In endemic areas around 33 per cent of the people affected currently receive this preventive therapy on a regular basis.

Onchocerciasis – or river blindness – caused by the threadworm (Onchocerca volvulus) is widespread in 31 countries in Africa, but endemic foci also exist in Latin America. Transmission of the larvae of threadworms from person to person occurs through a bite from a blackfly. This fly breeds in fast-flowing rivers and streams in remote rural areas with fertile agricultural land. After infection takes place, the worm larvae form nodules in the subcutaneous tissue and develop into adult worms. On reaching sexual maturity the adult females produce new larvae, also known as microfilariae, which migrate through the connective tissue and eventually die. The physical symptoms brought on by the infection include severely itchy scaly or thickened skin and inflammation of the eyes which, left untreated, can lead to blindness.