Health-based selection for asthma, but not for chronic bronchitis, in pig farmers: an evidence-based hypothesis.

In cross-sectional surveys among pig farmers an elevated prevalence of chronic bronchitis is usually reported, but not of asthma, despite many factors promoting the manifestation of asthma. To investigate this apparent contradiction a postal questionnaire survey was performed among 239 pig farmers and 311 rural controls. Pig farmers reported an elevated prevalence of symptoms of chronic bronchitis (20.2 versus 7.7%, p<0.001), but not of asthma (5.9 versus 5.5% for chest tightness). Among pig farmers the use of disinfectants (prevalence odds ratio (POR) 9.4, 95% confidence interval (CI) 1.6-57.2 for quaternary ammonium compounds) and aspects of the disinfecting procedure were associated with the prevalence of asthma symptoms. Compared with controls, pig farmers reported fewer allergies to common allergens (4.6 versus 14.6%, p<0.001 for pollen) and fewer symptoms of atopy in childhood (9.9 versus 17.2, p<0.05 for one or more of four symptoms). Atopy in childhood was strongly associated with the prevalence of asthma symptoms (POR 4.1, 95% CI 2.2-7.7), but not with chronic bronchitis. Health-based selection of nonasthmatics for pig farming, which tends to mask a work-related hazard for asthma, is offered as an explanation for these results.