Onchocerciasis is infection with the roundworm Onchocerca volvulus. It causes itching, a rash, sometimes with scarring, as well as eye symptoms that may lead to blindness.
The infection is spread through the bite of female blackflies, which breed in streams.
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 a skin sample.
Administration of ivermectin once or twice a year in areas where the infection is common helps control the infection.
Treatment is a single dose of ivermectin, which is repeated every 6 to 12 months until symptoms are gone.
Worldwide, about 25 million people have onchocerciasis. About 300,000 of them are blind, and 800,000 are visually impaired. Onchocerciasis is the second leading cause of blindness worldwide. Onchocerciasis is most common in tropical and southern (sub-Saharan) areas of Africa. It occasionally occurs in Yemen, southern Mexico, Guatemala, Ecuador, Colombia, Venezuela, and Brazil (along the Amazon).
Onchocerciasis is spread though the bite of female blackflies that breed in swiftly flowing streams (hence, the term river blindness). The cycle of infection begins when a blackfly bites an infected person and is infected with immature forms of the worm called microfilariae. They develop into larvae in the fly. When the fly bites another person, larvae are passed into that person’s skin. The larvae move under the skin and form lumps (nodules), where they develop 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 microfilariae that leave the worm. A worm may produce 1,000 microfilariae each day. Thousands of microfilariae move through the tissues of the skin and eyes and are responsible for the disease.
Usually, many bites are necessary before the infection causes symptoms. Thus, the infection is much less likely to develop in visitors to affected areas.
Because the infection is transmitted near rivers, many people avoid those areas. Not being able to live or to work near a river affects their ability to raise crops. Thus, onchocerciasis can contribute to food shortages in some areas.
Onchocerciasis symptoms occur when the microfilariae die. Their death can cause intense itching, which may be the only symptom. A rash with redness may develop. Over time, the skin may thicken, roughen, and wrinkle. It may lose its pigment in patchy spots. Lymph nodes, including those in the genital area, may become inflamed and swollen. Nodules containing adult worms may be seen or felt under the skin.
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—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. Blindness can result in a decreased life span.
Usually, a sample of skin is removed and examined for microfilariae. Alternatives are to test blood, but these tests are not always reliable or available. Microfilariae may also be seen in the eye using a slit lamp.
Nodules can be removed and checked for adult worms, but this procedure is rarely necessary.
Theoretically, the following may help reduce the risk of infection:
Ivermectin given once or twice a year dramatically reduces the number of microfilariae, prevents the development of further disease, and helps control the infection in people who are repeatedly exposed to it.
For treatment of onchocerciasis, ivermectin is given as a single dose by mouth and is repeated every 6 to 12 months until symptoms are gone. Ivermectin kills microfilariae, reducing 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.
Sometimes doctors also give doxycycline (an antibiotic) for 4 to 6 weeks. It kills some of the bacteria that live inside the worms and that are essential to the worms' survival. As a result, the worms die. Side effects are usually mild.
In the past, nodules were surgically removed, but this treatment has been replaced by ivermectin.