Four-year Incidence of Allergic Sensitization Among Schoolchildren in a Community Where Allergy to Cat and Dog Dominates Sensitization: Report from the Obstructive Lung Disease in Northern Sweden Study Group

Background: Exposure to high levels of cat allergen might prevent sensitization.

Objective: We sought to measure the incidence of allergic sensitization among schoolchildren living in a dust mite- and cockroach-free environment and the associated risk factors.

Methods: In 1996, a longitudinal cohort was established in northern Sweden, including 2454 children aged 7 to 8 years. Children were skin tested, and the testing was repeated 4 years later. Questionnaires were completed yearly. Participation was 88% both in 1996 and 2000.

Results: The prevalence of positive skin test results increased from 20.6% at age 7 and 8 years to 30.4% at age 11 and 12 years, a cumulative incidence of 13.8%, and was significantly higher among boys. The incidence was highest for cat (6.0%), timothy grass (5.9%), dog (4.9%), and birch (3.6%). A family history of allergy was the major risk factor for both a positive skin test response at age 7 and 8 years (odds ratio [OR], 1.69; 95% CI, 1.36-2.10) and for development of a positive skin test response over the next 4 years (OR, 1.67; 95% CI, 1.23-2.28). A significant inverse association between cat and dog ownership and the prevalence of type 1 allergy was found, particularly for those children who had lived with a cat both before age 7 and 8 years and during the next 4 years (OR, 0.44; 95% CI, 0.31-0.61). A similar pattern, although not significant, was found for incident cases.

Conclusion: The high incidence of type 1 allergy at this age was similar to reports from communities with mite and cockroach allergen. Despite cat and dog being the most common allergens of sensitization, keeping these animals at home was not associated with an increased risk for sensitization.