Onchocerciasis (river blindness)
is a chronic filarial disease caused by the nematode Onchocerca volvulus.
It is one of the major causes of infectious blindness worldwide. It is
estimated that there are over 18 million infected persons worldwide out
of whom 2 million have some form of visual impairment and about 400,000
suffer from blindness. Endemic areas include Equatorial Africa and several
foci in Central America, South America, and the Arabian peninsula. Black
flies of the family Simuliidae (order Diptera), are the only known
vector for O. volvulus. The flies are found mostly near fast-flowing
rivers. Female black flies are blood feeders, and transmission of the parasites
to human occur during the blood meal.