The prevalence of psoriasis in the world

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 sub-Saharan blacks. The frequency of psoriasis in white Australians resembles that of the Western world (approximately 2.6%), whereas no psoriasis has been found in Australian Aborigines.7 The Aborigines have been in Australia for at least 30 000 years, originating in South-east Asia. In HLA typing of 177 members, of the Walbiri tribe of the desert area of the North-west Territory, HLA B 17 was found to be absent. Similar to the Aborigines, psoriasis has been reported to be extremely rare or absent in the pre-Colombian population of the New World. A comprehensive dermatological survey of more than 25 000 Andean Indians revealed not one case of psoriasis.8 An absence of psoriasis has also been observed in the Amerindians in the remote villages of the Amazon–Orinoco forest. Similar findings have been reported in Alaskan, Canadian and the Native Americans of the United States. Recent HLA determinations in many of these groups showed a lack or very low frequency of HLA B13/B17. The state of one’s health depends on an integrated relationship between hereditary factors, mind–body synchrony and environmental influences. Psoriasis is a multifactorial disease. Various exogenous and endogenous factors such as upper respiratory infection, psychological stress, humidity and cold weather are known to influence the clinical course of psoriasis. In this respect we want to share our observation of a pair of identical twins whom we followed for a long time. These twin brothers had psoriasis of equal severity from age 12 to 24. Following graduation, one brother worked as an engineer on a ship and led a relaxed quiet life. His psoriasis went into remission. The other brother was employed as a labour organizer and had a highly stressful life. He developed generalized psoriasis in the next few years. Higher rates of incidence for psoriasis are constantly observed in eastern Africa than in western Africa. As Cw6 distribution is not different in the eastern and western Africa black population it is possible either the non-Bantu speaking population of western Africa lack specific susceptibility genes or share genetic factors that promote resistance to psoriasis. Alternatively, high humid conditions of Nigeria and adjacent areas compared with the dry climates of Kenya and Uganda may contribute to the low frequency of psoriasis on the west cost of Africa. Low frequency of diabetes, coronary artery disease and psoriasis has been observed in the circumpolar population. The different groups that ranged from Alaska through Canada and Greenland to Siberia depend on land and marine animals for their food, which include seal, walrus, whale, bear, caribou, musk ox, fish and birds. The traditional diet in this population is thus high in protein and fat and low in carbohydrate, ascorbic acid and tocopherols. The dietary intake of the Eskimos has been studied over the last couple of decades by nutritionists to establish a base for investigating the disease patterns in this race. The fat of the Arctic marine animals is the most unsaturated found in the animal kingdom, being particularly rich in essential fatty acids. It is believed in Eskimos that a high content of polyunsaturated fats in the diet and low plasma levels of arachidonic acid can contribute to the reduced prevalence of myocardial infarction and inflammatory diseases such as psoriasis. Because of these observations fish oil as a supplementary therapy for psoriasis has been suggested.