Onchocerciasis

Abstract

Onchocerciasis is a neglected tropical disease that has left hundreds of thousands of the world’s poorest people irreversibly blinded and has caused skin disease-induced misery for many more. The symptoms of the disease are caused by the death of skin-dwelling Onchocerca volvulus microfilariae. It is thought that the release of the filarial parasites’ intracellular bacteria Wolbachia is the principal cause of an immune response that can lead to a range of progressively worsening symptoms including intense itching, various forms and degrees of skin disease, and visual impairment. The level of exposure an individual has to O. volvulus - infected simuliid vector bites correlates with the level of microfilariae found in their skin, and this, microfilarial load, correlates with the severity of symptoms an individual suffers from. The oldest people in an onchocerciasis focus typically have the most severe symptoms, and the most hyperendemic foci tend to have the most severely debilitated individuals. For more than 50 years, the WHO has led international efforts to control the disease. To reduce transmission, the WHO initially targeted the juvenile stages of the disease’s simuliid vectors with the river application of the insecticide temephos. In the 1980s, however, the WHO’s control policy shifted to community-directed treatment with ivermectin, which both reduced the parasites’ transmission and benefited those treated by retarding the disease’s clinical progression. Following the success of past control programs, policy makers are now targeting the elimination of new O. volvulus infections and the eventual global elimination of the parasite. Novel onchocerciasis treatment programs (OTPs) are presently being developed and evaluated to meet the challenges posed by OTPs expanding into Loa loa endemic areas and suboptimal ivermectin patient responses.