Asthma Risk May Be Cut by Dirt, Study Says
Contact With Microbes May Aid Farm Youths' Immunity
By David Brown
Washington Post Staff Writer
Thursday, September 19, 2002; Page A03
Pigpen, the "Peanuts" cartoon character who spent his childhood in a swirling cloud of dirt, may have been on to something.
A team of European researchers has discovered that one reason farm children have puzzlingly low rates of asthma may be their exposure to dirt. In particular, contact with microbes found in the excrement of pigs, cows and horses seems to have salutary effects on the immune system.
Children who had relatively large amounts of microbial dust in their bed linens were only half as likely to be asthmatic as children whose sheets contained little of the residue. Furthermore, those who had spent their first year of life on a farm -- which presumably would have given them early contact with the bacterial substances -- appeared to be especially protected.
The findings are the latest evidence supporting the theory that modern man's obsession with cleanliness may be leading to a rise in disorders of the immune system, including asthma.
This "hygiene hypothesis" holds that our well-armed and hair-trigger immunity needs to be turned down and fine-tuned soon after birth. Early exposure to pathogens and other contaminants may be the best way for that to happen -- and it may not be happening enough in our overly fastidious world.
Previous research had hinted that exposure to bacteria might be playing a role in the experience of farm children. The new study, which examined the experience of about 800 Swiss, German and Austrian children, strengthened the observation and pointed to a particular kind of bacteria.
While the new research has no immediate implications for either medicine or child-rearing, it raises the possibility one or more "protective exposures" could be identified and provided to children in the future.
"It's important to narrow down the factors," said Fernando Martinez, a pediatrician and asthma expert at the University of Arizona who was not involved in the study, which is published in today's New England Journal of Medicine. "It could be there's a specific substance we could administer at the right time to children anywhere that would decrease their risk of developing asthma and allergy."
Both the frequency and severity of childhood asthma have been rising in the last generation. Today, about 8 percent of American children aged six to 13 are asthmatic, although prevalence varies widely by race and demographic variable. Twelve percent to 15 percent of inner-city black children are asthmatic. The rate in farm families is about half the national average.
In the new research, Charlotte Braun-Fahrlander of the University of Basel in Switzerland and colleagues from Germany and Austria interviewed parents about the health of their children and drew blood from the children for various tests. All of the children lived in rural areas, with about 300 in farm households and about 500 off the farm.
As a measure of exposure to dirt, the researchers vacuumed the bedding of each child and measured the amount of a compound called endotoxin. Endotoxin is found in the cell wall of many bacteria, and is especially prevalent in microbes living in animals' intestines.
Children whose beds were in the top one-quarter of all beds in terms of amount of endotoxin per area of sheet had about 50 percent the risk of being asthmatic as children whose beds were in the bottom quarter. (The same was true when rural children living off the farm were considered.) The protective effect was even stronger in children who had lived on a farm before their first birthdays.
Blood tests showed that children exposed to the most endotoxin or with the earliest residence on farms had immune system cells that were less reactive to allergic stimulants such as grass pollen and cat dander. The cells also produced fewer of the immune system hormones (called cytokines) characteristic of allergic reactions.
"It fits rather well with a number of studies that suggest that early animal exposure is protective, presumably because of increased exposure to endotoxin in the home," said Dennis R. Ownby, a researcher at the Medical College of Georgia.
Last month, Ownby and his colleagues published a study in the Journal of the American Medical Association reporting that among 500 suburban Detroit children followed since birth, those whose homes included two or more cats or dogs had half the likelihood of being allergic in various skin tests.
The prevalence of endotoxin in inner-city homes is unknown, but pet ownership there is much lower than in the suburbs, Ownby said. According to the hygiene theory, more and more people are entering adulthood with what are essentially over-reactive immune systems. Proponents of the theory argue it could explain not only more allergy and asthma, but also higher rates of autoimmune diseases, such as multiple sclerosis and childhood-onset ("type 1") diabetes.
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