The prevalence of psoriasis in the world

Abstract

Psoriasis is a worldwide disease. It affects most races, does not have any sexual predilections and can manifest at any age of life.1, 2 However, psoriasis is more frequent in certain racial groups and geographical areas. Reasons for these variations are likely to be both genetic and environmental. A reliable epidemiological survey depends on proper sample recruitment and accurate interpretation. It is, however, out of scope of this article to address these aspects of the individual reports. Keeping this limitation in mind we will be discussing the various studies on the prevalence of psoriasis currently available in the literature. In general, psoriasis is more common in colder northern climates than in the tropical regions. Series of studies carried out in Europe, North America and Australia suggest that Caucasians are more affected than other races. Lomholt in his landmark study at the Faroe Islands examined nearly two-thirds of the population of this island. In these secluded islands among 10 984 inhabitants he observed that 2.8% had psoriasis.3 It has been reported for psoriasis to occur at the rate of 6.5% in Germany, 5.5% in Ireland, 4.8% in Scotland, 3.7% in Spain, 2.3% in Sweden, 4.8% in Norway, 2.0% in the former USSR, 2.2–4.6% in the USA and 4.7% in Canada.1 A high prevalence of psoriasis at the rate of 11.8% has been reported from Kazach’ye, located in the Arctic region of the former Soviet Union.1 Evidence for the genetic contribution in the pathogenesis of psoriasis comes from the high concordance observed in twin studies carried out by various authors including the 61 twin pairs studied by Farber et al.4 Subsequently, a strong association of psoriasis susceptibility with certain HLA antigens has substantiated the genetic basis of this disease. A large number of studies mainly carried out in Europe, North America and Japan have demonstrated the association of psoriasis with various HLA loci such as Al, B13, B17, Cw6, DR7 and many others. Most of the studies agree that strongest association is with HLA Cw6. HLA B and DR alleles are in strong linkage disequilibrium with HLA CW6 and can form extended haplotypes in different populations. This suggests that HLA Cw6 primarily determines the susceptibility of psoriasis, and the multiple HLA associations may be the result of linkage disequilibrium. Among all the continents, Asia has the largest population with numerous races and subraces. So far epidemiology of psoriasis is considered the number of subjects studied are much less compared with the huge population in that part of the world. The prevalence of psoriasis in India ranges from 0.5% to 1.5%, 4–5.5% in Malaysia, 0.29–1.18% in Japan and 3.1% in Kuwait.1 Several studies carried out in China, including Hong Kong, showed that the prevalence of psoriasis in different regions varied between 0.2% and 1.5%.1 It is believed that the lower frequency of HLA-Cw6 in the Chinese population may be factor for the relatively lower occurrence of psoriasis in this race. In Central America the population is of mixed ethnicity comprising of Native Indians, Caucasians and African descendants. Prevalence of psoriasis in this region has been reported to be as common as other parts of the world. Failmezger examined inhabitants in various Central and South American regions of white, black and American Indian ancestry and found that in an overall study population of 3140 persons with skin diseases, psoriasis was seen in 0.7% in Guatemala, 0.7% in Honduras and 0.9–0.2% in different sites of Nicaragua.5 Prevalence of psoriasis reported in some other countries from South America is as follows: 1.3% in Brazil, 3% in Mexico, 2% in Venezuela and 4.2% in Paraguay.1 In the Caribbean islands one study observed 6% of its subjects had psoriasis. The Caribbean population is of mixed ethnicity consisting of African descendents, East Indians and Caucasians. Psoriasis prevalence rates reported for ethnically mixed populations of Egypt and South Africa are 3.0% and 4–5%, respectively. Modern Bantu-speaking Africans constitute a majority in most populations of eastern, central and southern Africa, where psoriasis incidence rates vary widely. In western Africa, however, where typical psoriasis incidence rates are lower than elsewhere in Africa, non-Bantu speaking peoples are most populous. The incidence of psoriasis in African Americans who are mostly genetically linked to West African Blacks is also lower. In the dry, rainless countries of eastern Africa such as Kenya, Uganda and Tanzania the prevalence of psoriasis was reported to be 3.5%, 2.8% and 3%, respectively.6 In contrast in the hot, humid and rainy climates of western Africa psoriasis has been reported to be 0.08–0.4% in Nigeria, 0.05% in Mali and 0.3% in Angola.6 HLA phenotypes do not provide a simple interpretation of the psoriasis prevalence in sub-Saharan Africa. Both HLA-Cw6 and HLA-B17 are reported to be of higher frequency compared with the Caucasians in several races of