Epidemiology & Risk Factors
Onchocercal infections are found in tropical climates. The main burden is in 30 countries in sub-Saharan Africa, though the parasite is found in limited areas in the Americas and in Yemen in the Middle East. In the Americas, transmission of onchocerciasis has been interrupted in 11 of the 13 foci. Interventions to limit transmission stopped in Colombia in 2008, in Ecuador in 2010, in Mexico in 2012, and in Guatemala in 2012. Transmission and interventions to limit transmission continue in one area in Venezuela and one area in Brazil. The World Health Organization (WHO) estimates that at least 25 million people are infected with O. volvulus worldwide; of these people 300,000 are blind and 800,000 have some sort of visual impairment. Some 123 million people are at risk for becoming infected with the parasite.
The people most at risk for acquiring onchocerciasis are those who live near streams or rivers where there are Simulium blackflies. Most of the areas where the blackflies are found are rural agricultural areas in sub-Saharan Africa. Usually, many bites are needed before being infected, so people who travel for short periods of time (generally less than 3 months) to areas where the parasite is found have a low chance of becoming infected with O. volvulus. Those travelers to at-risk areas most likely to become infected are long-term missionaries, Peace Corps and other long-term volunteers, and field researchers.