Children living on farms have a lower prevalence of asthma, likely due to a protective effect of their early exposure to a greater variety of environmental bacteria and fungi compared with what other children are exposed to, according to a report in the Feb. 24 issue of the New England Journal of Medicine.
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Children who live on farms have a lower prevalence of asthma than other children, suggests a new study.
In an epidemiologic analysis of data from two large observational studies of school-aged children in rural areas of central Europe, environmental samples from farmhouses showed a greater diversity of microbes than those from other homes. "The central finding of this analysis was the inverse association of the diversity scores with asthma [prevalence], which was not confounded by living on a farm," said Dr. Markus J. Ege of University Children’s Hospital, Munich, and his associates.
The analysis included data from a cross-sectional survey of 6,963 school children (aged 6-13 years), approximately half of whom lived on farms and the other half of whom lived in rural and suburban areas of Bavaria. Dust samples were collected from the mattresses of a randomly selected subgroup of 489 of the participants. Data also were obtained on the children’s respiratory and allergic symptoms, medical diagnoses, and potential confounders.
Children living on farms had a lower prevalence of asthma than did other children, with an adjusted odds ratio of 0.49. The percentage of dust samples that were positive for bacteria was significantly higher among farm dwellers, and the risk of asthma decreased significantly with an increasing number of detectable bands of bacterial DNA.
The analysis also included data from a second cross-sectional study involving 9,668 children attending elementary schools in rural areas of southern Germany, Switzerland, and Austria. Airborne dust samples were collected from the children’s bedrooms.
Again, children living on farms had a lower prevalence of asthma than did other children, with an odds ratio of 0.76. All bacterial and fungal taxa cultured from the dust samples were more prevalent among children living on farms than among other children, and the risk of asthma decreased significantly with an increasing number of fungal taxa, Dr. Ege and his colleagues said (N. Engl. J. Med. 2011;364:701-9).
In both studies, the diversity of microbes explained a substantial portion of the protective effect of the farming environment on asthma risk.
"Our methods do not allow us to identify specific microbes that may confer protection, but they have allowed us to identify broad families of species within microbial taxa that could be responsible for the effect of the farming environment.
"The challenge will be to identify these species with the precision needed to allow specific tests of the relationship between microbial exposure and protection against asthma," the investigators noted.
The analysis also could not determine the mechanism underlying this protective effect – how the diversity of microbial stimuli protects against asthma – but the researchers agreed with the prevailing view that perhaps micro-organisms trigger the innate immune system, which then bolsters resistance to asthma.
An alternative explanation may be that exposure to a broad rather than a narrow range of micro-organisms may prevent colonization of the lower airways with harmful bacteria. "Balanced colonization of the airways may parallel the beneficial effects of a diverse microbiome at other surfaces, such as the gut and skin," they added.
This study was funded by the Deutsche Forschungsgemeinschaft and the European Commission. Dr. Ege reported having a planned patent on asthma-protective bacteria. His associates reported ties to GlaxoSmithKline, Novartis, Protectimum, and ALK.