About Onchocerciasis

Key points

  • Onchocerciasis is also known as river blindness.
  • It is found mostly in Africa and spread by blackfly bites.
  • Treat onchocerciasis with antiparasitic drugs.
Posterior view of O. volvulvus


Onchocerciasis, or river blindness, is a skin and eye disease. It is caused by the parasite Onchocerca volvulus, a microscopic worm. A parasite is an organism (a living thing) that lives on or inside another organism. Onchocerciasis can cause vision loss, intense itching, rashes, and lumps under the skin.

Onchocerciasis is considered a neglected tropical disease (NTD) because it inflicts tremendous disability and illness among the affected populations. These communities often lack access to basic or adequate resources. Safe and effective medicines can control NTDs through mass drug administration (MDA) programs in the affected communities.


Many people infected with the parasitic worms that cause onchocerciasis never show any symptoms. In those who do, the most common are

  • Vision loss
  • Itchy skin rashes
  • Lumps, known as nodules, under the skin

Risk factors

The parasitic worms that cause river blindness are found in tropical climates, mostly in rural sub-Saharan Africa. They are also found in a small area along the border of Brazil and Venezuela, in South America and in Yemen, in the Middle East.

Onchocerciasis became known as river blindness because the people most likely to get infected live or work in rural areas, near streams or rivers, where blackflies breed.

Onchocerciasis rarely occurs in casual travelers. Unlike malaria, it takes repeated bites from infectious blackflies for people to become infected. Travelers to at-risk areas are most likely to become infected if they stay in these areas for long periods of time, such as missionaries, Peace Corps volunteers, or field researchers.

How it spreads

Onchocerciasis spreads through the bite of an infectious blackfly. When a blackfly bites a person with onchocerciasis, the blackfly consumes microscopic immature worms, or microfilariae, from the infected person's skin. The microfilaria develop into larvae, which overtime become infectious to people.

Blackfly, vector of O. volvulus.
The blackfly spreads the Onchocerca volvulus parasite.

When the infectious blackfly bites another person, the parasite enters the skin through the bite wound. The parasite will develop, grow into the adult form, and reproduce. When another blackfly bites that person, the insect can consume the microfilariae. This continues the spread of onchocerciasis.


The best way to prevent onchocerciasis is to avoid blackfly bites. To protect yourself

  • Use Environmental Protection Agency (EPA)-registered insect repellents containing DEET or another EPA-registered active ingredient.
  • Wear loose-fitting, long-sleeved shirts and pants, especially during the day when blackflies bite.
  • Keep windows and doors closed or covered with screens to keep biting insects out.
  • Repair broken screening on windows, doors, porches, and patios.

There is no vaccine or drug to prevent onchocerciasis infection.

Onchocerciasis is considered a neglected tropical disease, or NTD. There is a global campaign to eliminate onchocerciasis transmission in endemic areas. The campaign focuses on controlling blackflies and giving medicine that kills the microscopic worms to entire affected communities. Successful campaigns to eliminate onchocerciasis have taken place in some countries, including countries in the Western Hemisphere.


If a healthcare provider thinks you might have onchocerciasis, there are several ways to confirm the diagnosis:

  • Take and examine a skin snip—a thin skin biopsy—to look for immature worms through a microscope. This is the most common way to diagnose onchocerciasis.
  • Surgical removal of a nodule (lump) in your skin and examine it for adult worms.
  • Examine your eyes for signs of damage caused by immature worms, or for the worms themselves.
  • Take a blood test to look for signs of antibodies your immune system produced in response to an infection.


There are antiparasitic drugs that can kill the immature worms (microfilariae). This will prevent symptoms of the disease from developing. There is also a medication which can kill the adult worms. Talk with your healthcare provider if you traveled to an area where onchocerciasis spreads and experience any of the symptoms.