Human disease caused by a filarial worm native to Africa but also found in parts of tropical America and transmitted by several blackflies. It is so called because the flies that transmit the disease breed on rivers and mostly affect riverine populations. Blindness is caused by dead microfilariaethe larvae that can be produced for some 15–18 years by adult wormsinside the eye. River blindness is common in savannah areas of Africa and in Guatemala and Mexico. In 1987 the World Health Organization began to distribute the drug ivermectin (originally developed for use against livestock parasites), which eliminates the microfilariae, though it does not kill the adult parasite.