Onchocerca volvulus

Onchocerca volvulus is an arthropod-borne filarial nematode (roundworm) that causes onchocerciasis (river blindness), and is the second-leading cause of blindness due to infection worldwide after trachoma. It is one of the 20 neglected tropical diseases listed by the World Health Organization, with elimination from certain countries expected by 2025.

John O'Neill, an Irish surgeon, first described Onchocerca volvulus in 1874, when he found it to be the causative agent of 'craw-craw', a skin disease found in West Africa. A Guatemalan doctor, Rodolfo Robles, first linked it to visual impairment in 1917. The parasite was formally described and named by German zoologist and parasitologist Rudolf Leuckart in 1893.

Onchocerca volvulus is primarily found in sub-Saharan Africa, and there is also disease transmission in some South American nations, as well as Yemen (see global map under Epidemiology). It is spread from person to person via female biting blackflies of the genus Simulium, and humans are the only known definitive host.