Global Distribution

Blackflies and onchocerciasis are commonly found near the rivers
that flow through these wooded valleys. Photo courtesy of WHO/TDR.

Of Disease
Currently, 35 countries around the world are affected by onchocerciasis. It is an ancient disease of Africa and, accordingly, 28 of the affected countries and over 90% of all infected people are in Africa. Onchocerciasis has become endemic to Central and South America as well, affecting 70,000 people in Guatemala and Mexico alone. Other small, isolated endemic foci occur in Venezuela, Colombia, Brazil, Ecuador. Most recently, areas of Yemen and Saudia Arabia on the Arabian Peninsula have been declared endemic. [2,d,j,m,o]

Of Clinical Manifestations
Geographic differences in the disease manifestations may result from variations in: [a,d,o]
· different forms or species of the vector with different life cycles and habits
· different parasite strains with varying pathology and antigenic makeup
· the degree of the host’s susceptibility
· acquired immune tolerance in utero in endemic areas
· altered or suppressed immunity of the host from malnutrition

Summary of Vector and Disease Distribution
Region Major Vector(s) Disease Manifestation
African savanna S. damnosum - lives in hilly or sparsely wooded areas, breeds in streams of all sizes, tends to bite lower body;[d,e,o]
S. neavei - lives in Ethiopia, Central & East Africa [d,e]
Disease foci are widespread swathes of infected areas across countries with islands of particularly high prevalence. High rate of eye lesions, blindness, hanging groin, elephantiasis, high microfilaria levels, fewer nodules and gross skin lesions, and in Uganda the Nakalanga syndrome (low body weight, dwarfism, absence of secondary sex characteristics, skeletal deformation, dental caries, mental retardation) [a,d,o]
African forestland S. neavei - tend to bite lower body [o] Disease foci are widespread swathes of infected areas across countries with islands of particularly high prevalence. Fewer eye lesions, more gross skin and gland lesions, abundant nodules, moderate microfilaria levels, hanging groin, elephantiasis, lower rates of sclerosing keratitis [d,e,o]
Latin America

Vectors live at elevations up to 3000 m, but particularly inhabitat mountainsides where rivers flow faster [e];
S. ochraceum - in Guatemala & Mexico, lives near small streams in hilly and usually densely forested areas, tends to bite upper body;
S. metallicum & S. exiguum - in Venezuela & elsewhere, live in hilly but sparsely wooded areas, tends to bite lower body [d,e,o,h]

Completely isolated disease foci, particularly common in Native Americans who work in coffee plantations in the highlands. Mild skin lesions, with the exception of the now rare erisipela de la costa, which affects the faces of heavily infected children. Symmetric dermititis of limbs & trunk. Numberous microfilaria in skin snips. In Guatemala & Mexico - infections of high intensity, with eye lesions, facial dermititis, nodules on head and torso. Venezuela & elsewhere - less intense infections, bites and symptoms mostly of lower body, eye involvement less common. [d,e,o]
Arabian peninsula n/a Microfilaria mostly found in lower legs and ankles. "Sowda" refers to a reactive form of dermititis leading to hyperpigmented, hypertrophic skin of a single limb and pruritis, swelling papules, edema, and regional lymphadenitis. [d,o]