The Global Problem
Onchocerciasis is endemic in 36 countries across Africa, Latin America and Yemen.  About 85.5 million people live in endemic areas and about 50% of these reside in Nigeria.  Currently, more than 18 million people are infected with onchocerciasis and 99% of all infected people live in Africa.  Another 120 million people are at risk for contracting the disease.  Due to the vector’s breeding habit, the disease is more severe along the major rivers in the northern and central areas of the continent.  The disease exists in isolated foci in Latin America (6 countries) and the Middle East.  Ocular onchocerciasis has been found in more than 1 million individuals worldwide, although the rate of blindness varies by region.  (14, 17, 18, 19)


Morbidity and Mortality
According to a 2002 WHO report, Onchocerciasis has not caused a single death, but its global burden is 987,000 Disability Adjusted Life Years (DALYs).  The severe pruritis alone accounts for 60% of the DALYs.  The microfilariae cause a range of debilitating symptoms including blindness, lymphadenitis, and dermatitis.  Additionally, infection reduces the host’s immunity and resistance to other diseases.  This results in an estimated reduction in life expectancy of 13 years. (17, 19, 20)


Local Differences
It is thought that Onchocerciasis was brought from Africa to South America through the slave trade.  The African and American forms of onchocerciasis have distinct characteristics, which may be related to vector biting habits or strain differences.  The flies tend to bite the upper body more in the Americas whereas in Africa they show a propensity to bite the lower body, which leads to the difference in nodule distribution.  The fly vectors in Colombia are zoophillic, meaning that human infection has all but died out in areas where domestic livestock are kept.  (7)


Location of Endemic Areas
Simuliid flies have larval stages that are aquatic, meaning that in order for the flies to complete their lifecycle, they require fast-flowing and aerated water in which the larvae and pupae can attach to submerged rocks or vegetation.  Due to this aspect of the vector’s development, endemic areas usually coincide with the path of rivers and streams.  This fact has been confirmed with Onchocerciasis mapping. (7)