Important foci exist in Mexico, Guatemala, Venezuela, and most of Western Africa.
Human onchocerciasis is caused by the filarial parasite Onchocerca volvulus .
Accordingly onchocerciasis and the blindness it can lead to are associated with fast flowing rivers with rapids and onchocerciasis is often referred to as 'river blindness'. The infective larvae of Onchocerca enter the body through the wound made by the bite of its host fly. After mating the female ovivipariously gives birth to microfilariae 300 mm in length and 0.8 mm in diameter. The microfilariae are sheathless with sharply pointed recurved tails. The microfilariae can be found free in the fluid within the nodules and in the dermal layers of the skin spreading centrifugally from the area where an adult lies.