between 1974 and 2002, disease caused by onchocerciasis was brought under control in West Africa through the work of the Onchocerciasis Control Programme (OCP), using mainly the spraying of insecticides against blackfly larvae (vector control) by helicopters and airplanes. This was later 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.
The African Programme 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 (CDTI) and vector control with environmentally-safe methods where appropriate. In APOC’s final year, more than 119 million people were treated with ivermectin, and many countries had greatly decreased the morbidity associated with onchocerciasis. More than 800,000 people in Uganda and 120,000 people in Sudan no longer required ivermectin by the time that APOC closed.
In 2016, the Expanded Special Project for the Elimination of Neglected Tropical Diseases in Africa (ESPEN), was set up to cover the five preventive chemotherapy NTDs with 4 core objectives:
1. Scale up treatments towards the achievement of 100% geographic coverage, 2. Scale down: stopping treatments once transmission has been interrupted or control achieved, 3. Strengthen information systems for evidence-based action, and 4. Improve the effective use of donated medicines through enhance supply chain management. ESPEN is housed in the WHO Regional Office for Africa.
With support from ESPEN, ivermectin treatments continued to scale up, reaching 152.9 million people in 2019, but due to COVID-19 disruptions, the number of people treated declined by 26.9% in 2020.
The Onchocerciasis Elimination Program of the Americas (OEPA) began in 1992 with the objective of eliminating ocular morbidity and interruption of transmission throughout the Americas by 2015 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 11 of the 13 foci. Elimination efforts are now focused on the Yanomami people living in Brazil and Venezuela (Bolivarian Republic of), representing a total population at risk of only 35,228 people.
On 5 April 2013, the Director-General of WHO issued an official letter confirming that Colombia has achieved elimination of onchocerciasis. Colombia was the first country in the world to be verified and declared free of onchocerciasis by WHO. This has been followed by Ecuador in September 2014, Mexico in July 2015, and Guatemala in July 2016. More than 500 000 people no longer need ivermectin in the Americas.
Treatment
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 may need to be adjusted. Loa loa is a parasitic filarial worm that is endemic in Angola, Equatorial Guinea, Gabon, Cameroon, the Central African Republic, the Republic of Congo, the Democratic Republic of the Congo, Nigeria , Tchad and South Sudan. Treatment of individuals with high levels of L. loa in the blood can sometimes result in severe adverse events. Affected countries, should follow the Mectizan Expert Committee (MEC)/APOC recommendations for the prevention and management of severe adverse events.
WHO response
WHO provides administrative, technical and operational research support to three regions where onchocerciasis is transmitted.
The Onchocerciasis Technical Advisory Subgroup (OTS) setup by WHO in 2017 is providing guidance and oversight for operational research to identify endemic areas that require MDA. In areas co-endemic for lymphatic filariasis, research efforts are focused on developing strategies for co-evaluation of onchocerciasis and lymphatic filariasis, in order to support proper decision making regarding the stopping of MDA.
Diagnosis of onchocerciasis remains a challenge for programmes. The NTD Diagnostics Technical Advisory Group identified development of new diagnostic tools for onchocerciasis as a specific priority. A subgroup was convened to prepare 2 target product profiles (TPPs) for new diagnostics for mapping onchocerciasis and for a confirmatory test for deciding to stop MDA. The TPPs have now been posted on the WHO website.
Through the OEPA partnership, WHO collaborates with endemic countries and international partners in the WHO Region of the Americas. Although there is no official programme to coordinate activities in the WHO Eastern Mediterranean Region, the two countries in the region collaborate on elimination activities and receive the support of ESPEN to achieve elimination of river blindness.
With the shift from control to elimination, large areas in Africa require mapping to assess whether transmission is active and treatment required. A sampling strategy name Onchocerciasis elimination mapping has been developed to help countries conduct
those assessments and start treatment where needed.
Research priorities
To achieve elimination goals for onchocerciasis, an ambitious research agenda will be needed to support programme progress. Specific research needs include:
- Optimizing strategies to reach marginalized and migratory populations.
- Validating mapping and safe intervention strategies in settings where onchocerciasis and loiasis are con-endemic.
- Defining starting and stopping thresholds for MDA.
- Development of robust diagnostics tools to support programme decision-making.
- Demonstrating the programmatic utility of vector control measures.
- Testing new therapeutic regimens.
- Optimizing survey design through the use of new geostatistical tools.
- Developing post-verification strategies.
- Exploring opportunities to integrate surveillance.