Large families and animals keep allergies at bay

Cohort study of sibling effect, infectious diseases, and risk of atopic dermatitis during first 18 months of life BMJ Online First

Having siblings, keeping a pet, or living on a farm helps protect infants against the development of atopic (allergic) diseases, but early infections increase the risk, according to new research from Denmark. This study will be available on bmj.com on Friday 30 May 2004.

Researchers conducted four separate interviews with over 24,000 pregnant women (twice during their pregnancy and again when their child was 6 and 18 months old).

Information on atopic dermatitis and infections before 6 months of age were recorded. Data on factors associated with exposure to microbes, such as number of siblings, pet keeping, living on a farm, and attending day care, were also collected.

The risk of atopic dermatitis increased with each infectious disease before 6 months of age. This contrasts with the common belief that infectious diseases early in life may protect against the development of allergic diseases.

However, the risk of atopic dermatitis decreased with each additional exposure to three or more siblings, day care, pet ownership, and farm residence. This protective effect remained after adjusting for number of infectious diseases, suggesting that it is established independently and very early in life, say the authors.

These findings support the importance of microbial exposure for preventing atopic dermatitis, but challenge the hypothesis that infectious diseases in infancy protect against its development, they conclude.