DETROIT—Conventional wisdom suggests that exposing children to allergens promotes the development of atopy and allergic asthma. While this might apply to dust mites, the opposite now appears to be true for our better-liked household companions: Exposure to cats or dogs in the first year of life may protect against allergies and asthma in at least some children.[1-3] Further, contact with pets may guard against sensitization to other airborne allergens as well.[1]

“The presence of multiple animals in the home in the first year of life tends to dramatically reduce the risk that a child becomes allergically sensitive,” said Dennis R. Ownby, MD, lead author of a prospective study of 474 children.[1] “That’s totally contrary to what we’ve been taught for 20 or 30 years,” he emphasized.

Cats or dogs provided benefit even for outdoor allergens. “They were protective against allergies to things such as ragweed, timothy grass, and Alternaria, as well as to pets,” noted Christine Cole Johnson, PhD, Senior Research Epidemiologist with Henry Ford Health System in Detroit and one of the study’s coauthors. And the more pets, the better: “It really took two to make the drop in allergy and atopy [rates],” she added.

After controlling for several factors (eg, parental history, household allergens, sex, birth order), Drs. Ownby and Johnson and colleague Edward L. Peterson, PhD, found that, compared with petless study participants, children exposed to two or more pets in their first year of life were 77% less likely to have positive skin tests to any allergen at age 6 or 7; they were also 67% less likely to have allergen-specific immunoglobulin E (IgE).[1] Similarly, a survey of 3,431 first- and second-grade Swedish children demonstrated that living with a cat halved the rate of physician-diagnosed asthma and reduced the risk of a positive skin test to cat allergens.[2] The three-year cumulative incidence of physician-diagnosed asthma among cat owners was one quarter that among cat-deprived children of similar ages.

The findings raise practical concerns in addition to theoretical questions. For families who are expecting babies, “have pets, and are worried about whether there’s a risk of keeping the pets in the house, … our data would say there’s not. In fact, if anything, there may be a slight benefit,” argued Dr. Ownby, Chief of the Section of Allergy and Immunology at the Medical College of Georgia in Augusta. Thus, the new results challenge traditional thinking about how to limit atopy in children.

WHAT ABOUT FAMILY HISTORY?

“[H]aving a cat in the home during a child’s early life reduces the risk of developing asthma and allergies later in childhood in most instances,” concurred Juan C. Celedón, MD, DPH, first author of a study that followed 448 children with at least one atopic parent during their first five years of life.[3] There may be exceptions, though. According to Dr. Celedón, an instructor in medicine at Harvard Medical School in Boston, “Exposure to a cat in early life is not advisable … for children whose mothers have asthma or are known to be allergic to a cat.” He and colleagues demonstrated that among children of asthmatic mothers, early cat exposure did not significantly alter IgE titers at age 2 and increased the risk of wheezing at age 3.[3] Dr. Celedón explained, “Since we had sufficient numbers of asthmatic parents (due to our study design), we were able to look at the relationship between exposure to a cat stratified by either paternal or maternal history of asthma,” thereby permitting an observation others missed.

However, the survey of Swedish schoolchildren showed a somewhat different result: Only those children having a parent or sibling with asthma history were significantly protected against asthma and wheezing by ever owning a cat. Furthermore, “in comparison to Dr. Celedón’s findings, ... we found the opposite,” noted Matthew S. Perzanowski, MPH, of the University of Virginia Health Sciences Center in Charlottesville and first author of that study. “In Sweden, the protective effect of having a cat at home against developing asthma by age 7 was actually stronger for those children with a maternal risk of asthma compared to those with an asthmatic father.” Similarly, when Dr. Ownby and colleagues considered family history, pet exposure significantly benefited only children with an asthmatic parent.[1]

Thus, whether or when pet ownership is helpful or harmful to children of allergic parents remains unclear. Furthermore, measures to protect children from developing allergy and asthma may conflict with the needs of allergic family members to avoid pet exposure.

BIGGER BENEFIT FOR BOYS

When all children were considered, pet exposure generally reduced prevalence of a positive skin test to any allergen, to outdoor and indoor allergens, and to dog allergens, as well as prevalence of allergen-specific IgE.[1] But, said Dr. Ownby, “We know that asthma is very different by gender: Boys are more apt to have asthma—2:1—prior to puberty.” Additionally, he said, “The immune systems of boys and girls are indeed developing differently: By 2 years of age we saw significant differences in total serum IgE between the boys and the girls in this cohort, with the boys’ [levels] being higher.”

And taken separately, girls and boys did not benefit equally from multiple pet exposure: Total IgE decreased significantly in boys (from 43.8 to 23.1 IU/mL) but not in girls. Furthermore, said Dr. Ownby, a clear and consistent effect on lung function was seen in boys but not in girls: Prevalence of methacholine airway responsiveness among boys significantly dropped with exposure to two or more pets (5.1% vs 25.5% with no pets); however, no comparable effect was found among girls. Similarly, “for the boys, lung function actually improved, … whereas it didn’t change in the girls,” he said; forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) increased with pet exposure in boys but not girls.

The difference in sex might easily have been missed by other studies, said Dr. Ownby. “Many people have adjusted in multivariate analyses for gender, but often they haven’t stratified by gender in their analyses,” he pointed out.

THE DIRT ON DOGS

“The most likely reason for our results is that children are born with a bias toward an allergic-type immune response and that early exposure to endotoxins decreases that bias,” posited Dr. Ownby. And, he noted, “Homes with pets in them have higher levels of endotoxin.” Thus, exposure to pets and associated endotoxins might reduce a child’s propensity for allergic sensitization.

Thomas A. E. Platts-Mills, MD, PhD, a coauthor of the Swedish study[2] and author of an editorial[4] published with Dr. Ownby’s paper, offered an endotoxin-free alternative to explain the reduction in sensitization. In his study, “many of the children with negative skin prick test responses had serum IgG antibodies to the cat allergen Fel d 1,” particularly IgG4, he noted. Dr. Celedón explained, “Some individuals exposed to a cat develop an IgG response; they do not have an allergic [IgE] response to a cat. Others develop an IgE response to a cat and are more likely to develop symptoms when exposed to a cat.” A response without IgE production might amount to a “modified TH2 response,” which represents tolerization rather than sensitization, Dr. Platts-Mills suggested.

Such an argument might explain the reduced prevalence of cat allergies but falls short of explaining the decreased atopy to other indoor and outdoor allergens among pet owners, Dr. Ownby maintained. “Since our finding was across all of the allergens, I think that the endotoxin explanation makes more sense than saying that you can push immune tolerance and get it to apply to multiple allergens,” he reasoned. “Most of the time, high-dose immune tolerance is immune specific—it doesn’t generalize.”

—Mimi Zucker, PhD