A meta-analysis indicates that early exposure to dogs, but not cats, decreases risk for childhood atopic dermatitis by 25%.
The hygiene hypothesis was proposed in 1989 as a possible explanation for the rapidly increasing prevalence of atopic dermatitis (AD) and other allergic and autoimmune diseases in developed nations. This hypothesis posits that reduced exposure to infectious agents in early life alters immune system maturation. Findings on pet exposure and risk for (AD) in children are inconsistent. Therefore, researchers conducted a systematic review and meta-analysis of pooled data from 21 birth cohort studies (26 publications) that examined exposure to dogs, cats, or other pets during infancy or childhood and AD during infancy and childhood.
The pooled relative risk of AD was significantly reduced for exposure versus no exposure to dogs (0.72, 15 studies) and exposure to pets overall (0.75, 11 studies), but not for exposure to cats (0.94, 13 studies). Studies were moderately heterogeneous. Mitigating factors included geographic heterogeneity (Europe, 14 studies; U.S., 3 studies; Oceania, 3 studies; and Japan, 1 study), undefined pet-keeping practices (degree of exposure), inability to adjust for pet-avoidance behavior of parents (e.g., because of a family history of allergy), presence or absence of smoking, and education and income.
The conflicting results regarding a link between pet exposure and childhood atopic diseases (allergic rhinitis, asthma, atopic dermatitis) probably stem from different methodologies in the individual studies and various meta-analyses. Furthermore, pet exposure appears to affect the risk for the three atopic diseases differently. This careful meta-analysis indicates that early exposure to dogs significantly reduces risk for childhood atopic dermatitis. Although the protective effect of dogs and pets overall is consistent with the hygiene hypothesis, the lack of a protective effect from cat exposure indicates that the association is complex.
by Mary Wu Chang, MD in Journal Watch, NEJM