River blindness, or onchocerciasis, is an infection of the skin by a threadlike worm, Onchocerca volvulus. It is one of the filariasis diseases, a group of infections caused by various roundworms that are spread by a genus of blackflies, Simulium, which thrive near fast-running rivers. Prevalent in parts of Africa, Central America, and South America, it is characterized by skin irritation, skin nodules, and eye damage. The adult worms, which lie within the skin nodules, mate and produce larval forms called microfilariae, which invade the surrounding tissues and cause inflammation. Their metabolic products can cause hypersensitivity in eye parts and may lead to blindness. The World Health Organization (WHO) approximates that in some tropical African villages more than 15 percent of the adults have been so blinded.
Treatment of river blindness consists of surgically removing as many nodules as possible. Drugs such as diethylcarbamazine and suramin have also been administered to kill remaining worms, but they have potentially fatal side effects. The drug ivermectin, introduced in 1981, has only mild side effects, and one or two doses a year provide protection. Ivermectin is manufactured by Merck & Co., Inc., which distributes the drug free in developing countries.