Ronmark E, Perzanowski M, Platts-Mills TAE, Lundback B. J Allergy Clin Immunol. 2003;112:747–754
Purpose of the Study.
To evaluate the incidence of type 1 sensitization during a 4-year period among schoolchildren in northern Sweden (a dust mite- and cockroach-free environment) and to examine the risk factors for sensitization, including cat and dog ownership.
A total of 1870 schoolchildren, 7 to 8 years of age at the beginning of the study, were derived from a longitudinal cohort of 2454 children living in 2 municipalities in northern Sweden in 1996.
Skin prick tests for tree, grass, weed, dust mite, mold, cat, dog, and horse allergens were conducted. The same children were retested 4 years later. A positive reaction was recorded if the wheal was ≥3 mm after 15 minutes. The participation rate was 88% during both collection periods. CAP system immunoglobulin E antibodies to cat, dog, tree, and grass allergens were determined in sera collected from 923 children during the second data collection period. A parental questionnaire was used to screen for risk factors at the onset of the study and again annually, to determine the status of dog and cat ownership from year to year.
The prevalence of sensitization to any allergen increased from 20.6% at 7 and 8 years of age to 30.4% at 11 and 12 years of age. In both age periods, the most common allergen was cat allergen, followed by dog, birch, and timothy grass allergens. The cumulative incidence during the 4-year period was 13.8%. The strongest risk factor for sensitization at 7 to 8 years, as well as development of sensitization during the 4-year period, was a family history of allergy. A significant inverse association between cat and dog ownership and the prevalence of type 1 allergy was found, especially for children living with cats and/or dogs before 7 and 8 years of age, and was most pronounced among children who persistently had lived in a house with a cat.
The authors concluded that, despite the presence of a cockroach- and dust mite-free environment, the incidence of allergic sensitization between the ages of 7 to 8 years and 11 to 12 years was high in northern Sweden. The major risk factor for incident and prevalent cases was a family history of allergy. Although cat allergen was the most common allergen of sensitization, keeping a cat or dog at home was not related to an increased risk of sensitization to these allergens. Therefore, avoiding cats and dogs at home is not protective against sensitization.
It should be noted that steps were taken in the analysis of the data to avoid risk for bias of pet ownership selection, in terms of primary prevention (avoidance of pets in a family with a known history of allergy) as well as secondary prevention (removal of pets after allergy diagnosis). Furthermore, although only sensitization was addressed in this article, additional data concerning clinical outcomes within the same longitudinal cohort were published and showed a lower prevalence of asthma among children with cats in the home.