A study at Vancouver General Hospital in British Columbia, Canada followed 380 children who were at increased asthma risk because at least one first-degree relative asthma or two or more first-degree relatives had other allergies, such as eczema or hay fever. At the outset, roughly half of the families were encouraged to breast feed for at least four months, and limit their children's exposure to dust mites, pets and tobacco smoke, all interventions which can limit asthma.

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

The researchers 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. 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 although children with high dust-mite exposure were more likely to be sensitised.

Why dogs were related to a higher risk of asthma, while cats were not, is not entirely clear but the researchers believe that endotoxin, a substance produced by bacteria that is known to trigger inflammation in the airways and which is at far higher levels in dogs than in cats, may be relevant. But the children exposed to dog allergen were not at increased risk of developing an sensitisation 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.

However the researchers did not feel that the findings were clear enough to advise asthma-prone families against having a dog if they really wanted one.

Pediatric Allergy and Immunology, online March 19, 2010.

Courtesy of Reuters

First Published in March 2010

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