The Journal of Allergy and Clinical Immunology
Volume 112, Issue 2 , Pages 263-264, August 2003

Pets—good or bad for individuals with atopic predisposition?☆☆☆

Department of Pneumology and Immunology, Charitè Campus Vir-chow, University Children's Hospital, Humboldt University, Berlin. Berlin, Germany

Received 6 May 2003; accepted 8 May 2003.

Article Outline

 

It is still unclear how early life exposure to pets is related to children's risk of developing atopy-related diseases. Controversial data have been published during the last 4 years regarding the potential harmful or favorable effect of pet exposure in households. Clearly, in the hierarchy of evidence prospective studies are more reliable than cross-sectional studies.

Taking together all the information published to date, there seems to be more evidence that sensitization to cat is a risk factor for asthma in childhood and adulthood.1, 2, 3, 4 However, the British Isle of Wight study and the German Multicentre Allergy Study did not find differences in sensitization rates between families with and without pet keeping. Pet and pet allergen exposure in later life is associated with sensitization and respiratory symptoms,3, 5, 6, 7, 8 and the apparent protective effect of current pet ownership often is due to selective avoidance or a “healthy smoker effect.”5, 9

Avoidance of pet allergens as a measure of secondary and tertiary prevention is established in the treatment of allergic airway disease. A meta-analysis published by Apelberg et al10 came to the conclusion that pet keeping neither favors nor protects from allergic diseases in childhood.

However, it remains unclear whether avoidance of pet allergens early in infancy and childhood (during the first 1 to 2 years of life) can specifically prevent allergic sensitization or even reduce the risk for allergic airway disease. Some studies suggest that pet keeping during the first year of life can protect against sensitization and asthma at later school age probably through induction of tolerance.11 Similar findings were reported in another Swedish investigation.4 Although sensitization to cat was strongly associated with asthma, living with a cat was inversely related to both having a positive skin test result to cat and incidence of physician-diagnosed asthma (relative risk, 0.49; 95% confidence interval, 0.28 to 0.83). This effect was most pronounced among children with a family history of asthma. However, the effects were not always reaching significance as in a Norwegian publication,12 in which only the reduction of atopic eczema at the age of 0 to 6 months in the group of pet keeping families reached statistical significance (odds ratio, 0.7; 95% confidence interval, 0.5 to 0.9). Ownby et al13 found a dose-response relationship in which protection against allergic sensitization was only observed if more than 2 dogs or cats were kept in the households early in infancy (reduction of specific sensitization rate from 34% to 15.4% compared to families without pets or with only 1 pet). Other authors could not prove that early exposure to cat or cat allergens is protective in prospective birth cohort studies and observed that for children with atopic family history cat or higher cat allergen exposure is a risk factor for sensitization.1, 14, 15 Things become even more complicated because of the fact that pet allergens, especially cat allergen, are ubiquitous allergens and can also be found in homes without pet keeping and in public places. Therefore sensitization can also occur outside children's homes. It is possible that the sometimes observed protective effect of pet keeping might not be caused by the induction of tolerance as a result of very high exposure and the induction of IgG4 as suggested by Platts-Mills et al16 in 2001 but by bacterial compounds being secreted by animals such as endotoxin, ie, LPS from gram-negative bacteria.17 Some authors also postulated independent effects of exposure to pets and endotoxin.18 There is increasing evidence from trials in farming communities that early contact with cattle in stables during infancy is protective for allergic diseases and asthma.19, 20, 21

In this issue you will find an article by Svanes et al22 that presents results from participants of the European Community Respiratory Health Survey analyzing the association between pet keeping and adult asthma and hay fever. Although information about pet keeping early in childhood was given retrospectively in a structured interview, the large number of cases (n = 18,530) makes the analysis reliable. Interestingly, the authors found that the effect of pet keeping in childhood varied according to the type of pet and the presence of cats in the community, ie, in the wider environment. Cats owned in childhood were associated with more asthma in sensitized adults who grew up in areas with low community prevalence to cats. Dogs owned in childhood appeared to protect against adult allergic disease but promote nonallergic asthma. The first finding could be proof of the concept suggested by Platts-Mills et al16 that continuously very high exposure in a community can probably induce tolerance. The second finding can be an effect of LPS exposure favoring a TH1 response early in life but promoting an inflammation in the lung later in life in susceptible individuals. The observation that very low and very high concentrations of the cat allergen Fel d 1 can be protective was reported by Custovic et al,23 showing a “bell-shaped curve” correlating exposure and sensitization rates in adults. However, in most of the communities in Europe except the United Kingdom and Sweden, these very high concentrations of Fel d 1 (greater than 1000 μg/g of dust) cannot be found in the environment; therefore the avoidance of pets seems still to be recommendable in families with atopic predisposition. The findings of farming communities, in which a reduced risk of developing atopy among children raised on farms with lifestock was observed, cannot be directly transferred to the urban population. It is unclear whether LPS is the only protective factor in a farming environment or whether it is the different lifestyle in general influencing the immune system. In adults, observations in occupational medicine suggested that contact with endotoxin is a risk factor for bronchial symptoms.24 Intervention studies are now needed to prove the concept of immune deviation via bacterial products in patients at risk for atopy in a clinical setting.

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References 

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☆ Reprint requests: Ulrich Wahn, MD, Professor and Director, Charitè Campus Virchow, University Children's Hospital, Department of Pneumology and Immunology, Humboldt University, Augustenburger Platz 1, 13353 Berlin, Germany.

☆☆ J Allergy Clin Immunol 2003;112:263-4.

PII: S0091-6749(03)01706-8

doi:10.1067/mai.2003.1668

The Journal of Allergy and Clinical Immunology
Volume 112, Issue 2 , Pages 263-264, August 2003

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