Is the prevalence of psoriasis increasing? A 30-year followup of a population-based cohort

Background There is indication of an increasing prevalence of psoriasis in some western populations. However, the results are not conclusive. Objectives To analyse trends in the prevalence of psoriasis over the past 30 years, separating age, birth cohort and time period effects. Methods Five population-based surveys in North Norway, the Tromso Studies 2-6, collected between 1979 and 2008, were studied. Participants aged 20-79 years with self-reported psoriasis data in at least one of the surveys were included, yielding a total of 69 539 observations from 33 387 unique individuals born between 1915 and 1977. Trends in psoriasis prevalence were examined using cross-sectional, time lag and longitudinal designs of graphical plots. Observed trends were further evaluated in generalized linear-regression models. Results The self-reported lifetime prevalence of psoriasis increased from 4.8% in 1979-1980 to 11.4% in 2007-2008. Graphical plots showed an increasing prevalence of psoriasis with each consecutive survey in all examined age groups and birth cohorts, leaving time period effects as the explanation for the increase. The odds for psoriasis in the cohort were 2.5 times higher in 2007-2008 than in 1979-1980 (adjusted odds ratio 2.49, 95% confidence interval 2.08-2.99). The prevalence of persons reporting a doctor's diagnosis of psoriasis was 9.9% in the last survey. In subgroups of the study population, psoriasis was associated with higher body mass index, lower physical activity during work and leisure time, lower educational level and smoking. Conclusions Our findings indicate an increasing prevalence of self-reported psoriasis. This could represent a true increase in prevalence, possibly due to changes in lifestyle and environmental factors, or an increased awareness of the disease.