Onchocerciasis, or river blindness, is a disease caused by Onchocerca volvulus, a parasitic worm. The parasite spreads through infected flies that breed near rivers and streams in remote areas of Africa and South America. Symptoms include a potentially disfiguring skin rash and vision problems, even blindness.
Onchocerciasis (pronounced “on-koh-ser-kai-uh-sis”), also called “river blindness,” is a disease caused by Onchocerca volvulus, a parasitic worm. The parasite can spread to humans through repeat bites of infected blackflies of the Simulium species. The flies breed by fast-flowing rivers and streams in certain remote, tropical, agricultural areas.
The disease can cause severe problems with your skin and vision, even blindness.
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Onchocerciasis is common throughout sub-Saharan Africa and certain areas of Brazil and Venezuela in South America. The Pan American Health Organization estimates that there are 18 million infected people worldwide, including 270,000 who’ve lost their sight.
Some countries have eliminated the disease by spraying insecticides and giving medicine to large portions of the population.
Infected Simulium flies live by fast-flowing streams and rivers near remote, rural, agricultural villages in Africa and South America. People at risk of onchocerciasis include those who live in these areas and those who visit for long lengths of time. Examples include field researchers, missionaries and Peace Corps volunteers.
When a blackfly bites a person with onchocerciasis, it ingests the microscopic worm larvae (juvenile form of the worm). Inside the fly, the larvae develop and grow.
Then, when the same fly bites another human, it can transmit the parasite as larvae. Several bites are generally necessary for infection. Inside the new host, the larvae mature into adults, mate and produce more larvae, spreading to skin, eyes and other organs.
As the larvae population swells, the person’s body launches an inflammatory response, which causes symptoms.
Onchocerciasis affects your skin and eyes. Symptoms include:
A person infected with onchocerciasis will usually feel fine until the larvae reproduce in large numbers. This process may take 12 to 18 months.
Onchocerciasis itself isn’t contagious because it can’t spread directly from one person to another. But the disease can spread when blackflies bite infected people, ingest the larvae and then bite people who aren’t infected.
To diagnose onchocerciasis, a healthcare provider performs skin biopsies. They take skin samples from various parts of your body and look at them under a microscope.
To diagnose an eye infection, your healthcare provider examines your eyes for lesions. They use a slit lamp, a microscope attached to a bright light.
In addition, they may order blood tests to identify antibodies the body makes to fight infection.
Treatment for river blindness is ivermectin, an oral antiparasitic medication. Ivermectin kills the larvae but doesn’t kill the adults. You’ll need to take it at least once a year for 10 to 15 years. That’s because each adult female worm can live approximately 10 to 15 years, producing millions of new larvae during her lifetime.
The best way to prevent onchocerciasis is to avoid visiting places where infection still exists. But if you must go to those areas:
Onchocerciasis is the second-leading infectious cause of blindness in the world. It can cause disfiguring skin issues and permanent blindness. But you can help prevent severe infections by getting medical treatment as soon as possible after infection.
If onchocerciasis leads to blindness, it isn’t reversible.
If you’ve been bitten by blackflies in affected regions or have signs of onchocerciasis, seek medical attention. A healthcare provider can test your skin and blood to look for infection.
A note from Cleveland Clinic
Onchocerciasis, or “river blindness,” is a disease caused by a parasite named Onchocerca volvulus. The parasite spreads through infected flies that breed near rivers and streams in remote areas of Africa and South America. If you’ve been to those areas and were bitten by blackflies, talk to your healthcare provider.
Last reviewed by a Cleveland Clinic medical professional on 03/29/2023.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy