Early Animal Exposure Decreases Childhood Asthma Risk

Jennifer Garcia

November 03, 2015

Exposure to dogs and farm animals during the first year of life decreased the risk for asthma in children at 6 years of age, according to a registry-based cohort study published in the November 2 issue of JAMA Pediatrics.

These new data support the “hygiene hypothesis” that early exposure to animals decreases the risk for childhood asthma and "might be helpful in decision making for families and physicians on the appropriateness and timing of early animal exposure," write Tove Fall, PhD, from Uppsala University in Sweden, and colleagues. The incidence of childhood asthma has been increasing around the world.

Researchers reviewed registry data from all children born in Sweden between January 1, 2001, and December 31, 2010 — more than 1 million overall. The researchers evaluated information regarding presumptive dog and farm animal exposure, asthma diagnosis and medication, and parental asthma history. The researchers considered exposure to have occurred if, during the child's entire first year, the parents either were registered dog owners or were registered as "animal producers and related workers."

In their analysis, which included 376,638 preschool-age children (53,460 exposed to dogs and 1729 exposed to farm animals), the researchers found that 18,799 (5.0%) of the children in this cohort experienced an asthmatic event before age 6 years. The authors identified a statistically significant risk reduction for preschool children aged 3 years or older who were exposed to dogs early in life (hazard ratio [HR], 0.90; 95% confidence interval [CI], 0.83 - 0.99). Early exposure to farm animals also appeared to reduce the risk of developing asthma (HR, 0.69; 95% CI, 0.56 - 0.84), and parental asthma history did not modify the effect. The results were similar when the researchers restricted the sensitivity analysis to firstborn children to avoid bias because of older siblings with asthma.

The study authors identified 276,298 school-aged children, 11,585 (4.2%) of whom had experienced an asthma event during their seventh year of life. Within this cohort, 22,629 children were exposed to dogs and 958 were exposed to farm animals. The researchers found that children exposed to dogs during their first year of life had a decreased risk of developing asthma (odds ratio [OR], 0.92; 95% CI, 0.86 - 0.99), and that this inverse association was irrespective of parental asthma history. Early exposure to farm animals showed similar results (OR, 0.47; 95% CI, 0.30 - 0.74) and remained unchanged when the researchers adjusted for confounders including parent age, parity, birth municipality, number of parents with asthma, dog exposure, farm animal exposure, parents' income, parents' educational level, and parents' birth country.

The authors did find, however, an increased risk for pneumonia (HR, 1.13; 95% CI, 1.06 - 1.20), as well as other lower respiratory disease (HR, 1.06; 95% CI, 1.00 - 1.13), among preschool-aged children exposed to dogs, but not among those exposed to farm animals. The researchers hypothesize that exposure to animals may increase a child's overall exposure to microorganisms, some of which may increase the risk for respiratory tract infections, and others that may modulate the immune system to reduce the risk for asthma.

The authors posit that differences in study design, study populations, and age at outcome measurements may explain why previous studies examining the association between dog exposure and childhood asthma may have shown contradictory results. In addition to drawing from a larger population size in the present study, the authors also attempted to control for factors such as avoidance bias. "In an attempt to account for the possibility that families with asthma in older siblings avoid dogs, we also reanalyzed the data including only first-born children and noted comparable results."

The authors note that although their findings are "robust," there are limitations in the study design, such as inability to account for exposure to animals outside the home or incomplete information regarding parental asthma history.

Funding for this study was provided through grants from the Swedish Research Council, the Stockholm County Council, the Strategic Research Program in Epidemiology at Karolinska Institutet, the Swedish Heart and Lung Foundation, Bror Hjerpstedts stiftelse, and Tore Nilssons stiftelse. One coauthor reported being a shareholder of BioCistronix AB. The authors have disclosed no other relevant financial relationships.

JAMA Pediatrics. 2015;169:e153219. Abstract

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