THE VARIABLE INCIDENCE OF PSORIASIS IN SUB-SAHARAN AFRICA

Background Surveys of skin disorders have previously provided informat ion about the prevalence and incidence of psoriasis in sub-Saharan Afr ica; however, the geographic and ethnic trends which may be drawn from these surveys have not been fully described in previous studies, whic h considered only a fraction of the available data. Methods A critical review of clinic-based surveys of psoriasis incidence and population- based studies of psoriasis prevalence is presented. The incidence of p soriasis is adjusted, wherever possible, to factor out the widely vari able incidence of infectious skin conditions seen in African skin clin ics. 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 la nguage groups across the regions surveyed in sub-Saharan Africa. Resul ts Higher psoriasis incidence rates are consistently observed in easte rn Africa than in western Africa, consistent with more limited data on the prevalence of psoriasis in western Africa, Neither rainfall/humid ity levels nor HLA frequencies can simply account for these difference s; however, the ethnicities of sub-Saharan African peoples may be obse rved to parallel roughly the trend in psoriasis incidence. Western Afr ican countries, such as Nigeria, Mall, Senegal, and Sierra Leone, wher e lower rates of psoriasis incidence have been reported (less than 1.0 % of skin disorders), are populated mainly by non-Bantu-speaking ethni c groups. Bantu-speaking peoples constitute a majority in the populati ons of most countries in eastern and southern sub-Saharan Africa, wher e the incidence of psoriasis varies widely. African Americans, whose l argely non-Bantu-speaking African ancestry is shared with modern weste rn Africans, also have relatively low psoriasis incidence rates by com parison with North American Caucasians. Conclusions Ethnic correlation s both within Africa and between North America and Africa suggest that unidentified genetic factors, which differ between eastern and wester n sub-Saharan Africans, may govern the differential incidence of psori asis.