Which aspects of the farming lifestyle explain the inverse association with childhood allergy?

BACKGROUND: Farmers' children have a reduced prevalence of allergic disorders. The specific protective environmental factors responsible are not yet identified.
OBJECTIVE: We sought to determine whether farmers' children in the rural county of Shropshire, England, have a reduced risk of atopy and, if so, to identify the factors responsible.
METHODS: The Study of Asthma and Allergy in Shropshire was a 2-stage cross-sectional study. In stage 1 a questionnaire to elicit allergic status, diet, and farming exposure was completed by the parents of 4767 children. In stage 2 a stratified subsample of 879 children underwent skin prick testing and measurement of domestic endotoxin.
RESULTS: Compared with rural nonfarming children, farmers' children had significantly less current asthma symptoms (adjusted odds ratio (OR), 0.67; 95% CI, 0.49-0.91; P = .01) and current seasonal allergic rhinitis (adjusted OR, 0.50; 95% CI, 0.33-0.77; P = .002) but not current eczema symptoms (adjusted OR, 0.91; 95% CI, 0.68-1.21; P = .53) or atopy (adjusted OR, 0.68; 95% CI, 0.40-1.16; P = .15). In contrast, current unpasteurized milk consumption was associated with significantly less current eczema symptoms (adjusted OR, 0.59; 95% CI, 0.40-0.87; P = .008) and a greater reduction in atopy (adjusted OR, 0.24; 95% CI, 0.10-0.53; P = .001). The effect was seen in all children, independent of farming status. Unpasteurized milk consumption was associated with a 59% reduction in total IgE levels (P < .001) and higher production of whole blood stimulated IFN-gamma (P = .02).
CONCLUSION: Unpasteurized milk consumption was the exposure mediating the protective effect on skin prick test positivity. The effect was independent of farming status and present with consumption of infrequent amounts of unpasteurized milk. CLINICAL IMPLICATIONS: Unpasteurized milk might be a modifiable influence on allergic sensitization in children.