The variable incidence of psoriasis in sub-Saharan Africa

Surveys of skin disorders have previously provided information about the prevalence and incidence of psoriasis in sub-Saharan Africa; however, the geographic and ethnic trends which may be drawn from these surveys have not been fully described in previous studies, which considered only a fraction of the available data.

A critical review of clinic-based surveys of psoriasis incidence and population-based studies of psoriasis prevalence is presented. The incidence of psoriasis is adjusted, wherever possible, to factor out the widely variable incidence of infectious skin conditions seen in African skin clinics. To distinguish between genetic and environmental factors that may be responsible for the variability of psoriasis incidence, attention is drawn to climate, human leukocyte antigen (HLA) frequencies, and language groups across the regions surveyed in sub-Saharan Africa.

Higher psoriasis incidence rates are consistently observed in eastern Africa than in western Africa, consistent with more limited data on the prevalence of psoriasis in western Africa. Neither rainfall/humidity levels nor HLA frequencies can simply account for these differences; however, the ethnicities of sub-Saharan African peoples may be observed to parallel roughly the trend in psoriasis incidence. Western African countries, such as Nigeria, Mali, Senegal, and Sierra Leone, where lower rates of psoriasis incidence have been reported (less than 1.0% of skin disorders), are populated mainly by non-Bantu-speaking ethnic groups. Bantu-speaking peoples constitute a majority in the populations of most countries in eastern and southern sub-Saharan Africa, where the incidence of psoriasis varies widely. African Americans, whose largely non-Bantu-speaking African ancestry is shared with modern western Africans, also have relatively low psoriasis incidence rates by comparison with North American Caucasians.

Ethnic correlations both within Africa and between North America and Africa suggest that unidentified genetic factors, which differ between eastern and western sub-Saharan Africans, may govern the differential incidence of psoriasis.