Onchocerciasis, also known as river blindness, is the second leading cause of preventable blindness after trachoma.

Onchocerciasis is an eye and skin infection caused by the parasitic worm, Onchocerca volvulus, which is transmitted by the bite of an infective blackfly (genus Simulium). Because the insect that spreads the disease breeds and lives near fast-flowing rivers and streams, onchocerciasis is sometime referred to as river blindness. Chronically infected people often develop skin discoloration and severe, burning itching skin and/or eye lesions.

During a blood meal, an infective blackfly deposits third-stage filarial larvae onto the skin of the human host, where they penetrate into the wound and migrate through the subcutaneous tissues and develop into adult worms. These adults commonly reside in nests called “nodules,” where they can live for roughly 15 years. While most nodules have 3–5 worms, up to 50 worms can live in one nodule. Adult female worms can produce up to 1,000 offspring, called microfilariae, in a single day. These microfilariae can live for up to 2 years and migrate through the subcutaneous tissue in hopes of being taken up by another blackfly. Most microfilariae die in the human host, causing an inflammatory reaction where, depending on their location, they can produce itching and lesions associated with the disease.

More than 99 percent of all cases of onchocerciasis and onchocerciasis-related blindness are found in Africa. Isolated areas of the disease also exist in Yemen and in six countries in Central and South America. The World Health Organization (WHO) estimates that more than 25 million people are infected with onchocerciasis, of whom 265,000 are blind, 746,000 are visually impaired, and as many as 160 million are at risk of infection. People who live in rural areas near fast-flowing rivers and streams where there are Simulium blackflies are most at risk of contracting onchocerciasis.


Symptoms of onchocerciasis are caused by dead and dying microfilariae, which can cause skin reactions that range from mild to intense itching and can eventually lead to severe dermatitis.

While the disease is not a direct cause of mortality, the socioeconomic consequences of onchocerciasis are profound and extend beyond the individual, affecting families, communities, and countries as a whole. According to the World Health Organization (WHO), onchocerciasis is the second leading cause of preventable blindness, after trachoma. During the Onchocerciasis Control Programme in West Africa (OCP) era of the 1970s, it was estimated that onchocerciasis in western Africa alone caused an economic cost of $30 million annually (more than $135 million in 2015 dollars).

Learn more at the WHO Onchocerciasis page

On a global level, sustained mass drug administration of ivermectin is currently the primary strategy for control or elimination of onchocerciasis. Merck & Co., Inc., has established the Mectizan Donation Program, to provide free drugs to any governmental or non-governmental organization that demonstrates the need and the capability to distribute ivermectin.

Until recently, the strategy in Africa has focused on controlling the morbidity associated with onchocerciasis by providing treatment only to those communities that were at risk of skin and eye disease. With the demonstrated success of elimination of transmission of onchocerciasis in the Americas, many countries in Africa are now making a shift from control to elimination of the disease. This is being achieved through expanded treatment to cover not just those at risk of morbidity, but also those contributing to transmission and through more rigorous monitoring and evaluation.

In the Americas, elimination of transmission of onchocerciasis has always been the established goal, and, in 1993, Onchocerciasis Elimination Program for the Americas (OEPA) was founded to oversee and coordinate efforts in the six countries endemic for the disease. Twice yearly treatment with ivermectin anywhere onchocerciasis exists has long been the strategy for interrupting transmission, and in some, foci treatment is given as many as four times per year. As of 2016, four of the six countries had been certified free of onchocerciasis with only a small foci in the Amazon jungle crossing into both Brazil and Venezuela remaining.

In January, 2016, WHO updated their Guidelines for Stopping Mass Drug Administration and Verifying Elimination of Human Onchocerciasis. These guidelines updated the original 2001 guidelines to incorporate new diagnostic tools and evaluation models. While these guidelines provide the indicators for when onchocerciasis transmission has been successfully interrupted, they do not provide an operational guidance on how to reach these indicators. In order to assist countries in developing national plans of action, the U.S. Agency for International Development is supporting the formation of technical Onchocerciasis Expert Committees, comprised of domestic and international experts, who can advise ministries of health on how to proceed with transmission interruption.


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