Decreased prevalence of asthma among farm-reared children compared with those who are rural but not farm-reared.

BACKGROUND: Farm exposure has been associated with decreased asthma prevalence.
OBJECTIVES: We compared asthma prevalence among rural farm-reared and non-farm-reared children and examined farm demographic and environmental factors.
METHODS: We performed a cross-sectional, population-based survey among 36,500 rural kindergarten through 12th grade school children. Surveys were distributed through schools and returned by mail.
RESULTS: Of the 4152 participants, 18% had lived or were currently living on a farm. Compared to other rural children, farm children had more siblings (3.0 vs 2.5; P < .015), were more likely to be breast-fed (64% vs 58%; P < .002), to have pets (88% vs 79%; P < .001), and were less likely to have attended daycare (39% vs 50%; P < .001). Farm-reared children were less likely to have had a history of wheezing (28% vs 34%; P < .003) or a diagnosis of asthma (22% vs 26%; P < .002). This effect was greater among children younger than 10 years of age than among older children. There was no difference in the frequency of either asthma or non-asthma allergy symptoms during the previous 12 months. When analyzed by age and sex, decreased asthma prevalence was associated with farm rearing among younger children more than among adolescents. Farm residence beginning during the first 5 years, but not later, was associated with decreased rates of ever asthma (23.7% vs 33.7%; P < .005).
CONCLUSIONS: Asthma, but not other manifestations of allergy, is less commonly reported among farm-reared children. Early exposures may be more important than those occurring later. Without ongoing exposures, their effects on disease expression may diminish over time.