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- Material and methods
Background: Domestic furred animals are important sources of allergens in asthma, but few clinical studies have investigated the effects of exposure to or elimination of these allergens. The objective of this study was to investigate whether pet-sensitized asthmatics with pets at home have worsened asthma, compared with similar asthmatics without pets.
Methods: In a cross-sectional epidemiologic study, 129 adult asthmatics sensitized to cats or dogs were investigated by interviews, skin prick tests, specific IgE, eosinophil counts, and bronchial methacholine challenge tests. Patients with a cat or dog at home (n=39) were compared with subjects without pets (n=90).
Results: More subjects with pets had symptoms, inhaled or oral steroids, abnormal peak-flow records, higher eosinophil counts, and a higher degree of bronchial hyperresponsiveness than subjects without pets. Asthmatics with pets more frequently ascribed their symptoms to their home environment, but fewer of them reported that their symptoms were elicited by contact with cats or dogs.
Conclusions: This study strongly indicates that living with a cat or a dog aggravates symptoms and biomarkers of airway inflammation in asthmatics sensitized to cats or dogs. The study indicates that avoidance of these allergens may be an important measure in the treatment of sensitized asthmatics.
A reduction of exposure to allergens should be an obvious cornerstone in the treatment of allergic diseases such as atopic asthma and allergic rhinitis (1, 2), but there are few studies of the importance of such reductions. Cats and dogs are important sensitizers in patients visiting emergency rooms with asthma (3, 4), and in Sweden, sensitization to pets is very common (5). Pet allergens are the allergens most closely related to asthma in Sweden and parts of the USA (6, 7). To our knowledge, there are no studies of allergen avoidance in asthmatics sensitized to pets. However, in asthmatics sensitized to house-dust mite (HDM) improvements such as reduction in sputum eosinophils and bronchial hyperresponsiveness (BHR) are seen when exposure to HDM is reduced (8–11).
The aim of this study was to investigate whether asthma was worsened by exposure to cats or dogs at home in persons sensitized to pets. In a cross-sectional epidemiologic study, the prevalence of symptoms and clinical findings was compared between sensitized asthmatics with and without pets.
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- Material and methods
This study clearly indicates that asthmatics sensitized to cats or dogs would improve if their exposure to pets were reduced. In addition, we found that sensitized pet owners less often associated their symptoms with pet exposure, although their asthma was aggravated by their home environment.
However, this is an epidemiologic study, and an experimental, randomized study would be the method of choice, although such a study would not be ethical since it is generally accepted that exposure to allergens should be lowered in sensitized asthmatics. Furthermore, pet owners who associate their symptoms with pets may avoid exposure to them, making randomization difficult.
Aspects of validity
It is important to determine whether selection and information bias influenced the results. The difference in some outcomes, e.g., occurrence of nocturnal bronchial symptoms and use of steroids, was large, and only substantial confounding may explain such differences. Subjects with pets did not have a higher frequency of sensitization to other allergens such as house-dust mites or pollens. Furthermore, the groups were similar in terms of the prevalence of rhinitis, type of housing, sex, age, and duration of asthma. Pet owners were less frequently lifelong nonsmokers and less frequently had a history of asthma or bronchitis in childhood. It is possible that patients who had childhood asthma are more aware of their allergy and less inclined to be pet owners. It is also possible that the allergic symptoms differed between patients with previous asthma in childhood and primarily adult-onset asthmatics. We controlled for these differences in the adjusted analysis, and the differences between the groups persisted. It does not seem likely that the most severe asthmatics would be the least inclined to remove their pets and thus bias our results. The opposite is more probable. Thus, it is not probable that selection bias would explain more severe asthma in pet owners. Pet owners were less aware of their allergy to pets and less often associated symptoms with pets (Fig. 2), but pet owners had more symptoms in general, used more steroids, and had more often abnormal peak flow. One might assume that pet owners liked their pets and underestimated the symptoms associated with them. Thus, information bias seems to be a very improbable cause of more severe asthma in pet owners.
Reduction of pet allergens
In our study, we have assumed that pet owners have a higher exposure to pet allergens than asthmatics without pets. It would have improved our study if we had measured the levels of cat and dog allergens in the homes of patients. In particular, it would have been of interest to know the allergen levels in patients who had removed their pets. However, at the time of the study, we did not have the necessary resources to determine this.
The allergens may persist for a very long time after the removal of a pet, and allergens from furred animals are widespread in our environment (21–26). However, Wood et al. found reduced allergen levels after the removal of cats (21). In addition, studies of allergen levels have shown the highest levels in homes with pets (27). In our study, fewer than 20% of the asthmatics who had removed their pets had symptoms elicited by dust at home or by their home environment, but 70% had symptoms when visiting friends with pets. This indicates that the allergen levels in their house dust had been significantly reduced after removal of their pets. Experimental studies have shown reduced levels of cat allergen with the cat in situ after repeated washings, with uncarpeted floors and air cleaning (28–30). The effect of these procedures on the asthma itself has not been shown. In clinical practice, it might help some patients who are reluctant to remove their pets. However, this type of advice might also prolong the delay of effective allergen avoidance, i.e., removing the animal from the home, and increase the risk that the asthma severity would progress.
It was expected that pet-sensitized asthmatics would have airway symptoms elicited by pets and problems visiting friends with pets. The lower frequency of pet-elicited symptoms among pet owners could hypothetically be explained by neglect of symptoms elicited by pets. Pet owners had nasal symptoms elicited by dust at home more frequently than patients without pets. This could support some degree of neglect – dust, and not the pet, was seen as the offending agent. Another hypothetic explanation could be that pet owners did not experience immediate allergic reactions as often as the asthmatics without chronic allergen exposure. It is possible that repeated or continuous inhalations of a relatively high dose of allergens create a reaction pattern dominated by late-phase reactions (31). As a result, they might have become less aware of the importance of their allergy and consequently less prone to remove the animal. It is also possible that some of the asthmatic pet owners could tolerate their pets in spite of their positive allergy test, because they had only latent allergy. In particular, in older subjects, the sensitization could be a trace left from previous clinical allergy without present importance. However, young subjects could have an allergy with increasing clinical relevance. Eight of the 39 patients with pets had no or mild daily bronchial symptoms, no nocturnal symptoms, and no abnormal peak-flow records, and their only medication was a short-acting β-agonist. They were young (mean age 23 years), five had a cat, three had a dog, and one had both animals at home. Four of them reported bronchial symptoms, and six reported nasal symptoms elicited by pets. However, six of these subjects had pronounced hyperresponsiveness (PC20≤1 mg/ml methacholine), and a longer observation period might have shown worsening of their asthma (32).
Importance of allergen exposure and allergen avoidance
In experimental studies, Ihre & Zetterström found an increase in BHR after repeated low-dose inhalations of allergens (33). In addition, bronchial variability with low peak-flow values was observed several days after a late-phase reaction (34). Gibson et al. found that that an increase in BHR after allergen inhalation was associated with an increase in circulating eosinophils after 24 h (35). The high eosinophil counts, the pronounced BHR, and the peak-flow variability among the pet owners in our study could therefore be a consequence of repeated or continuous allergen stimulation. There were also non-pet owners with asthma of considerable severity. In some cases, the major cause could be other allergens such as HDM or nonallergic factors. However, very sensitive patients could be affected by indirect exposure to pet allergens spread in the environment by pet owners (22–26).
If the pet owners in our study should have the same severity of disease as the asthmatics without pets, the use of inhaled steroids would decrease from a prevalence of around 50% to about 25%, a considerable reduction (Fig. 1). Ten percent of the pet owners used oral steroids, but hardly any would need them if their disease was similar to that of the non-pet owners. Furthermore, the prevalence of pet owners with moderate to severe daytime symptoms would be reduced from about 30% to 10%.
These results indicate that allergen avoidance in the form of removing the pet from the home should be recommended in cat- or dog-sensitized asthmatics. Some patients with pets might not be aware of their sensitization. Others know about their allergy, but might underestimate its importance, because they do not directly experience a worsening as a result of their pet.