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Farm exposure in utero may protect against asthma, hay fever and eczema

¹ Centre for Public Health Research, ³ Research Centre for Mori Health and Development, Research School of Public Health, Massey University, ⁴ Malaghan Institute for Medical Research, Victoria University, Wellington, and ² Epicentre, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand, ⁵ Dr von Hauner Children’s Hospital, University of Munich, Munich, Germany.

CORRESPONDENCE: J. Douwes, Private Box 756, Wellington, New Zealand, Fax: 64 43800600. E-mail: j.douwes{at}massey.ac.nz

Keywords: Asthma, farming, hygiene hypothesis, prenatal, timing

Received: March 21, 2007
Accepted April 15, 2008

The aim of the present study was to assess which factors contribute to the lower prevalence of allergic diseases in farmers’ children, and the importance of timing of exposure.

In a cross-sectional questionnaire survey, asthma symptoms, hay fever and eczema were assessed, as well as current, early and prenatal farm-related exposures in 1,333 farmers’ children and 566 reference children aged 5–17 yrs.

Farmers’ children had a lower incidence of asthma symptoms and eczema. Current and maternal exposure during pregnancy to animals and/or grain and hay reduced the risk of asthma symptoms, hay fever and eczema. The exposure–response association for maternal exposure was nonlinear for most outcomes. After mutual adjustment, the effects of prenatal exposure remained unchanged whereas current exposure remained protective only for asthma medication, asthma ever and hay fever. Exposure during the first 2 yrs was not associated with symptoms, after controlling for prenatal exposure. A combination of prenatal and current exposure was most strongly associated with wheeze (odds ratio (OR) 0.48, 95% confidence interval (CI) 0.28–0.80), asthma medication (OR 0.50, 95% CI 0.30–0.82), asthma ever (OR 0.50, 95% CI 0.33–0.76), hay fever (OR 0.47, 95% CI 0.30–0.73) and eczema (OR 0.46, 95% CI 0.30–0.70).

Prenatal exposure may contribute to the low prevalence of asthma, hay fever and eczema in farmers’ children, but continued exposure may be required to maintain optimal protection.

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