Background The prevalence of atopic dermatitis (AD) is increasing in Western societies. The hygiene hypothesis proposes that this is due to reduced exposure to environmental allergens and infections during early life.
Objectives To examine factors associated with a diagnosis of AD at 3·5 years of age, especially those factors implicated by the hygiene hypothesis.
Methods The Auckland Birthweight Collaborative study is a case–control study of risk factors for small for gestational age babies. Cases were born at term with birthweight ≤ 10th centile; controls were appropriate for gestational age, with birthweight > 10th centile. The infants were assessed at birth, 1 year and 3·5 years of age. Data were collected by parental interview and examination of the child. AD was defined as the presence of an itchy rash in the past 12 months with three or more of the following: history of flexural involvement; history of generally dry skin; history of atopic disease in parents or siblings; and visible flexural dermatitis as per photographic protocol. Statistical analyses took into account the disproportionate sampling of the study population.
Results Analysis was restricted to European subjects. Eight hundred and seventy-one children were enrolled at birth, 744 (85·4%) participated at 1 year, and 550 (63·2%) at 3·5 years. AD was diagnosed in 87 (15·8%) children seen at 3·5 years. The prevalence of AD did not differ by birthweight. AD at 3·5 years was associated with raised serum IgE > 200 kU L−1, and wheezing, asthma, rash or eczema at 1 year. In multivariate analysis, adjusted for parental atopy and breastfeeding, AD at 3·5 years was associated with atopic disease in the parents: maternal atopy only, adjusted odds ratio (OR) 3·83, 95% confidence interval (CI) 1·20–12·23; paternal atopy only, adjusted OR 3·59, 95% CI 1·09–11·75; both parents atopic, adjusted OR 6·12, 95% CI 2·02–18·50. There was a higher risk of AD with longer duration of breastfeeding: < 6 months, adjusted OR 6·13, 95% CI 1·45–25·86; ≥ 6 months, adjusted OR 9·70, 95% CI 2·47–38·15 compared with never breastfed. These findings remained significant after adjusting for environmental factors and a personal history of atopy. AD at 3·5 years was associated with owning a cat at 3·5 years (adjusted OR 0·45, 95% CI 0·21–0·97) but not with owning a dog at 3·5 years, pets at 1 year, nor with older siblings. Furthermore, AD at 3·5 years was not associated with gender, socioeconomic status, maternal smoking, parity, damp, mould, immunizations, body mass index or antibiotic use in first year of life.
Conclusions A personal and a parental history of atopic disease are risk factors for AD at 3·5 years. Duration of breastfeeding was associated with an increased risk of AD. No association was found with those factors implicated by the hygiene hypothesis. This study suggests that breastfeeding should not be recommended for the prevention of AD.