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, mostly near remote rural villages, and the disease that is caused by O. volvulus following repeated bites from infected blackflies can lead to 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 Global Burden of Disease Study estimated that in 2017 there were at least 20.9 million people infected worldwide, of which 14.6 million had skin disease and 1.15 million had vision loss. More than 99% of infected persons live in Africa; the remainder live in foci in Yemen and Latin America. Onchocerciasis is commonly treated with an oral medicine called ivermectin.
Onchocerciasis is locally transmitted in 31 countries of Africa and in foci in Yemen and South America. The list of 31 African countries can be found on the World Health Organization (WHO) Onchocerciasis Fact Sheetexternal icon website. Four countries have been verified by the World Health Organization as free from onchocerciasis: Colombia, Ecuador, Mexico, and Guatemala. Only a single small transmission zone remains in South America, crossing the border between the Bolivarian Republic of Venezuela and 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.
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 are ingested by the blackfly. The larvae develop over approximately one week 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.
Those most at risk are people who live or work near rapidly flowing streams or rivers where there are Simulium blackflies, followed by long-term missionaries, Peace Corps volunteers, field researchers, and other long-term travelers who are at greater risk for being bitten multiple times by blackflies infected by the parasite. The disease is most intensely transmitted in remote rural African agricultural villages which are located near rapidly flowing streams and rivers.
Most of the areas where the blackflies are found are rural agricultural areas in sub-Saharan Africa in the countries mentioned previously. Usually, many bites are needed before becoming infected.
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 measures and treatment.
Infected persons may not have symptoms. Those with symptoms will usually have one or more of three manifestations: skin rash (usually itchy), eye disease, or nodules under the skin. The most serious manifestation consists of lesions in the eye that can lead to visual impairment or blindness.
After a blackfly bite, it can take up to 12–18 months for the larvae to develop into mature adult worms inside the human body that are capable of mating and producing new larvae (also called microfilariae) that can be found in the skin. Each adult female worm, which can live approximately 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 infected with O. volvulus feel well until after the adult worms start producing large numbers of new larvae.
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.
There are treatments, such as ivermectin, available to kill the larvae in your body and thus prevent or reduce the symptoms of the disease, such as skin rash and visual impairment. 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.
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.
No, there is neither a vaccine nor a recommended drug available to prevent onchocerciasis.