Atopic dermatitis and the hygiene hypothesis: A case control study

Description/Abstract

The notion that a reduced exposure to microbial pathogens in early
life increases the chances of the expression of atopic disease has come
to be known as the hygiene hypothesis1. It has arisen from
observations of the rapidly rising prevalence of atopic diseases in
recent decades, the lower prevalence of atopy in lower socio-economic
groups and the lower prevalence of atopy with rising birth order.
Direct evidence for the hypothesis to date has been inconsistent.
This case-control study involving 602 children aged one to four
years in Norfolk, UK was designed to test the hypothesis with respect
to atopic dermatitis (AD). Cases and matched controls were selected
randomly from primary care databases and carefully defined using the
UK Diagnostic Criteria for AD. Exposure to infection during infancy
was measured with a range of direct and indirect methods including
assays of salivary antibodies to Epstein Barr virus and Varicella Zoster
virus, data extracted from primary care records and a questionnaire
administered to parents. Odds ratios (OR) for the effect of these
measures on AD were calculated using logistic regression with
adjustment for possible biological and social confounding factors.
Reduced odds of AD were associated with rising birth order (OR for
one older sibling 0Æ59, 95% CI 0Æ42 to 0Æ84 and for two or more older
siblings 0Æ49, 95% CI 0Æ31 to 0Æ77). None of the measures of exposure
to infection reduced the odds of AD, either in the unadjusted or
adjusted analyses. Some measures showed a weak association
between exposure and greater odds of AD, in the opposite direction
to that predicted by the hygiene hypothesis.
These data confirm a significantly reduced risk of AD in second and
subsequent siblings but this sibling effect of reduced odds with rising
birth order does not appear to be explained by exposure to infection in
early life. More generally, these data cast doubt on the hygiene
hypothesis as a causal explanation for AD in young children.