Onchocerciasis is an infection with the roundworm (nematode) Onchocerca volvulus.
The infection is transmitted to people by infected female blackflies.
The infection may cause only intense itching but sometimes causes a rash, swollen lymph nodes, impaired vision, or complete blindness.
Usually, doctors diagnose the infection by identifying an immature form of the worm in skin samples.
If onchocerciasis causes symptoms, treatment is a single dose of a medication used to treat worms called ivermectin.
Ivermectin given once or twice a year to people living in areas where onchocerciasis is common can help control the infection.
Helminths are parasitic worms that can infect humans and animals. There are 3 types of helminths: flukes (trematodes), tapeworms (cestodes), and roundworms (nematodes). Onchocerca volvulus is a type of roundworm called a filarial worm.
Worldwide, approximately 21 million people have onchocerciasis. Of those infected, approximately 14.6 million have skin disease and 1.15 million have vision problems or blindness. Onchocerciasis is the second leading cause of infectious blindness worldwide (after an eye infection called trachoma).
Onchocerciasis is most common in tropical and southern (sub-Saharan) areas of Africa. It occasionally occurs in Yemen and in a small transmission zone in South America along the border of Venezuela and Brazil. Colombia, Ecuador, Mexico, and Guatemala have been declared free of onchocerciasis by the World Health Organization (WHO).
People in these regions who live or work near rapidly flowing streams or rivers are the most likely to be infected. Long-term travelers to these regions, such as missionaries, volunteers, or field researchers, are also at risk.
(See also Overview of Parasitic Infections.)
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Transmission of Onchocerciasis
Onchocerciasis is transmitted through the bite of female blackflies that breed in swiftly flowing streams (hence the term "river blindness").
The cycle of infection begins when an infected blackfly bites a person and deposits larvae of the worm in the skin. The larvae enter the person's body through the bite wound, move to the tissues under the skin, and form lumps (nodules). In the nodules, the larvae mature into adult worms in 12 to 18 months. Adult female worms may live up to 15 years in these nodules. After mating, mature female worms produce eggs, which develop into immature worm larvae (microfilariae). The microfilariae travel mainly through the skin and invade the eyes. The infection is spread when the infected person is bitten by a blackfly and it ingests the microfilariae.
Usually, many bites are necessary before the infection causes symptoms.
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.
Symptoms of Onchocerciasis
Onchocerciasis symptoms occur when the microfilariae die.
In the skin, their death can cause intense itching, which may be the only symptom. A red rash may develop. Over time, the skin may thicken, roughen, and wrinkle. It may lose its elasticity and pigment in patchy spots. In severe cases, people may develop long folds of skin that hang down over their lower abdomen and upper thighs ("hanging groin"). Lymph nodes, including those in the genital area, may become inflamed and swollen. Lumps (nodules) containing adult worms form and may be seen or felt under the skin. Usually, these lumps do not cause symptoms.
Effects on vision range from mild impairment (blurring) to complete blindness. The eye may become inflamed and appear red. Exposure to bright light may cause pain. Without treatment, the cornea may become completely opaque and may scar, which is the cause of blindness. Other structures in the eye, including the iris, pupil, and retina, may be affected. The optic nerve may become inflamed and degenerate.
Diagnosis of Onchocerciasis
Examination of skin samples
Examination of the eyes
To diagnose onchocerciasis, doctors typically remove samples of skin and examine them for microfilariae.
Doctors may use a slit lamp to look for microfilariae in the eye.
Blood tests to check for evidence of the infection may be done, but these tests are not always reliable or available.
Nodules can be removed and checked for adult worms, but this procedure is rarely necessary.
Treatment of Onchocerciasis
Ivermectin
Sometimes doxycycline
To treat onchocerciasis, doctors prescribe ivermectin. This medication is known as an anthelmintic. It is given as a single dose by mouth and is repeated every 6 to 12 months until symptoms are gone.
Ivermectin reduces the number of microfilariae in the skin and eyes and reduces production of microfilariae by adult worms for several months. It does not kill adult worms, but repeated doses decrease their fertility.
If people with onchocerciasis live in areas of Africa where another filarial worm called Loa loa is common, doctors check them for loiasis before giving them ivermectin because ivermectin can cause severe reactions in people who have loiasis and onchocerciasis at the same time.
Sometimes doctors also prescribe the antibiotic doxycycline to treat onchocerciasis. This medication is taken by mouth for 6 weeks.Doxycycline kills bacteria that live inside the worms and that are essential to the worms' survival. As a result, many of the adult female worms die, and others produce fewer or no microfilariae.
In the past, nodules were surgically removed, but this treatment has been replaced by ivermectin.
Prevention of Onchocerciasis
The following may help reduce the chances of being bitten by a blackfly and thus reduce the risk of onchocerciasis:
Avoiding blackfly-infested areas
Wearing protective clothing
Liberally using insect repellents
Ivermectin given once or twice a year helps control the infection in people who are repeatedly exposed to it, and this may decrease transmission. This community-based preventive approach has resulted in the elimination of onchocerciasis from some areas where onchocerciasis is common. Preventive treatment withivermectin continues in areas where the infection remains.
