The breeding and biting habits of the vector blackfly influence the prevalence and type of infection. In Africa, the flies tend to bite low on the body and breed in fast-running streams and rivers. Eye lesions are more prevalent in the African savannah. Gross skin lesions are more prevalent in the rain forest. In America, the flies breed in very small streams. The vector blackfly in Guatemala bites the upper body parts and results in great numbers of head nodules and eye lesions.
Onchocerciasis occurs in 34 countries of tropical Africa and the Americas, especially Mexico, Guatemala, Venezuela, northern Brazil and Colombia. At 1993, 18 million people were infected and a further 90 million were at risk. 90% of the infected people live in a wide belt south of the Sahara from Angola in the west to Tanzania in the east. In the Americas, important foci exist in Mexico, Guatemala, Colombia and Venezuela, and also among small human groups of the Amazonian forest. Blindness rates may attain dramatic proportions in heavily infected villages, particularly in the older age groups. In some areas up to 20% of villagers aged between 30 and 60 years are blind, and half the village ultimately goes blind. The greatest economic losses occur when horses and cattle are affected. In 1990 it was estimated that 40 million people were infected by leprosy, Chagas' disease and river blindness.