Onchocerciasis is a rare tropical parasitic disease transmitted by a black fly. Infection by the parasite can cause eye and skin problems. In humans, the parasitic worms live under the skin (subcutaneous nodules) and produce larvae (microfilariae). The larvae are found throughout the body, but especially in the skin and eyes. Repeated bites by infected flies increase the number of adult worms and larvae. Chronic skin Onchocerciasis (onchodermatitis) causes itching, a rash with small pimples (papular rash), scarring, and thickened, leathery skin (lichenification). Other symptoms may develop over time and might include saggy skin (such as ''hanging groin''), patchy areas of much lighter colored skin (leopard skin), ichthyosis-like lesions (''lizard skin''), darkening of the skin, and very severe itching. The eye infection is known as river blindness because the blackfly carrying the parasite lives near fast-flowing waters. Symptoms of an eye infection may include itchy eyes, red eyes, and sensitivity to light (photophobia). In some cases, the larvae may infect the optic nerve. As the common name suggests, infection of the eye by Onchocerciasis can lead to vision loss and eventually blindness. The disease is caused by the parasitic worm Onchocerca volvulus and is transmitted by the bite of an infected black fly. About 90% of the cases occur in Africa.