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Michele E Murdoch, BSc, FRCP
Section Editor
Peter F Weller, MD, FACP
Deputy Editor
Elinor L Baron, MD, DTMH


Onchocerciasis is caused by the filarial nematode Onchocerca volvulus. It is also known as "river blindness" because the blackfly vector breeds near fast-flowing streams and rivers. The disease affects rural communities and is a major cause of blindness and skin disease in endemic areas with serious socioeconomic consequences.


The number of infected people worldwide was estimated in 2006 to be 37 million [1]. More than 99 percent of cases occur in 27 countries in sub-Saharan Africa (figure 1). Overall, 120 million people live at risk of infection in endemic countries in Africa. Smaller foci of infection have been found in Yemen and Central and Southern America (Mexico, Guatemala, Ecuador, Colombia, Venezuela, and Brazil). Transmission has now been eliminated or interrupted in eleven of the thirteen foci in the Americas and is ongoing in one focus in Venezuela and one in Brazil (figure 2).

Onchocerciasis is the second-leading infectious cause of blindness worldwide: approximately 500,000 people are blind due to onchocerciasis [2]. The epidemiologic patterns of infection differ between savanna and forest regions.

In West African savanna areas, ocular onchocerciasis is common; it particularly affects the anterior segment of the eye, though the posterior eye segment can also be affected. The risks of visual impairment increase, in part, as the prevalence and intensity of infection in a community rises [3]. The prevalence of infection can vary between villages and was historically as high as 80 to 100 percent by the age of 20 years in some areas, with blindness peaking at 40 to 50 years of age. Prior to control activities, hyperendemic regions were frequently depopulated because of high rates of blindness.

In African forest areas with a comparable intensity of onchocerciasis as savanna areas, onchocercal skin disease predominates, with much less blindness. Furthermore, ocular lesions, when present, usually involve the posterior eye segment. A multi-country study in highly endemic forest communities found that itching affected 42 percent of the population aged ≥20 years, and onchocercal skin lesions affected 28 percent of the population aged ≥5 years. Strong associations were found between the prevalence of skin lesions and troublesome itching and onchocercal endemicity [4]. By extrapolation, it is estimated that 6 million people worldwide suffer from onchocercal skin disease [5].


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