People exposed to pets during their first year of life were less likely to develop allergies in adolescence and adulthood, new data indicate.

Children who were raised in homes with cats during their first year were about half as likely to develop allergies specific to that animal in adulthood than those who were not (relative risk=0.52; 95% CI: 0.31-0.90), U.S. researchers found. The same was true for boys who grew up with an indoor dog during this same first-year time frame (RR=0.50; 95% CI: 0.27-0.92).

Both boys and girls that were delivered by cesarean section and exposed to dogs during the first year of life also had a significantly reduced risk for later sensitization to dogs (RR=0.33; 95% CI: 0.07 to 0.97) compared with children born vaginally.

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“This research provides further evidence that experiences in the first year of life are associated with health status later in life, and that early life pet exposure does not put most children at risk of being sensitized to these animals later in life,” study researcher Ganesa Wegienka, PhD, of Henry Ford Hospital in Detroit, Mich., said in a press release.

Results from the analysis, which included data from an 18-year follow-up study that involved 566 participants enrolled in the Detroit Childhood Allergy Study between 1987 and 1989, were published this week in Clinical & Experimental Allergy.

The researchers collected blood samples and performed interviews to determine pet history. They asked specifically about lifetime indoor cat and dog exposure over the participant’s first year, by age group and cumulative lifetime. The researchers also inquired about family allergy history, birth delivery method and demographic information.

Blood tests revealed participant sensitization to dogs or cats using animal-specific immunoglobulin E (IgE) ≥0.35 kU/L. In addition to the first year of life, the researchers examined animal-exposure at ages 1 to 5 years, 6 to 12 years and ≥ 13 years. They did not find an association between cumulative exposure to either animal and overall allergy development in any age group.

Although the researchers acknowledged that, “the mechanisms that mediate the role of pet exposure in allergy development are still unexplained,” the finding that children born by cesarean section who were exposed to pets during the first year developed fewer allergies than those born vaginally, suggests that the two groups may develop immunity differently.

“Perhaps children born via c-section acquire a higher proportion of their microbes from household (dust) exposures compared with children born vaginally, because vaginally born children have already been exposed to a broad array of microbes during birth,” they wrote.

Study limitations included incomplete follow-up data for all participants in the original study, maternal reports of children’s allergic history, lack of a lifetime measurement of the route by which participants were actually exposed to dog and cat allergens, and exposure recall from 6 to 18 years.

The researchers called for more study on childhood pet immunity development during smaller time frames within the first year of life.

Wegienka G. Clin Exp Allergy. 2011; DOI: 10.1111/j.1365-2222.2011.03747.x.