Does early exposure to cat or dog protect against later allergy development?

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Abstract
It is unknown which factors in modern western society that have caused the current increase in prevalence of allergic diseases. Improved hygiene, smaller families, altered exposure to allergens have been suggested. To assess the relationship between exposure to pets in early life, family size, allergic manifestations and allergic sensitization at 7–9 and 12–13 years of age. The prevalence of allergic diseases and various background factors were assessed in 1991 and 1996 by questionnaire studies. In 1991, the study comprised representative samples of children from the Göteborg area on the Swedish west coast (7 years old, n = 1649) and the inland town Kiruna in northern Sweden (7–9 years old, n = 832). In 1992, a validation interview and skin prick test (SPT) were performed in a stratified sub-sample of 412 children. In 1996, this subgroup was followed up with identical questions about clinical symptoms as in 1991, detailed questions about early pet exposure were added and SPT performed. Children exposed to pets during the first year of life had a lower frequency of allergic rhinitis at 7–9 years of age and of asthma at 12–13 years. Children exposed to cat during the first year of life were less often SPT positive to cat at 12–13 years. The results were similar when those children were excluded, whose parents had actively decided against pet keeping during infancy because of allergy in the family. There was a negative correlation between the number of siblings and development of asthma and allergic rhinitis. Pet exposure during the first year of life and increasing number of siblings were both associated with a lower prevalence of allergic rhinitis and asthma in school children.