Loa loa is a blood dwelling nematode that is parasitic in humans. The adult worm wanders through the subcutaneous tissue but is most obvious as it crosses the conjunctiva of the eye hence leading to its common name, the African eye worm. Like all roundworms, Loa loa is sexual so a male and female worm must be present in the same host for a full infection to ensue. Upon reproduction the female worm produces sheathed eggs called microfilariae which circulate in the blood stream.

Loa loa is endemic to parts of Western Africa, especially in the rainforests of the Congo and Sudan. Symptoms are less serious in natives of these areas with complications occurring mostly in visitors and tourists.

Infection with Loa loa is spread by biting mango flies, a member of the genus Chysops. The American deer fly, Chysops atlanticus, had been reported to be a competent intermediate host of Loa loa and able to spread the worm to monkeys. This is of some public health concern but so far Loa loa has remained isolated to Africa.