Young children with a family history of allergies may be less likely to develop the allergic skin condition eczema if they live with a dog starting in infancy, a new study suggests.
On the other hand, researchers found, living with a cat may increase those odds -- but only among children who have a specific sensitivity to cat allergen.
The findings, published in the Journal of Pediatrics, do not prove that puppies are protective and kitties are bad for allergy-prone children. Instead, they add to what appears to be a complex, and often confusing, relationship between the family pet and kids' allergy risks.
The issue remains a bit of a gray area, and it is too early to give parents specific advice on whether they should or should not have a dog or cat in the house, said Dr. Tolly G. Epstein, an assistant professor at Ohio's University of Cincinnati College of Medicine and one of the researchers on the new study.
However, she told Reuters Health, a number of studies have now found that when it comes to eczema, young children who live with dogs may be at lower risk than those who do not.
The current study included 636 children who were enrolled as infants in a long-term study of environmental exposures and allergy risk. All were considered to be at increased risk of allergies because they had a parent with a history of asthma, nasal allergies or eczema.
When the children were younger than 1 year, researchers visited their homes and collected dust samples. The children also underwent yearly exams, including a skin-prick test to see whether they were sensitized to any number of allergens; sensitization means that the immune system, after being exposed to an allergen such as pet dander or mold, has produced antibodies against that allergen.
Overall, Epstein's team found, 14 percent of the children had eczema at the age of 4. But that rate was lower -- 9 percent -- among the 184 children who'd had a dog in the home during infancy.
What's more, among children who had a sensitivity to dog allergen, having a family dog was linked to a substantially lower eczema risk: of 14 children who met both those conditions, two (14 percent) developed eczema. That compared with 17 of 30 children (57 percent) who were sensitized to dogs but had no dog at home early in life.
When it came to cats, the issue became more complex. There was no clear overall relationship between having a cat in the house during infancy and subsequent eczema risk. But when Epstein's team looked only at children sensitized to cats, there was a connection.
Among 13 children who were sensitized to cats and had lived with one during infancy, 54 percent developed eczema by age 4. That rate was 33 percent among sensitized children who had not lived with a cat, and 11 percent among children with no cat sensitivity who had lived with a cat before the age of 1.
There is no solid explanation for the seemingly protective effects of dogs, according to Epstein. But she speculated that early exposure to dog allergen (substances in pet dander, saliva and urine) affects children's immune system development in such a way that eczema becomes less likely to develop.
"It may be that these children develop a tolerance, but we don't know that for sure," Epstein said.
Cat allergen, she noted, may, in theory, have distinct effects on immune system development.
The findings would seem to stand in contrast to those of another recent study that found that among children at increased allergy risk, those with a dog in the house at the age of 7 were more likely to have asthma than children in dog-free homes. In contrast, having a cat in the house was unrelated to asthma risk.
However, the lead researcher on that work said that there are a number of differences between that study and the current one.
For one, they looked at two different allergic conditions, eczema and asthma, Dr. Chris Carlsten, of Vancouver General Hospital in British Columbia, Canada, told Reuters Health in an e-mail.
They also differed in the length of time they followed the children, and in other specifics of their study methods, he noted. "These are complex studies with multiple views on exposure and allergy-related conditions," Carlsten said, "and I am not convinced that they reach fundamentally contradictory conclusions overall."
It is not clear what biological factors might also underlie the differences in the studies' results, according to Carlsten.
But he noted that his study was consistent with the idea that dogs might contribute to asthma in susceptible kids by exposing them to higher levels of endotoxin -- a substance produced by bacteria that is known to trigger inflammation in the airways.
Dogs carry more endotoxin than cats do, by virtue of being dogs.
Epstein agreed that the differences in the studies' methodology, and the fact that they looked at two different allergic conditions, mean that the findings are not necessarily conflicting. She also pointed out that in her team's study, household endotoxin levels -- measured from the collected dust samples -- did not appear to be involved in the pet-eczema relationships they found.
So it is possible that pet allergens and pet-related endotoxin exposure each have a distinct effect on children's allergy and asthma development.
Given the complexity of the whole matter, Epstein said that for now, it is hard to give parents specific advice on pets. But they can be aware that, as far as eczema, there is a consistent relationship between dog ownership and lower risk.
Epstein also pointed out that the findings apply only to children at increased risk of allergies due to parents' history. Little is known about how the family pet might affect allergy and asthma development in children at average risk of the conditions.