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Diversity, not quantity, of germs cuts asthma risk

farm.jpgChildren living on farms and exposed to a wide variety of bugs have a lower chance of developing asthma, hay fever and other such alergies than city kids, according to new research published in the New England Journal of Medicine.

But having a dirty house with thriving communities of microbes, fungi, and other bugs is not the way to develop a normal immune system; the composition of the bugs in the house also matter. The best preventative is prolonged exposure to microbes in a farm environment, suggests the study.

The work builds upon the hygiene hypothesis, which states that lack of exposure to bugs during childhood can impair development of a normal immune system, and lead to an increased risk of allergies.

Researchers analyzed two different studies with large sample sizes. The first study tested samples of mattress dust from the rooms of 6,843 children living in rural South Germany. About half of them lived on farms. It analyzed bacterial DNA to detect the types of bugs living there. The second study tested environmental dust in the rooms of 9,668 Bavarian children living in rural areas of Austria, Switzerland and South Germany. About 16% of them lived on farms.

The scientists found that the microbes and fungi are more diverse on farms than in other households. They correlated this to fewer cases of asthma in children living on farms. The strength of association between greater microbial diversity and lower asthma, also known as the “odds ratio” was 0.62 in the first study. The odds ratio in the second study was 0.86.

The authors speculate that greater diversity of exposure likely protects children against asthma. Previous research has shown that microbes trigger the innate immune system by activating signaling pathways leading to T-cell induction. Many microbes trigger the type 1 helper T cell pathway, but in asthma, the type 2 helper T cell pathway is hyperactive. The hypothesis is that insufficient stimulation of the type 1 pathway in an ultra-clean environment leads to an overactive type 2 pathway and causes allergic diseases.

The authors suggest that the certain microbial exposures on farms could be inducing type 1 helper T cells, rather than the Type 2 helper T cells associated with asthma.

They say it is also possible that if children have a variety of microbes living in their lower respiratory tract, creating a diverse microbiome like in the gut, this could protect them from infections by crowding out harmful bacteria associated with asthma.

The scientists also sought to broadly identify the microbial families that may be protecting the farm children. In kids in South Germany, bacteria from the staphylococcaceae family, and another family, were prevalent in the environmental samples. In Bavarian children, fungi from two genera, eurotium and penicillium, seemed relevant.

Image: photo by 0_kelso_0 via Flickr under Creative Commons.


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