Paradoxical Effect of Domestic Animals on Asthma and Allergic Sensitization

Over the last 20 years, the relationship between exposure to allergens and asthma has been intensively investigated because the prevalence and severity of the disease have increased, and because sensitization to common indoor allergens is strongly associated with asthma.^1- 2 Studies on exposure to dust mite allergens have consistently shown a direct relationship to sensitization, as judged by positive skin prick test responses, as well as a strong relationship between sensitization and asthma.^3- 6 By contrast, several studies of farming communities in Europe have indicated that early exposure to farm animals has a protective effect against both sensitization and asthma.⁷ Recent evidence has suggested that this effect might be mediated by high exposure to bacterial endotoxins.^7- 8 Farm animals have not been common in the big cities of America or Europe since before 1900; on the other hand, domestic pets are extremely common, a prolific source of allergens, and sensitization to these allergens is strongly associated with asthma.^9- 10 It was therefore of considerable interest when reports from Europe suggested that the presence of a cat in the home could decrease the risk of sensitization to cat allergens.^11- 12 Initial speculation that this effect could be secondary to decisions by families with allergic disease not to have pets has been made unlikely by reports that the same effect occurs in countries where domestic animals are equally common in the homes of families with a history of asthma as in the homes of families without a history of asthma.¹³ Furthermore, recent evidence from Sweden has shown that the presence of a cat in the home is associated with decreased incidence and prevalence of asthma among preteens.¹⁴