Opinion: A Cure for the Allergy Epidemic?

WILL the cure for allergies come from the cowshed?

Allergies are often seen as an accident. Your immune system misinterprets a harmless protein like dust or peanuts as a threat, and when you encounter it, you pay the price with sneezing, wheezing, and in the worst cases, death.

What prompts some immune systems to err like this, while others never do? Some of the vulnerability is surely genetic. But comparative studies highlight the importance of environment, beginning, it seems, in the womb. Microbes are one intriguing protective factor. Certain ones seem to stimulate a mother's immune system during pregnancy, preventing allergic disease in children.

By emulating this naturally occurring phenomenon, scientists may one day devise a way to prevent allergies.

This task, though still in its infancy, has some urgency. Depending on the study and population, the prevalence of allergic disease and asthma increased between two- and threefold in the late 20th century, a mysterious trend often called the "allergy epidemic."

These days, one in five American children have a respiratory allergy like hay fever, and nearly one in 10 have asthma.

Nine people die daily from asthma attacks. While the increase in respiratory allergies shows some signs of leveling off, the prevalence of food and skin allergies continues to rise. Five percent of children are allergic to peanuts, milk and other foods, half again as many as 15 years ago. And each new generation seems to have more severe, potentially life-threatening allergic reactions than the last.

Some time ago, I visited a place where seemingly protective microbes occurred spontaneously. It wasn't a spotless laboratory in some university somewhere. It was a manure-spattered cowshed in Indiana's Amish country.

My guide was Mark Holbreich, an allergist in Indianapolis. He'd recently discovered that the Amish people who lived in the northern part of the state were remarkably free of allergies and asthma.

About half of Americans have evidence of allergic sensitization, which increases the risk of allergic disease. But judging from skin-prick tests, just 7.2 percent of the 138 Amish children who Dr. Holbreich tested were sensitized to tree pollens and other allergens. That yawning difference positions the Indiana Amish among the least allergic populations ever described in the developed world.

This invulnerability isn't likely to be genetic. The Amish originally came to the United States from the German-speaking part of Switzerland, and these days Swiss children, a genetically similar population, are about as allergic as Americans.

Ninety-two percent of the Amish children Dr. Holbreich tested either lived on farms or visited one frequently. Farming, Dr. Holbreich thinks, is the Amish secret. This idea has some history. Since the late 1990s, European scientists have investigated what they call the "farm effect."

The working hypothesis is that innocuous cowshed microbes, plant material and raw milk protect farming children by favorably stimulating their immune systems throughout life, particularly early on. That spring morning, Dr. Holbreich gave me a tour of the bonanza of immune stimuli under consideration.

We found our hosts, Andrew Mast and his wife, Laura, hard at work milking cows in the predawn chill.

Dr. Holbreich, slight and bespectacled, peppered them with questions. At what age did Mr. Mast begin working in the cowshed? "My first memory is of milking," he said, at about the age of 5. What about his children, two straw-haired girls, then ages 2 and 3; did they spend time in the cowshed? The elder girl came to the barn at 3 months of age, he said. "People learn to walk in here." Do expectant mothers work in the barn? "Yes," Laura said. "We work."

Dr. Holbreich had made his point: whatever forces were acting here, they were chronic, and they began before birth. As the sun rose, Dr. Holbreich and I sniffed the damp, fermented feed (slightly malty); shoveled fresh cow manure ("Liquid gold," Dr. Holbreich said only half-jokingly, "the best medicine you could think of"); and marveled at the detritus floating in the air. Extrapolating from previous research, with each breath we were inhaling perhaps 1,000 times more microbes than usual. By breakfast time, grime had collected under our nails, hay clung to our clothes, and muck to our boots. "There's got to be bacteria, mold and plant material," Dr. Holbreich said. "You do this every day for 30 years, 365 days a year, you can see there are so many exposures."

The challenge of identifying the important exposures — and getting them into a bottle — is a pressing one. In parts of the developing world, where allergic disease was once considered rare, scientists have noted an uptick, especially in urban areas. China offers a dramatic case in point. A 2009 study found a more than threefold difference in allergic sensitization (as judged by skin-prick tests) between schoolchildren in rural areas around Beijing and children in the city proper. Doctor-diagnosed asthma differed sixfold. Maybe not coincidentally, 40 percent of the rural children had lived on farms their whole lives.

Immigrants from the developing world to the developed tend to be less allergic than average. But the longer they reside in their adopted countries, the more allergic they become. And their native-born children seem to gain the vulnerability to asthma, sometimes surpassing it. All of which highlights a longstanding question in the allergy field. As Dr. Holbreich puts it, "What is it about westernization that makes people allergic?"

Moises Velasquez-Manoff is a science writer and the author of "An Epidemic of Absence."