German scientists identified bacteria present in cow digestion and poop that seem to help children exposed to them avoid developing asthma. The catch is they have to inhale the germs.

As rates of allergies, asthma, eczema and hay fever rose in industrial civilizations, children raised on farms did not seem to be affected. By the 1990s, research suggested that certain childhood infections helped the body tune its immune system, instead of overreacting to dust, pollen and other allergens. It became known as the “hygiene hypothesis” — staying too clean had a cost.

Now, researchers at LMU University Hospital and the Institute of Asthma and Allergy Prevention at Helmholtz Munich in Germany measured the bacteria that children tend to inhale when playing or working in cow barns and traced the mechanism that helps fight inflammation in their lungs.

“Girls and boys who grow up on farms and are exposed to a wider variety of microbes have the problem far less often,” epidemiologist Markus Ege said in a news release. “With our new study, we can make a much stronger case, because we can identify the individual links in the proposed causal chain: the bacteria, the relevant microbial metabolic products, and the human receptors.”

The team analyzed data from more than 1,000 children participating in European health studies in rural areas. They compared nasal swabs and samples of mattress dust, as well as airborne samples from cowsheds for 47 farm children. They published their findings in the July 28 issue of the New England Journal of Medicine.

Within the microbial stew of a farm environment, his team’s study identified nine gram-positive environmental bacterial genera. Together, those microbes explained two-thirds of the association of farm exposure and reduced rates of asthma. The findings ruled out fungus as a contributing factor.

Those bacteria produced chemicals that could switch on certain proteins in the human body to help control the immune system and inflammation. These chemicals include Kynurenine and xanthine, which activate a protein called the aryl hydrocarbon receptor, and α-Linolenic acid and stearidonic acid, two types of healthy fatty acids which activate another protein called PPAR-γ. Both receptors are common on human airway cells

High levels of stimulation are detrimental, the researchers wrote, whereas moderate levels indicating small amounts of time spent in cowsheds seemed to be beneficial.

The effect of the cow bacteria doesn’t benefit everyone. Small variations in genetics, especially those genes involved in sensing molecules produced in the body or influenced by the gut biome, could affect the results.

The effect is common enough, Ege said, to suggest new medical treatments to prevent asthma in children who do not have access to cowsheds.

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