Onchocerciasis is endemic in 27 countries in Africa. Approximately 90 million people are at risk, with 37 million people infected. About 270, 000 individuals developed blindness from onchocerciasis and another 500 000 have severe visual disability. Global disease burden in 2002 was 0.95 million disability-adjusted life years (Remme, 2004) Over 99% of cases occur in sub-Saharan Africa. In hyper endemic villages of Africa, infection rates commonly approach 100%.In Latin America, distribution is focal and vector transmission is at least 10-fold less intense. Thus, infected individuals in Latin America have low worm burdens and consequently few people develop blindness. Forest-strain O.volvulus, found in rainforest coastal regions of sub-Saharan Africa are genetically different from the savanna strain, (Flockhart , Cibulskis , Karam , et al., 1986) only causes mild ocular diseases but rarely blindness, even in heavily infected persons. (Dadzie , Remme , Rolland , et al., 1989). In contrast, in endemic savanna regions of Africa, a linear relationship between a derived parameter, the community microfilarial load, and the incidence of blindness in that community exists (Remme , Dadzie , Rolland , et al., 1989).
On an individual basis, the likelihood of blindness is positively associated with increasing microfilarial burden (Little , Basanez , et al., 2004). Other factors influencing the epidemiology of onchocerciasis are less well defined. Host immunity seems to play a role in limiting total worm loads with increasing age.
RIVER BLINDNESS IN SUB-SAHARA
Also, children of O. volvulus infected mothers have a higher risk of infection and become infected at a younger age and with increased microfilaria intensity, suggesting intrauterine O. volvulus specific immunosuppressant (Kirch , Duerr , Boatin , et al., 2003).
In regions outside the savanna, skin manifestations are the main complications of disease. The profound psychosocial implications of unremitting pruritus and disfiguring skin lesions make onchocercal skin disease a major public health problem and more than 50% of Daly’s lost due to onchocerciasis are due not to blindness but to skin disease (Murdoch , Asuzu , Hagan , et al., 2002).
On a community basis, there is a strong correlation between prevalence of pruritus and O. volvulus endemicity in the community. In communities hyper endemic for O. volvulus (defined as >60% of people with microfilardermia and >30% with palpable nodules (onchocercomas), 30 –40% have symptomatic skin disease and 50% of children aged 5–9 are already infected. Adult persons may harbor up to 50 adult worms and >100 microfilariae/mg skin. In mesoendemic areas, 30 –60% of people and in hypoendemic areas <30% have microfilardermia. RIVER BLINDNESS: EPIDEMIOLOGY OF RIVER BLINDNESS IN TROP