Swedish infants living with dogs or farm animals had decreased risk for asthma at ages 3 to 6 years.
The hygiene hypothesis posits that exposure to a greater variety of microorganisms during childhood may decrease the risk for chronic inflammatory diseases. However, previous studies of the associations between animal exposure and childhood asthma risk have been contradictory, with the exception of living in a farm environment, which seems protective. In a new study, researchers used Swedish registry data to assess whether exposure to dogs or farm animals in infancy was associated with childhood asthma.
In a cohort of roughly 276,000 children, 8.2% were exposed to dogs and 0.3% to farm animals (mostly cattle and sheep) during the first year of life. At age 6 years, 4.2% of the cohort had asthma. Adjusting for location, socioeconomic factors, and parental history of asthma, asthma at age 6 years was significantly inversely associated with exposure to dogs (odds ratio, 0.87) and farm animals (OR, 0.48).
In a separate cohort of roughly 377,000 children followed through the preschool years, exposure to dogs during infancy was associated with a significantly lower risk for developing asthma at age 3 to 5 years (adjusted hazard ratio, 0.90), but not at 1 or 2 years. Children exposed to farm animals during infancy had a significantly lower risk at all preschool ages (adjusted hazard ratio, 0.69). Dog exposure in the preschool cohort was associated with increased risk for pneumonia (hazard ratio, 1.13).
This large prospective study confirms a notable reduction in asthma risk among children who grow up in farm environments. Dogs also appear to be somewhat protective, whether or not there is a family history of asthma. These results are likely generalizable to other European and North American societies. When parents of infants ask about the pros and cons of having a dog, we can talk about a potential mild decrease in asthma risk.
Cornelius W. Van Niel, MD reviewing Fall T et al. JAMA Pediatr 2015 Nov 2.