Close contact with farm animals' microbes, bolster the immunity of children who grow up on traditional Amish farms in the United States and protects them from asthma.
The study in the August 4 edition of the New England Journal of Medicine
compared two similar communities -- the Amish of Indiana and the Hutterites of South Dakota -- which have different methods of farming.
‘Dust in Amish homes was much richer in microbial products, and engages the children's innate immune system to fight off asthma.’
The Amish live on single-family dairy farms and use horses for fieldwork and transport.
Hutterites use modern, industrialized farm machinery to work their larger, communal farms, which distances them somewhat from the daily exposure to animals.
Otherwise, many aspects of the two groups are similar. They have similar genetic ancestry, having come from Central European immigrants, and follow a traditional Germanic farming diet.
They also drink raw milk, get their children vaccinated, breastfeed their babies and do not allow indoor pets. But their asthma rates are vastly different.
About five percent of Amish schoolchildren have asthma -- about half the US average.
Meanwhile, Hutterite children have an unusually high prevalence of asthma -- 21.3 percent -- said the study.
"Over a decade ago, our colleague Erika von Mutius discovered that growing up on a farm can protect against asthma," said study co-author Carole Ober, professor and chairman of human genetics at the University of Chicago.
"Our new study, building on her work, now shows that it's not simply farming, and has narrowed in on what the specific protection might be."
Researchers found that the difference came down to the kind of dust inside their homes.
Dust in Amish homes was "much richer in microbial products," said the study.
"Neither the Amish nor the Hutterites have dirty homes," explained Ober.
"Both are tidy. The Amish barns, however, are much closer to their homes. Their children run in and out of them, often barefoot, all day long. There's no obvious dirt in the Amish homes, no lapse of cleanliness. It's just in the air, and in the dust."
This dust engages the children's innate immune system to fight off asthma, the study found.
Blood tests done on 30 Amish children -- aged seven to 14 -- and 30 Hutterite children of the same age range showed the Amish had more blood cells crucial to fighting infections, known as neutrophils.
The Amish also had fewer blood cells that promote allergic inflammation, known as eosinophils.
When researchers exposed lab mice to the Amish house dust, they found the rodents were protected against asthma-like responses to allergens. Similar experiments with the Hutterite house dust conveyed no protection.
"We hope that our findings will allow the identification of relevant substances that will lead to completely novel strategies to prevent asthma and allergy," said co-author Erika von Mutius, professor at Dr. von Hauner University's Children Hospital in Munich, Germany.