Onchocerciasis, or river blindness, is a neglected tropical disease (NTD) caused by the parasitic worm Onchocerca volvulus. It is transmitted through repeated bites by blackflies of the genus Simulium. The disease is called river blindness because the blackfly that transmits the infection lives and breeds near fast-flowing streams and rivers, mostly near remote rural villages. The infection can result in visual impairment and sometimes blindness. Additionally, onchocerciasis can cause skin disease, including intense itching, rashes, or nodules under the skin. Worldwide onchocerciasis is second only to trachoma as an infectious cause of blindness.

What is onchocerciasis?

Onchocerciasis is an infection caused by the parasitic worm Onchocerca volvulus, spread by the bite of an infected Simulium blackfly. It is also called River Blindness because the fly that transmits infection breeds in rapidly flowing streams and the infection can cause blindness. Persons with heavy infections will usually have one or more of three conditions: skin rash (usually itchy), eye disease, or nodules under the skin. The World Health Organization’s (WHO) expert committee on onchocerciasis estimates that at least 25 million people are infected and 123 million people live in areas that put them at risk of infection. About 300,000 people are blind because of the parasite and another 800,000 have visual impairment. Nearly 99% of infected persons live in Africa; the remainder lives in Yemen and six countries in the Americas. Onchocerciasis is commonly treated with an oral medicine called ivermectin.

In what parts of the world am I more likely to get onchocerciasis?

Onchocerciasis is locally transmitted in 30 countries of Africa and in Yemen in the Middle East. Local transmission occurred in 13 focal areas in 6 countries in the Americas (Mexico, Guatemala, Ecuador, Colombia, Venezuela, and Brazil), but transmission has now stopped in 11 of these focal areas. Transmission continues in one focal area in Venezuela and one in Brazil. Onchocerciasis in casual travelers is rare; the infection is transmitted in remote rural areas and, unlike malaria, contracting onchocerciasis often requires more than one infectious bite. Thus, risk of infection is greater in adventure travelers, missionaries, and Peace Corps and other long-term volunteers who are likely to have more intense or sustained exposure to blackfly bites. Given the low rate of transmission in the Americas, the likelihood is very low that any travelers in this region (even missionaries and long-term volunteers) would ever get infected.

How is onchocerciasis spread?

The disease spreads by the bite of an infectious blackfly. When a blackfly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person’s skin enter and infect the blackfly. The larvae develop over 2 weeks in the fly to a stage that is infectious to humans. An infectious blackfly will typically drop larvae when biting a person. The larvae then penetrate the skin to infect the person. Because the worms reproduce only in humans but need to complete some of their development inside the blackfly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual. Blindness is usually seen in the setting of longstanding and intense infection.

Who is most at risk for onchocerciasis?

Those most at risk are people who live in areas where the parasite is spread followed by adventure travelers, missionaries, and Peace Corps volunteers who are exposed for long periods of time (generally more than 3 months) to blackfly bites in areas where the parasite exists. The disease is most intensely transmitted in remote rural African agricultural villages which are located near rapidly flowing streams.

What is the impact of onchocerciasis?

Onchocerciasis is the second leading infectious cause of blindness and can cause debilitating and disfiguring skin disease. However, the worldwide burden of onchocerciasis has been considerably reduced as the result of very successful disease control programs led by the World Health Organization (WHO). These programs are based on control of the blackfly population and/or mass administration to affected communities of an oral drug called ivermectin (Mectizan™), that is donated by Merck & Co., Inc. As a result of these programs, millions of people are at greatly reduced risk of debilitating itching, disfigurement, and blindness caused by onchocerciasis. Unfortunately, many people still do not have access to prevention and treatment measures.

What are the clinical manifestations of onchocerciasis?

Infected persons may be without symptoms. Those with symptoms will usually have one or more of the three manifestations: skin rash (usually itchy), eye disease, and nodules under the skin. The most serious manifestation consists of lesions in the eye that can lead to visual impairment and blindness.

How soon after infection will I have symptoms of onchocerciasis?

It can take up to one year for the larvae (also called microfilariae) to develop into an adult inside the human body and between 10 and 20 months before larvae can be found in the skin. Each adult female worm, which can live from 10–15 years, can produce millions of new larvae during her lifetime. As it is the larvae that cause most of the symptoms of onchocerciasis, most people feel well until after the adults start producing large numbers of new larvae.

What should I do if I think I may have onchocerciasis?

If you think you might have onchocerciasis, see your health care provider who may order skin biopsies (“snips”) or blood tests looking for antibodies to the parasite. However, examination of skin snips does not always show the parasites and positive blood tests do not necessarily indicate that you are still infected with O. volvulus.

What is the treatment for onchocerciasis?

There are treatments, such as ivermectin, available to kill the larvae in your body and thus prevent the symptoms of the disease, such as skin rash and blindness. Promising treatments, such as doxycycline, to kill the adult worms are currently being studied. You should discuss your treatment options with your health care provider.

How can I prevent onchocerciasis?

Blackflies bite during the day. The best prevention is avoiding being bitten by infected blackflies by using insecticides that contain N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeve shirts and pants, and wearing permethrin treated clothing.

Can I take medication to prevent onchocerciasis?

No, there is neither a vaccine nor recommended drug available to prevent onchocerciasis.

The blackflies that transmit the parasite bite during the day. Female blackflies need to ingest blood for ovulation, so they feed on humans. If a blackfly bites an infected person, onchocerciasis larvae can be ingested by the blackfly after which they migrate to the flight muscles. The larvae develop inside the blackfly and become infective for humans in about 10 to 12 days. They migrate to the biting parts of the fly where they can be transmitted back to humans when it bites again.

Humans become infected when blackflies deposit Onchocerca infective larvae into the skin when biting to extract blood. Once inside the human body, the larvae mature into adults in around 3 months to 1 year. Most adult female worms live in fibrous nodules under the skin and sometimes near muscles and joints. Adult male worms are usually found near the female worms. Nodules form around the worms as part of the interaction between the parasite and its human host. Inside the nodules the worms are relatively safe from the human immune response. As adults, female worms produce thousands of new larvae daily. The larvae become detectable in the skin 10 to 20 months after the initial infection. The adult worms can live up to 15 years inside the human body, and their larvae have a lifespan up to 2 years.

Some people do not experience symptoms while infected with O. volvulus, as the larvae can migrate through the human body without provoking a response from the immune system. But many people do have symptoms, which include itchy skin rashes, nodules under the skin, and vision changes. There can be non-painful swelling of lymph glands, but this is not common. Most symptoms of onchocerciasis are caused by the body’s response to dead or dying larvae. The inflammation caused in the skin, in addition to causing itching, can result in long-term damage to the skin. This can cause changes in the color of the skin that result in a “leopard skin” appearance, and can cause thinning of the skin with loss of elastic tissue that gives the skin a “cigarette-paper” appearance and can contribute to conditions such as “hanging groin.”. The inflammation caused by larvae that die in the eye results initially in reversible lesions on the cornea that without treatment progress to permanent clouding of the cornea, resulting in blindness. There can also be inflammation of the optic nerve resulting in vision loss, particularly peripheral vision, and eventually blindness.

Casual Agents

Onchocerciasis is the disease caused by the nematode (roundworm) Onchocerca volvulus when it inhabits subcutaneous tissues.

Life Cycle

Onchocercal infections are found in tropical climates. The main burden is in 30 countries in sub-Saharan Africa, though the parasite is found in limited areas in the Americas and in Yemen in the Middle East. In the Americas, transmission of onchocerciasis has been interrupted in 11 of the 13 foci. Interventions to limit transmission stopped in Colombia in 2008, in Ecuador in 2010, in Mexico in 2012, and in Guatemala in 2012. Transmission and interventions to limit transmission continue in one area in Venezuela and one area in Brazil. The World Health Organization (WHO) estimates that at least 25 million people are infected with O. volvulus worldwide; of these people 300,000 are blind and 800,000 have some sort of visual impairment. Some 123 million people are at risk for becoming infected with the parasite.

The people most at risk for acquiring onchocerciasis are those who live near streams or rivers where there are Simulium blackflies. Most of the areas where the blackflies are found are rural agricultural areas in sub-Saharan Africa. Usually, many bites are needed before being infected, so people who travel for short periods of time (generally less than 3 months) to areas where the parasite is found have a low chance of becoming infected with O. volvulus. Those travelers to at-risk areas most likely to become infected are long-term missionaries, Peace Corps and other long-term volunteers, and field researchers.

The diagnosis of onchocerciasis can be difficult in light infections, which are more common in persons who have travelled to but are not residents of affected areas. There are multiple ways that the diagnosis can be made:

• The most common method of diagnosis is the skin snip. A 1- to 2- mg shaving or biopsy of the skin is done to identify larvae, which emerge from the skin when it is put in physiologic solutions (e.g. normal saline). Typically 6 snips are taken from different areas of the body. Polymerase chain reaction (PCR) of the skin can allow for diagnosis if the larvae are not visualized.
• In patients with nodules in the skin, the nodule can be surgically removed and examined for adult worms.
• Infections in the eye can be diagnosed with a slit-lamp examination of the anterior part of the eye where the larvae or the lesions they cause are visible.
• Antibody tests have been developed to test for infection, though they are not widely available in the United States. These tests cannot distinguish between past and current infections, so they are not as useful in people who lived in areas where the parasite exists, but they are useful in visitors to these areas. Some of the tests are general tests for infection with any filarial parasite and some are more specific to onchocerciasis.

People who are found to be infected with O. volvulus should be treated in order to prevent the long-term skin damage and blindness. The recommended treatment is ivermectin, which will need to be given every 6 months for the life span of the adult worms or for as long as the infected person has evidence of skin or eye infection. Ivermectin kills the larvae and prevents them from causing damage but it does not kill the adults. There is a promising new treatment using doxycycline that kills the adult worms by killing the Wolbachia bacteria on which the adult worms depend in order to survive. If you are infected, it is possible that your doctor will want to treat you both with the ivermectin and the new treatment. Before any treatment is begun, however, you need to make sure that you are not also infected with Loa loa, another filarial parasite found in central Africa that is sometimes found in the same areas where O. volvulus is found, because Loa loa can be responsible for severe side effects to the medications used to treat onchocerciasis.

There are no vaccines or medications available to prevent becoming infected with O. volvulus. The best prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeves and long pants during the day when blackflies bite, and wearing permethrin- treated clothing. For a description of the CDC’s information for preventing insect bites, refer to CDC’s Yellow Book. Mass distribution of ivermectin, donated by Merck & Co., Inc., to all people living in many areas where O. volvulus is found is being given to control onchocerciasis.