Increase of allergic diseases in schoolchildren. The significance of pets and other environmental components

Abstract: Objectives: 1) To compare the prevalence of allergic diseases in schoolchildren between 1979 and 1991 in two Swedish regions with differing climates, Kiruna in the northern inland mountains, north of the Arctic Circle, and Göteborg on the south-west coast. 2) To study the changes in allergic diseases and sensitisation during a 5 year follow-up period. 3) To evaluate the influence of parental history, residential characteristics, pet exposure during first year of life and other background factors on allergy development.Methods: Two cross-sectional questionnaire based studies, one in 1979 including 4255 7-year-olds and one in 1991 including 2481 7-9 year old children. The study in 1991 was followed by a validating interview and skin-prick tests in a stratified and population based sub-sample of 412 children, 7-8 year old. The sub-sample was followed-up after five years (1996) with a new questionnaire, interview and skin prick tests. The same questions about disease were used at all occasions, in 1979, 1991 and 1996. In 1991 new questions focused on parental history and different background factors. In 1996, it focused on early pet-keeping and factors influencing a family's decision not to have pets. Results: The prevalence of asthma increased from 2.5% to 5.7% between 1979 and 1991. Allergic rhino-conjunctivitis (ARC) from 5.4% to 8.1% and eczema from 7.0% to 18.3%. In 1991, at 7-9 years of age, a higher prevalence of eczema and sensitisation was observed in Kiruna. At the follow-up in 1996, at 12-13 years of age, similar prevalence figures were seen in the two regions for asthma (8.5%), ARC (17.2%) and eczema (23.4%). Sensitisation remained more common in Kiruna (38.1%) than in Göteborg (22.5%). Risk factors for allergy development were parental history of allergy, damp dwellings and birth during autumn/winter. Siblings and pet-keeping during first year of life were associated with less allergy. Boys were more often sensitised to horse, despite a low frequency in horseback riding as compared to girls. Conclusions: The prevalence of allergic diseases in schoolchildren doubled during the 12 year period from 1979 to 1991. Damp dwellings and birth during autumn/winter were associated with an increased risk of allergy development; highlighting the association between indoor climate and allergy development. Less allergy was seen in children with siblings and in children with direct animal contacts, especially during first year of life. The findings are interpreted as if factors associated with "less hygiene" and high allergen exposure may have protective effects.