Understanding

Prevalence

Atopic dermatitis is a highly prevalent disease, affecting 15–30% of children and 2–10% of adults, in developed countries.⁴ Incidences have trebled over the past 30 years in industrialised societies which an estimated 20% of children in northern and western Europe, Australia and the United States being diagnosed. In contrast incidence rates remain much lower in countries with predominantly rural or agricultural areas.⁵ The graph below (figure 2) demonstrates the prevalence of atopic dermatitis in Europe and shows the percentages of children aged 6–7 years and 13–14 years diagnosed with the disease.⁶

Figure 2. Prevalence estimates of atopic dermatitis in European countries (1999)⁶

It is estimated that 60% of atopic dermatitis patients develop symptoms in the first year of life and 85% develop symptoms before the age of 5.⁴ It is less common for atopic dermatitis to first appear in adulthood but there is a tendency for the symptoms to be more severe in such patients. Once the condition has manifested itself, flares of varying severity and duration can occur at any age.

Several theories attempting to explain the dramatic increase in atopic dermatitis prevalence have been proposed. In 1989, Strachan suggested that allergic diseases were less common in children from larger families, due to an increased exposure to infectious agents from siblings, than in children from families with only one child.⁷ Increases in the use of antibiotics, antimicrobial cleaning agents and general improvements in living conditions have also been implicated as key factors responsible for the rise in the number of atopic dermatitis cases in developed nations. Reducing childhood exposure to pathogens was considered the principle reason for the increasing prevalence of atopic dermatitis. The underlying biological interpretation of this theory, known as the ‘hygiene hypothesis’, states that reduced exposure to microbial antigens affects the development of the immune response during early growth.^8,9

Another mechanism thought to account for the growing prevalence of atopic dermatitis is an increase in gene-environment interactions. Individuals with specific variations in susceptibility genes are at higher risk of developing severe allergies and/or atopy when specific environmental factors are present.¹⁰ A more detailed description of the genetic factors and immune dysfunctions associated with atopic dermatitis predisposition are provided under the pathophysiology heading in this section.