SAN FRANCISCO, CA -- March 22, 2004 -- The so-called "hygiene hypothesis," that early exposure to pets may protect against the development of allergies, is only valid if the environment is free of tobacco smoke, according to findings presented here March

"This study shows that exposure to environmental tobacco affects the developing immune system and increases the risk of a child's developing atopy [and asthma]," said principal investigator Dennis R. Ownby, MD, Professor of Paediatrics and Internal Medicine, Medical College of Georgia, Augusta, Georgia, United States. "Future studies that test the protective value of pet exposure need to take parental smoking into account." He and his co-investigators were interested in the inter-relationship between passive cigarette exposure, early pet exposure, and children's subsequent development of atopy. To determine the relationship between exposure to cats and dogs during the children's first year of life and the risk of developing atopy by the age of 6 to 7 years, therefore, they analysed data from the Childhood Allergy Study, which involved 835 children from the suburbs of Detroit, Michigan, United States, who were recruited into the study at birth. The investigators stratified the data by parental smoking history, which had been obtained when the mothers were pregnant. The study design classified the children's exposure to cats and dogs during the first year of life. The study defined atopy as one or more positive skin tests to any of 6 common inhalant allergens: Dermatophagoides farinae, D. pteronyssinus, timothy grass, ragweed, cat and dog. Sero-atopy was defined as 1 or more positive test results for allergen-specific immunoglobulin E to the same allergens. Results show that among children of non-smoking parents, 14% of those who were exposed to 2 or more cats or dogs developed allergies, compared to 37.5% of those exposed to 1 pet and 36.8% of those with no pet exposure (P = .006). Among children of smoking parents, 18.5% of those exposed to 2 or more cats or dogs developed atopy, compared to 28.8% of those with 1 pet and 24.6% of those with no pets (P = .588). Dr. Ownby said that he and his co-investigators observed the same pattern when they examined sero-atopy and compared the significance of pet exposure for children of non-smoking parents (P =.01) to that for children of smoking parents (P = .271) "These findings show that exposure to parental cigarette smoke reduces the apparent protective effect of early exposure to cats and dogs against the subsequent development of atopy and sero-atopy," Dr. Ownby said. He noted that one of the study's limitations was that the children were primarily white and from middle-class backgrounds.

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