Markers of microbial exposure lower the incidence of atopic dermatitis

Background: The hygiene hypothesis proposes that reduced exposure to infectious agents in early life would explain the increase of allergic and autoimmune diseases observed over the past decades in high-income countries. Methods: We conducted a matched case-control study on incident atopic dermatitis (AD). Cases were 426 outpatient children with a first diagnosis of incident AD. Controls were 426 children attending a pediatric/dermatological visit for nonatopic disorders matched to cases (1:1). Particular attention was paid to the time elapsed between the markers of microbial exposure and disease onset, and we considered for controls the same time window of exposures from birth as his/her matched case. Odds ratios (ORs) were computed using multivariable conditional logistic regression models, according to center, sex, age, and period of enrollment, and including as potential confounders a family history of any allergy in parents, type of delivery, having siblings, keeping pets, age at weaning, and having had ≥4 infections. Results: The OR of AD first occurrence was 0.35 (P-value =.039) for children who had experienced ≥4 infections compared with those with no infections. A decreasing trend in risk was observed with increasing number of siblings (P-value =.023), the protective effect reaching about 40% for children with 2 or more siblings (OR = 0.62; P-value =.048). Pet keeping, in particular daily contact with dogs, was inversely associated with AD risk (OR = 0.40; P-value =.004). Conclusions: These results support the hygiene hypothesis in its broad sense. Early-life environmental exposures, including pathogens and commensals, act as “microbes contact carriers” influencing immune system balance early in life.