For a long time it’s been clear that living on a farm protects children against asthma (compared to city life). A new study, done in Germany, tried to go a little further than that: to ask if it was microbial exposure that made the difference. Non-American scientists have been far more interested in the environmental causes of disease than American scientists.
They measured microbial exposure by studying mattress dust. One branch of the study used DNA techniques to measure microbial diversity of the dust; the other branch measured microbial diversity by seeing how many microbes could be cultured from the dust.
They found the usual farm/city difference in asthma: The city kids had roughly twice as much. They found the expected farm/city difference in microbial diversity of mattress dust: For a given species of bacteria or fungi, there was roughly twice as much chance of finding it in the farm dust.
Did the microbial difference explain the asthma difference? To find out they corrected for the farm difference. I think this means they looked within the farm kids to see if in this restricted group there was a microbial diversity/asthma risk correlation. In one branch of the study, there was a significant correlation. In the other branch, the correlation was nearly significant.
In all, the results support the idea that differences in microbial exposure explain the farm/city asthma difference. The biggest strength of this study is that they gathered useful evidence related to a major problem (asthma). The biggest weakness is how difficult it was. It involved about 15,000 kids and probably cost more than a million dollars.
Thanks to Aaron Blaisdell.