River blindness: A disease caused by a parasitic worm (Onchocerca volvulus) that is transmitted by biting blackflies 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, causing blindness. The drug ivermectin (brand name: Stromectol), taken in a single oral dose administered once a year, prevents the accumulation of microfilariae in persons at risk. Treatment is with antibiotics that can kill the adult worms in the body. Also known as onchocerciasis.