Turns out children who grow up on farms have a significantly lower incidence of asthma and allergies like hay fever than children whose parents choose a non-agrarian path. OK, but why? A new paper by Markus Ege of Germany’s University Children’s Hospital Munich and his colleagues, published in the New England Journal of Medicine, suggests a surprising answer and it relates to living in a sterile environment … or, more precisely, the disadvantages of a sterile environment.

The Campaign Against Microbes

In many ways we have become a society obsessed with bacteria and fungi, obsessed with wiping them out. And we’ve gone a whole lot farther than following our mother’s dictum to wash our hands before dinner.

In the supermarket we are presented with myriad bactericides: for our kitchen counters, our hands, even our toothpaste (even if that means brushing our teeth with pesticides).

It’s not unheard of to spend thousands of dollars to wipe out the mold growing in our air ducts or quietly thriving in our basements or crawl spaces or even our carpets. Surely all that scrubbing and spraying and scraping helps keep us healthy. Getting salmonella poisoning is a definite no-no, and I’m told that black mold in your home is bad news, maybe even life-threatening. But could there be a downside to all that sterility? The Ege et al paper suggests yes.

The Mix of Microbes, Asthma and Hay Fever: A Good Mix or a Bad Mix?

Ege et al found a correlation between the presence of microbes and asthma and hay fever but not the kind of correlation you might expect.

To investigate the relationship between the presence of bacteria, fungi, childhood asthma, and hay fever, Ege and his co-authors analyzed data from two large cross-sectional studies in Germany that used complementary study techniques:

The PARSIFAL study (short for Prevention of Allergy — Risk Factors for Sensitization in Children Related to Farming and Anthroposophic Lifestyle) included roughly 7,000 children from which a subset of 489 dust samples from mattresses were analyzed for bacterial DNA.

The GABRIELA study (short for Multidisciplinary Study to Identify the Genetic and Environmental Causes of Asthma in the European Community) included close to 10,000 children. Here, a subset of 444 children was chosen to have airborne samples of dust collected from their rooms and analyzed for types of bacteria and fungi.

The authors report that “in both studies, children who lived on farms had lower prevalences of asthma and were exposed to a greater variety of environmental microorganisms than the children in the reference group.” A key part of their analysis was the finding that diverse microbes are abundant not only in farm areas where one would expect them to be (like in barns) but also in indoor areas where farm children live. Clearly the microbes from the farm are being tracked into the farmhouse, and at least in the case of childhood asthma (meaning in the case of avoiding it), that is a good thing.

The authors conclude that “this exposure explains a substantial fraction of the inverse relation between asthma and growing up on a farm.”

OK, but why should exposure to microbes suppress the incidence of asthma?

I imagine the simple explanation is that our bodies have evolved to cope with a specific kind of environment — no doubt an agrarian one and one filled with apparently a healthy array of microbes— and moving out of it can cause problems.

The authors offer two explicit mechanisms for the connection between microbe exposure and asthma.

One is based on the idea that asthma is an instance of an autoimmune response — that is, an overactive immune system that, in the absence of actual foreign microbes to defend against, starts to attack the body itself, causing inflammation and all sorts of other mischief. It is possible, the authors speculate, that early exposure to microbes conditions the immune system to work properly, suppressing later destructive autoimmune responses.

For the other explanation the authors speculate that early exposure to microbes may allow for the colonization of airways with a beneficial array of microbes which crowd out harmful bacteria that have “been associated with an increased risk of asthma among children and adults.”

Whatever the reason, one has to marvel at the complex ways the world and our bodies work. For most of us Americans, life is unbelievably good; our standard of living is without precedence and I wouldn’t trade it for any other time or place. But there is a price for all that progress … asthma may be one of them.