Inverse association between farm animal contact and respiratory allergies in adulthood: protection, underreporting or selection?

Background: It has been argued that the inverse association between exposure to farm animals and nasal allergies observed in children and adults might be because of self-selection. Aims: We aimed to assess the health-based selection out of farming in adults. Material and methods: A cross-sectional study was carried out in a rural region. Overall, 4053 inhabitants (63%) aged 18-44 years responded to a questionnaire on respiratory diseases, life-time exposure to farming environments and potential confounders. For 2678 of these, specific immunoglobulin E to common allergens was available. The outcome was: (i) sensitization and symptoms of nasal allergies (symptomatic sensitization); (ii) sensitization without symptoms of nasal allergies (asymptomatic sensitization). Results: Farm animal contact in childhood was associated with a decreased risk of symptomatic and asymptomatic sensitization. Continued exposure to farm animals in adulthood further decreased the odds ratio of symptomatic (odds ratio 0.2; 95% confidence interval 0.1, 0.4) but not asymptomatic sensitization (0.7; 0.4, 1.1). Starting farm animal contact in adulthood even increased the odds ratio of asymptomatic sensitization (2.4; 1.1, 5.2). Conclusions: The preventive effect of childhood contact to farm animals against sensitization continues into adulthood. However, in adulthood self-selection based on symptoms and underreporting of symptoms might also play a role.