Incidence, Prevalence, & Associated Factors
World distribution of onchocerciasis. Map courtesy of WHO.
More than 17.7 million people worldwide are infected with onchocerciasis. Visual impairments occur in 500,000 of these people, including 270,00 cases of blindness. Over 90%per cent of all cases of onchocerciasis occur in Africa. In some communities in West Africa, 50% of the men over age 40 have been blinded by onchocerciasis. [2, d, h, i, and m (quoting k and l)]
Onchocerciasis is found in rural, often poor, populations living near fast-flowing rivers, rapids, and other turbulent waters where the blackfly vector breeds. Studies show a correlation between the proximity of villages to the water and the prevalence and intensity of onchocercal infection occurring in those villages, giving rise to the designations first-, second-, and third-line villages. [2,a]
While the women of most villages draw water from village wells, the men more often fish and farm near the fast-flowing rivers. Thus, males are more frequently infected, more severely infected, and more often blinded by onchocerciasis than females. [a,h]
The prevalence of infection is nearly zero at birth, since onchocerciasis can, but seldom does, undergo intrauterine transmission, and increases with age to a high plateau later in life, reaching 100% in hyperendemic areas. Similarly, intensity of infection increases with age to a plateau between ages 30 and 50 years. [2,a]