Editorial

There has been a marked increase in the prevalence of atopy-related diseases during the last 40 years (1, 2). There has also been a documented increase in speci®c sensitization to pets (2). There seem to be geographic differences; for example, children in Sundsvall, Sweden, have a higher prevalence of sensitization to pets than children in Konin, Poland, although it was as common to keep dogs in Poland as in Sundsvall, Sweden (3). The differences in the prevalence of hay fever and allergic sensitization between the former East Germany and West Germany have decreased without change in exposure (4). It is not clear what mechanisms are behind the increase in prevalence of atopic diseases, including sensitization to common airborne allergens. Exposure to environmental tobacco smoke, the length of breast-feeding, damp houses, and exposure to allergens during the ®rst years of life have been claimed to be important causes of the increased prevalence (5±10). A lower prevalence of atopic diseases and speci®c allergy, including allergy to pets, has been reported among children living on farms and children in families with an anthroposophic lifestyle (11±13). It has been debated whether a low-dose allergen exposure would increase the risk of sensitization while a higher dose of the same allergen would promote development of tolerance. All in all, it is quite unclear to what extent pet exposure during the ®rst years of life is important for sensitization or disease development later in life. Recently, a panel of 13 Swedish pediatric allergologists were asked to give their views on whether child exposure to pets should be avoided until 2 years of age in families with different risk backgrounds, and they were also asked to give scienti®c evidence for their views. There was no consensus for any of the risk categories. Seventeen references were given as support for their positions, of which two were editorials, 14 were original articles, and only 10 dealt with the question of exposure to pets and sensitization. Of these, seven were transectional studies with a retrospective design, and several of them were not designed to answer the question of exposure to pet animals. The results of the different studies varied considerably. Some of the most interesting studies were those by Burr et al. (14) and Wahn et al (15), since they both included prospective cohorts. The study by Burr et al. was not primarily designed to answer the question of exposure to pets and the risk of sensitization. They found a negative relationship between pet exposure and sensitization. In this study, there was a lack of description and details of exposure and outcome regarding speci®c sensitization. Furthermore, there was a very small group of sensitized children in the study as a whole. Wahn and coworkers performed a study primarily designed to answer the question about early exposure to pets and sensitization. Exposure to cat allergen during the ®rst years of life increased the risk of speci®c cat sensitization. Recently, the same authors have not been able to show any relationship between exposure to cat allergen at 6 and 18 months of age and asthma at 7 years (16). Selection before birth, lack of precision of exposure, and low allergen exposure may have in ̄uenced the results. However, the authors obviously restricted themselves to calculation of the odds ratios for exposure to cat allergen at the age of 6 months and asthma at 7 years of age, and no analysis was done of the rather clear dose-response relation for early exposure to cat allergen and health effect. Lindfors et al. (17) measured cat allergen in house dust in acute asthma among children and found a signi®cant relationship between exposure to cat allergen and sensitization. Other studies (18, 19) in the European Community Health Respiratory Health Survey found a negative relationship between exposure to cat or dog during childhood and sensitization measured at the adult age. It is dif®cult to perform strictly prospective Allergy 2001: 56: 267±269 Printed in UK. All rights reserved Copyright # Munksgaard 2001