The worms are transmitted to humans by biting flies that breed in fast flowing rivers. The disease mainly affects those who live near rivers.
Symptoms range from severe itching and skin lesions to total blindness.
In 1974, onchocerciasis was most devastating in central West Africa, where it was not unusual to find 60 percent of the adults in a river valley afflicted with the disease and 3 to 5 percent blind. Villagers were forced to abandon their communities en masse. The governments of 11 African nations and 24 donor agencies combined their resources and energies to spray the rivers where the black flies bred and to develop and distribute drugs for treatment or prevention of the disease. Villages once emptied by river blindness are now thriving.