River Blindness or onchocerciasis is caused by the prelarval (microfilaria) and adult stages of the filarial nematode Onchocerca volvulus. The disease is transmitted by the bite of certain species of female Simulium flies (black flies) that bite by day and are found near rapidly flowing rivers and streams. Onchocerciasis is endemic in more than 25 nations located in a broad band across the central part of Africa. Small endemic foci are also present in the Arabian Peninsula (Yemen) and in the Americas (Brazil, Colombia, Ecuador, Guatemala, southern Mexico, and Venezuela)