Onchocerciasis: River blindness, a disease caused by a parasitic worm (Onchocerca volvulus) which is transmitted to persons by biting blackflies (buffalo gnats) that breed in fast-flowing rivers. The adult worms can live for up to 15 years in nodules beneath the skin and in the muscles of infected persons, where they produce millions of worm embryos (microfilariae) that invade the skin and other tissues including the eyes. About 18 million persons are affected, mostly in Africa and also in Yemen and Latin America. Both living and dead microfilariae cause severe itching in the skin and sometimes blindness after many years.
Until the 1980s, the main control measure was to use larvicides to kill immature blackflies in rivers. This method was used effectively to reduce the incidence of the disease in part of West Africa but is expensive. Since 1987, the drug ivermectin (brand name: Stromectol) has been provided by the manufacturer (Merck) free of charge to control programs for treating persons with onchocerciasis. This treatment, effective in a single oral dose administered once a year, prevents the accumulation of microfilariae in persons at risk. No drug suitable for mass treatment can kill the adult worms in the body and onchocerciasis cannot be eradicated without such means. The blindness, however, can be eliminated.