NEW YORK – For children at higher-than-average risk of asthma, having a dog around the house may increase the chances of developing the lung disease, a new study suggests.

The study, which followed 380 children at increased risk of asthma due to family history, found that those exposed to relatively high levels of dog allergen at the age of 7 were more likely to have asthma.

In contrast, there was no relationship between cat-allergen exposure and a child's risk of asthma, according to findings published in the journal Pediatric Allergy and Immunology.

Exactly why dogs were related to a higher risk of asthma, while cats were not, is not entirely clear. But one factor may be endotoxin, a substance produced by bacteria that is known to trigger inflammation in the airways, explained lead researcher Dr. Chris Carlsten, of Vancouver General Hospital in British Columbia, Canada.

Carlsten and his colleagues found that children exposed to dog allergen at home were not at increased risk of developing an immune-system sensitization to dog allergen itself. Therefore, greater exposure to endotoxin may at least partly explain the association between having a dog in the home and a child's risk of asthma.

"Dogs tend to have a lot of endotoxin on them, because they're dogs," Carlsten told Reuters Health. In contrast, cats have much less, he said.

So should families with a history of asthma or allergies opt for a kitten over a puppy, or no fluffy pets at all?

"This study doesn't answer it," Carlsten said. "And in general, there is not enough evidence to recommend for or against pets."

He said that for now, his advice to parents is to base the decision on their family's desire to have a pet, rather than the potential effects on asthma risk.

The findings are based on 380 children who were at increased asthma risk because at least one first-degree relative (meaning a parent, sibling or child) had the lung disease or two or more first-degree relatives had other allergies, such as eczema or hay fever.

The children's mothers were recruited for the study during pregnancy, and researchers measured the levels of three allergens -- cat, dog and dust mite -- in the families' homes, periodically over the child's first year of life and again when they were 7 years old.

At the outset, roughly half of the families were randomly assigned to an intervention aimed at lowering the child's risk of developing allergies and asthma. That included encouraging mothers to breastfeed for at least four months, and having parents limit their children's exposure to dust mites, pets and tobacco smoke.

Carlsten's team found that exposure to higher levels of dog allergen -- at least 2 micrograms per gram of house dust -- at age 7 was associated with a nearly three-fold increase in the risk of asthma compared with lesser dog-allergen exposures. But that was only among children in the intervention group; 17 of 97 children exposed to higher levels of dog allergen at home had asthma at age 7.

Neither cat nor dust-mite exposure in infancy or at age 7 was related to the risk of asthma. Children with high dust-mite exposure were, however, more likely to show sensitization -- an immune system reaction during skin testing -- to dust-mite allergen.

According to Carlsten, the findings underscore the complexity of the relationship between indoor-allergen exposures and children's asthma risk. More research, he said, is still needed to understand those intricacies.

Because the current study included only children at elevated risk of asthma, it is not clear whether the findings might also apply to children at average risk.

SOURCE: Pediatric Allergy and Immunology, online March 19, 2010.