Onchocerciasis, or river blindness, is a neglected tropical disease that inflicts lifelong misery on sufferers through skin disease, visual impairment and blindness. It is caused by Onchocerca volvulus, a parasitic filarial nematode worm. The infection is transmitted through the bite of blackflies (Simulium spp) that breed in fast flowing streams and rivers; it is this association that gave rise to the common name of the disease.
According to the World Health Organisation (WHO, 2017), 21 million people are infected with O volvulus and 198 million are at risk of infection. More than 99% of onchocerciasis patients live in sub-Saharan Africa, although there are also small isolated foci in Latin America and Yemen.
The parasite life cycle (Figure 1)
Infection is initiated by L3 larvae (Figure 2) that enter the skin when blackflies take a blood meal. Over a period of about 1 year, L3 larvae mature into adults which are found in sub-cutaneous nodules over the pelvis, pectoral girdle and/or head (Figure 3). Female worms (Figure 4) live for about 15 years and can give birth to between 500 and 1500 microfilariae (L1 larvae, Figures 5 and 6) per day. Microfilariae migrate from the nodules to the skin and eyes where they can be detected within a year of the initial infection. The parasite’s life cycle is completed when microfilariae are ingested by a blackfly taking a blood meal.
Microfilariae can live up to two years in the skin but when they eventually die, either naturally or due to drug treatment, they evoke inflammatory responses, which are responsible for most symptoms of onchocerciasis (Figures 7 and 8). Itching is the most frequent early sign of infection, but this can lead to severe local or general and disfiguring dermatitis (Figure 7) and premature ageing of the skin. Skin disease has a disproportionate impact on women, first through social exclusion, including reducing prospects of marriage; and second, on their ability and willingness to breast-feed babies.
About 1% of individuals infected with O volvulus are blind and a further 10% visually impaired (Figure 8); however, up to 70% suffer from skin disease of varying severity.