Onchocerciasis, also known as river blindness, is one of the main causes of blindness affecting poorer countries and is the second most frequent infectious cause of blindness after trachoma.

What is onchocerciasis?

Onchocerciasis is a disease in which a parasitic infection with a worm called Onchocerca volvulus causes inflammation within the eye leading to severe inflammation, damage to critical structures and frequently to permanent blindness.

Infection with this worm also causes severe skin disease with terrible itching, rashes, skin nodules and other secondary skin changes and deformities.

It is a dreadful disease which causes very high levels of blindness and suffering in communities in which it is endemic. It often leads to community breakdown and dispersal because of its severe economic and social consequences.

The disease process is that an insect vector, the blackfly (Simulium damnosum) which feeds on human blood by biting through skin, transmits the tiny immature worms (infectious lavae) from an infected human to other humans.

These larvae infect the new host human and in due course mature and form tangles or clusters of adult worms under the skin as nodules.

These adult worms reproduce and each day release thousands of tiny worms called microfilariae, which migrate around the body by moving under the skin. When they reach the eye they can cause inflammation and scarring in the cornea and deeper tissues of the eye including the choroid and optic nerve.

Over time, the cumulative effects of inflammation and scarring can destroy all useful function in the eyes and the sufferer becomes blind.

Humans are the only known host for Onchocerca volvulus and the blackfly is the only known vector.

The blackfly breeds in broken flowing water with high oxygen content such as river rapids, so is often found in areas which are fertile for agriculture and attractive for human habitation.

Where does onchocerciasis occur?

Onchocerciasis occurs mainly in Africa in about 30 hot countries extending in a band across the sub-Saharan region. Several Central American countries with similar climates have zones where onchocerciasis occurs, and it is also known to occur in Yemen.

These are all areas which favour the survival of the blackfly vector and where person-to-person transmission via their bite is likely.

By far the majority of people who suffer from onchocerciasis live in Africa.

Who gets onchocerciasis?

In the countries where it occurs, at least 80 million people are known to be at risk of onchocerciasis. There are probably about 15 million people who are infected with Onchocerca volvulus and of these, studies suggest that about 300,000 people are blinded by it.

It is one of the main causes of blindness affecting poorer countries and is the second most frequent infectious cause of blindness after trachoma.

Those most affected are poor people who live by subsistence agriculture in areas where the blackfly is common. They are likely to be bitten and thus acquire the disease because they work outdoors and live and work in areas close to running water, where the best soil and the best water supplies for crops, animals and drinking are found.

In some of the worst affected areas, more than 50% of the adult population is blind from onchocerciasis - a terrible burden of suffering for areas where life is already very tough.

How is onchocerciasis managed?

People who are already blind from advanced damage to their eyes from onchocerciasis cannot usually be helped much. People with less advanced disease can be treated according to their actual symptoms and findings but the mainstay of management is the use of a drug, ivermectin (Mectizan™, Merck and Company), which kills the microfilaria which are the source of the inflammation.

Because the treatment kills the microfilaria, but not the adult worms which release them, treatment has to continue with doses of ivermectin being required at least annually for about 15 years, which is the lifespan of the adult worms.

Large-scale programs to treat communities and individuals who need it have been developed over the last 35 years. The main aim of these programs is to develop ongoing ‘community directed treatment with ivermectin' (CDTI) programs staffed, controlled and operated by community members. The Merck company has agreed to provide as much ivermectin as is needed for as long as it is needed to control and hopefully eliminate the disease.

In some limited areas it has been possible to gain reasonable control of the vector blackfly by spraying  extensively around areas where they breed, killing the larvae. This is not logistically possible in many areas so ivermectin treatment remains the mainstay.