Onchocerciasis is caused by a filarial nematode, the roundworm Onchocerca volvulus. Humans are infected by larval microfilariae transmitted by bloodfeeding female flies of the genus Simulium. Symptoms include skin damage, extreme itching, and ocular lesions, which can lead to permanent blindness. Synonyms include river blindness in West Africa, sowda in Yemen, and enfermedad de Robles in Latin America.

Distribution and Incidence

Onchocerciasis is widely distributed in Africa south of the Sahara, especially in the savanna grasslands from Senegal to Sudan. Its range extends southward into Kenya, Zaire, and Malawi. The region encompassing the headwaters of the Volta River system in northern Ghana, northeastern Ivory Coast, southern Burkina Faso (Upper Volta), and adjacent territories has been a major center for the disease. Onchocerciasis was almost certainly indigenous to Africa, but it has been transmitted by the slave trade to the Arabian Peninsula (Saudi Arabia and Yemen) and to the Caribbean basin, where scattered foci exist in Mexico, Guatemala, Colombia, Venezuela, Ecuador, and Brazil. The disease has a patchy distribution within its range; infection rates in particular villages may range from zero to virtually 100 percent. In the 700,000 square-kilometers of the Volta Basin region alone, the World Health Organization estimated that in the early 1970s, about 1 million of the 10 million inhabitants were infected, with about 70,000 classified as “economically blind.” In northern Ghana alone, surveys in the early 1950s determined that about 30,000 people, roughly 3 percent of the population, were totally blind because of onchocerciasis. In some West African villages, adult blindness rates of from 10 to 30 percent have been observed. Conversely, dermatologic symptoms predominate in Arabia, and ocular involvement is rare.