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 percent 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 South 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.