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http://www.nytimes.com/2002/09/19/health/19ASTH.html

Environment Rich in Germs May Reduce Risk of Asthma

By DENISE GRADY

hildren from extremely clean homes may be more likely to develop asthma and hay fever than those who grow up on farms or in families that allow a bit of dirt in the house, researchers are reporting.

Dirt and manure may be beneficial because they are swarming with bacteria, which can help an infant's immune system to mature and develop tolerance — instead of allergies — to environmental substances like pollen and animal dander.

The new findings, published today in The New England Journal of Medicine, add to a growing collection of evidence for the "hygiene hypothesis." This theory suggests that 20th century advances like indoor plumbing, antibiotics and cleaner homes may have contributed to recent increases in allergy, asthma and eczema by decreasing rates of childhood infection. Some infections early in life, the argument goes, help the immune system develop properly.

The new study, led by Dr. Charlotte Braun-Fahrländer of the Institute of Social and Preventive Medicine in Basel, Switzerland, found that farm children were less prone to allergies and asthma than nonfarm children. It also found that the more the children were exposed to a certain bacterial component, the less likely they were to have allergies.

Earlier studies also supported the hygiene hypothesis, suggesting that children who grow up with pets or in large families, or who attend day care — all rich sources of germs — are less likely to develop asthma.

But the studies do not mean that people should buy pets, abandon housecleaning, frequent farms or send their children to day care just to prevent allergies. Researchers say they still do not understand allergies and asthma well enough to make such recommendations. More than 25 genes may play a role, and some families have such a strong genetic tendency to allergies that, as one researcher said, not even living in a barn would protect them. For children who already have allergies, exposure to animals or dust can make things worse.

"It's not so simple that you can isolate one thing and say `If you're exposed to this, it's good, if you're exposed to that, it's bad,' " said Dr. Scott T. Weiss, a professor at Harvard Medical School who wrote an editorial accompanying the new study. "It's way too complicated."

About 17 million Americans have asthma, according to the American Academy of Allergy Asthma and Immunology. In 1999, 4,657 people died of it. From 1980 to 1994, the prevalence of asthma rose by 75 percent, and scientists do not know why. Improved detection and diagnosis cannot account for all of the increase.

The new study included 319 children from farms and 493 from nonfarming homes, all 6 to 13 years old and all from rural parts of Austria, Germany and Switzerland. Among the farm children, 4.1 percent had hay fever and 3.1 percent had allergy-related asthma. The rates were higher among the children not on farms: 10.5 percent had hay fever and 5.9 percent had allergic asthma. Other measures of allergy were also higher in the nonfarm children.

The researchers also measured the levels of a substance called endotoxin in dust from the children's mattresses. Endotoxin is a component of many types of bacteria, and levels tend to be high on farms. In the study, farms did have higher endotoxin levels than nonfarm homes, and the scientists found that in mattresses, the highest levels of endotoxin were associated with the lowest risk of asthma and allergy.

For instance, children whose mattresses had the lowest endotoxin levels had the highest hay fever rates, about 15 percent, while those with the most endotoxin in their mattresses had the lowest hay fever rates, about 2 or 3 percent.

The study showed only correlations between endotoxin levels and allergy; it did not prove cause and effect. Other compounds made by bacteria could be involved as well.

The research may eventually lead to new treatments to be given early in life, perhaps as vaccinations, to prevent asthma and allergies. But endotoxin itself would probably not work as a treatment, said Dr. Donald Leung, head of the pediatric allergy and immunology division of the National Jewish Medical and Research Center in Denver.

"If you give too much endotoxin, you get sick," Dr. Leung said. "Endotoxin is what causes people to go into septic shock. And at high concentrations, it can cause lung disease."

But researchers are looking for other substances that can help the immune system develop properly.

Dr. Weiss said: "We've made a lot of progress. The field is rapidly advancing, and in the next few years there may be specific recommendations or vaccinations."

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