River blindness is a disease responsible for a high incidence of partial or total blindness in parts of tropical Africa and Central America. Also called onchocerciasis, the disease is caused by infection with Onchocerca volvulus,a thread-shaped round worm (a nematode), which is transmitted between people by the biting blackfly Simulium. The larvae of Onchocerca develop into the infective stage, called L4, inside the blackfly and are introduced into humans by the bite of an infected blackfly.

Adult Onchocerca (2.5 ft or 0.76 m in length) develop in the connective tissue under the skin of humans. The adult worms lie coiled within subcutaneous nodules several inches in diameter. The nodules are painless and cause little damage, but can be cosmetically unattractive; fortunately, they are easily removed by simple surgery. The more serious health problems associated with onchocerciasis are caused by the release of masses of early-stage larvae, known...