Onchocerciasis is a parasitic disease caused by the filarial worm Onchocerca volvulus. It is transmitted through the bites of infected blackflies of Simulium species, which carry immature larval forms of the parasite from human to human. In the human body, the larvae form nodules in the subcutaneous tissue, where they mature to adult worms. After mating, the female adult worm can release up to 1000 microfilariae a day. These move through the body, and when they die they cause a variety of conditions, including blindness, skin rashes, lesions, intense itching and skin depigmentation. In a number of countries, onchocerciasis has been controlled through spraying of blackfly breeding sites with insecticide. In addition, a drug is available that kills the microfilariae, alleviating symptoms and reducing transmission. An international control effort aims to bring annual treatment with this drug to all populations at risk by the year 2010. When that is achieved, onchocerciasis may cease to be a public health problem.