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- Material and methods
Background: Earlier studies have shown a high prevalence of respiratory symptoms in farming communities and that storage mites constitute important allergens. We examined risk factors associated with asthma and rhinoconjunctivitis among Swedish farmers.
Methods: A population of 1015 small-scale dairy farmers was part of an epidemiologic survey. After selection based on symptom reports in a questionnaire, 461 of the farmers attended a medical examination, which comprised SPT, RAST analyses, and lung-function measurements. Risk factors for sensitization to different allergens, and development of asthma and rhinoconjunctivitis were assessed by multiple logistic regression.
Results: The prevalence of atopy was 26.7% among the farmers. For both asthma and rhinoconjunctivitis, sensitization to mites (OR=5.8 vs OR=3.8) and pollens (OR=10.3 vs OR=5.8) was a significant risk factor. There was a significant relationship between sensitization to mites and working time (OR=5.2). Environmental tobacco smoke and exposure to different animal species at the farm did not appear to affect the risk of allergen sensitization or respiratory symptoms. Farmers smoked less than the general population, but they more frequently had reduced FEV1.
Conclusions: Allergen sensitization, especially to mites and pollens, was significantly associated with asthma and rhinoconjunctivitis in a farming community. The results point to allergen avoidance as a major goal for the prevention of occupational respiratory diseases among the farming population.
An increased prevalence of asthma and rhinoconjunctivitis in industrialized countries has been reported during the last decade ( 1, 2), and this trend is not due merely to a change in diagnostic criteria but constitutes an increase per se ( 3). Recently, von Mutius et al. showed that the prevalence of atopy and respiratory disorders in two ethnically similar populations (children studied shortly after the reunification of Germany) was significantly lower among subjects living in the former East Germany than among subjects living in the former West Germany ( 4). The authors suggested that the Western lifestyle is a risk factor for the development of atopy, which in turn predisposes to asthma.
An association between occupation and development of allergy has been shown for several professions, and many agents are known to cause occupational rhinitis and asthma ( 5, 6) mediated by IgE antibodies. A high prevalence of respiratory symptoms among farmers has been demonstrated in studies from both Scotland and the Nordic countries ( 7–10); occupational asthma is the most common disease among farmers in Finland ( 11). Not only do farmers have a higher risk of developing asthma, but the asthma mortality rate among male farmers in Sweden is significantly higher than that in other occupational branches ( 12). Earlier studies have shown that farmers are more often sensitized to mites than the general population, and less to other common allergens, such as pollens and animal danders ( 8, 13, 14). Storage mites, together with cow epithelium and flour, have been reported as the most common causative agents of occupational asthma among farmers ( 11).
The aim of this epidemiologic study in a Swedish farming community was to determine whether exposure to common inhalant allergens and other risk factors is associated with atopy, asthma, and rhinoconjunctivitis.
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- Material and methods
Several studies have shown that farmers have an elevated risk of developing airway symptoms ( 5, 8, 10, 17). The role of storage mites as a causative factor in respiratory allergy has been well established in rural as well as in urban populations ( 8, 9, 13, 18, 19). The findings in this study confirm that mites are important allergens in the farming environment, with almost one-fifth of the farmers sensitized to mites. Furthermore, mites were the most prevalent allergens among farmers with asthma and rhinoconjunctivitis. Storage mites are most common in barns and other places where hay, straw, and grain are stored ( 20, 21), but they can also be found in the indoor home environment ( 22). A recent study from Sweden found L. destructor in 100% of the hay samples from 30 investigated farms and A. siro in more than 80% ( 23). As for the indoor environment in rural homes, high numbers of both house-dust mites and storage mites, as well as large amounts of the house-dust mite allergens Der p 1 and Der f 1, have been recorded ( 22, S. Parvaneh, in manuscript). Thus, storage mites as well as house-dust mites are part of the mite fauna of rural surroundings.
We observed a more than threefold increased risk of respiratory symptoms of some kind among farmers sensitized to mites or pollens and a more than fivefold elevated risk of asthma. Because asthma is a chronic disease which when inadequately treated may lead to a persistent impairment of lung function that may adversely affect the quality of life, it is desirable to prevent allergic sensitization.
In Finland, cow dander is reported to be an important cause of allergy among farmers. Terho et al. have demonstrated that 50 out of 70 farmers with allergic rhinitis had a positive reaction upon nasal challenge with cow dander ( 9). Among Swedish farmers, cow dander seems to be of less importance as an occupational allergen, although we found that cow was the most common cause of sensitization to animal danders, according to both SPT and RAST.
Various insect species can cause inhalant ( 24) as well as occupational allergies ( 25). Both the botfly (G. intestinalis) and the grain weevil (S. granarius) yielded nearly the same prevalence of sensitization as animal dander. A positive RAST against G. intestinalis was almost twice as frequent as against S. granarius, a finding which is in accordance with earlier findings from a farming community ( 26). Out of 34 subjects with positive RAST against either botfly or grain weevil, 23 were also sensitized to mites, a finding which probably reflects the fact that these insects and mites usually are found in the same habitat. The IgE-mediated response to timothy grass, wheat, and rye among the farmers was twice as high as the response to birch, but still remained low compared to the response in the general population ( 27). Most of the farmers who were RAST-positive to timothy grass were also positive to wheat and/or rye, probably because of allergenic similarities between the allergens. A taxonomic relationship exists among the three different types of pollens, and cross-reacting allergens have been demonstrated ( 28, 29).
We obtained an almost four times higher prevalence of reduced FEV1 than Bakke et al., who reported that 4.5% of a general population had a FEV1 below 80% of predicted ( 30). Several earlier studies have demonstrated that farmers more often have a reduced FEV1 and a more rapid decline in lung function than controls ( 31, 32). It is well documented that swine confinement workers in particular, but also grain and dairy farmers, have lower FEV1 than the general population ( 33–35). The significant association of a FEV1 below 80% of predicted value with asthma primarily represents a manifestation of the disease. We were not able to demonstrate any relation between swine-keeping and reduced lung function, possibly because most of the farms on Gotland are family farms with small numbers of swine. The relationship between smoking and reduction of FEV1 is well established ( 36). In the univariate model, we found that smoking and sensitization to mites were risk factors for decreased FEV1. The prevalence of smoking in rural communities has earlier been reported as almost 30% ( 37, 38). We found a lower prevalence of smoking (15.9%), which is more in accordance with the findings of Vergnenegre et al., who reported a prevalence of 18.1% of current smokers among agricultural workers in France ( 39). The proportion of smokers among the Gotland farmers is lower than among Swedes in general, in whom the prevalence is estimated to be about 25% ( 40).
The cross-sectional methodology used in this study is not optimal for assessment of causal relationships ( 41). For example, exposure to risk factors for allergy, such as environmental tobacco smoke and furred animals, as well as use of dust masks, may have changed after disease occurrence. Furthermore, the use of dust masks may be an indication of high exposure to dust in some cases and low exposure in others, making the interpretation difficult. On the other hand, substantial misclassification of pollen exposure is improbable since the pollen load was constant over several years ( 42, 43). In addition, two different studies performed by the authors in 1988 and 1992 revealed that the exposure to mites was constant ( 24, 44).
In conclusion, this study shows that in a small-scale, family-oriented farming community, sensitization to common allergens, especially mites and pollens, is associated with occurrence of asthma and rhinoconjunctivitis. However, no clear association was observed for exposure to environmental tobacco smoke or exposure to different animal species at the farm. The results point to allergen avoidance as a major goal for the prevention of occupational respiratory diseases among the farming population.