Prevention of Blindness and Visual Impairment

Onchocerciasis (river blindness) - disease information

What is onchocerciasis?

Photomicrograph of onchocerca volvulus (adult worms).
Photomicrograph of onchocerca volvulus (adult worms).

Onchocerciasis is an eye and skin disease caused by a worm (filaria) known scientifically as Onchocerca volvulus. It is transmitted to humans through the bite of a blackfly (simulium species). These flies breed in fast-flowing streams and rivers, increasing the risk of blindness to individuals living nearby, hence the commonly known name of "river blindness". Within the human body, the adult female worm (macrofilaria) produces thousands of baby or larval worms (microfilariae) which migrate in the skin and the eye.

What are the consequences of the disease?

An adult blackfly (Simulium damnosum) taking a bloodmeal on human skin.
An adult blackfly (Simulium damnosum) taking a bloodmeal on human skin.
Photo: WHO/TDR/Stammers

The death of microfilariae is very toxic to the skin and the eye, producing terrible itching and various eye manifestations (lesions). After repeated years of exposure, these lesions may lead to irreversible blindness and disfigurative skin diseases sometimes named "leopard" skin and "lizard" skin.

In some West African communities, about 50% of men over the age of 40 years had been blinded by the disease. Finally, people fled the fertile river valleys to settle in less productive upland country. Hence the annual economic losses were estimated, in the 1970s, at US$ 30 million.

Where is onchocerciasis distribued?

The distribution of onchocerciasis is linked to the location of blackflies which are naturally found close to the fast-running streams and rivers in the inter-tropical zones. Therefore, about 90% of the disease occurs in Africa. Onchocerciasis is also found in six countries in Latin America and in Yemen in the Arabian Peninsula, where the disease is believed to be exported by the slave trade.