The "too clean" theory of asthma rises again. It even makes the Wall St. Journal!

The germ theory has been in some eclipse in recent years because of some awkward epidemiological facts. For instance, Australian Aborigines often live in extraordinary squalor but don't seem to be protected from anything because of that. In fact they have quite high rates of autoimmune diseases such as diabetes

So how do we evaluate the findings below? It's a bit difficult as the article in NEJM has not yet appeared but there are at least two possibilities. The most favourable to the theory is that it is not the overall bacterial load that matters but rather just some bacteria. So southern German farmhouses might have the helpful bacteria but Aboriginal camps may not. That is not inherently absurd but would be very much in need of proof, considering that both populations have extensive contact with all the world's infective agents via the modern-day "global village".

The second much more skeptical possibility derives from the fact that we are only looking at epidemiology here -- so all causal links are speculative. For instance, it has recently been found that Paracetamol (Tylenol) use in children under 15 months doubles their chance of getting asthma. So maybe the "dirty" farms were less health conscious in general and so used fewer medications, including paracetamol. Isn't epidemiology wonderful?

The possibilities are endless, in fact. It was found last year, for instance, that that receptors for bitter tastes are not confined to the tongue but are also are found in the smooth muscles of the lungs and airways. And bitter tastes RELAX those airways. So in doing any epidemiological comparisons of asthma incidence, we would have to ask whether the different groups used in the research differed in their preferences for bitter drinks, including, of course, beer!

OK. I could go on but I will have mercy at that point

Children living on farms have a lower risk of asthma than children who don't because they are surrounded by a greater variety of germs, according to two large-scale studies published Wednesday.

The prevalence of asthma in the U.S. has doubled over the past 30 years, and one theory for the increase blames urban and suburban living environments that are too clean. The latest findings, published in the New England Journal of Medicine, bolster what is often known as the hygiene theory, which says that contact with bacteria and other microbes is necessary to building a normal immune system.

The key appears to be exposure to a diversity of bugs, not just more of them, according to Markus Ege, an epidemiologist at the Children's Hospital of Munich and first author on the paper that covered both studies. "Bacteria can be beneficial for asthma," said Dr. Ege. "You have to have microbes that educate the immune system. But you have to have the right ones."

Previous research, including some conducted by Dr. Ege's group, has found that children raised on farms exhibit substantially reduced risk for asthma and allergies—lower by 30% or more—than those raised elsewhere. Though scientists had hypothesized that the difference was linked to germs, they also had to determine whether it could be due to other elements of farm life such as fresh air, exposure to farm animals, or dietary factors like drinking raw milk.

The latest study helps untangle that question by providing evidence that the reduction in risk is indeed significantly related to the variety of bacteria and other bugs a child is exposed to, according to James Gern, a professor of pediatrics and medicine at the University of Wisconsin-Madison who wrote an editorial to accompany the paper in the journal but wasn't involved in the study.

In Wednesday's paper, the researchers surveyed and collected samples of house dust in two studies of children from Southern Germany, Austria and Switzerland. One study comprised 6,800 children, about half of whom lived on farms, and the other studied nearly 9,700 children, 16% of whom were raised on a farm. Researchers then examined the dust for presence and type of microbes.

Those living on farms were exposed to a greater variety of bugs and also had a lower risk of asthma. There was evidence that exposure to a particular type of bacteria, known as gram-negative rods, was also related to lower rates of allergic responses.

Identifying which microbes are beneficial to the immune system is important because those germs could help the development of new treatments or vaccines to prevent asthma, Dr. Ege said. His group is now studying some of the microbes in greater detail.

The findings don't yield much in the way of practical suggestions, however. Dr. Ege said it wouldn't help for parents to take their children to a farm two or three times a year or to get a dog or other pet for the purpose of exposing their children to microbes, since the biggest effect appeared to be related to prolonged exposure to cows and pigs.


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