In a European study that echoes the findings of other scientists, researchers have found that children who grow up on farms are less likely to develop childhood asthma.
The risk of asthma was reduced by as much as 51 percent for children living on farms, and researchers suspect that it's the diversity of exposure to different microbes that may offer protection against the airway disease.
"The risk of asthma decreased with an increase in the diversity of microbial exposure," said study author Dr. Markus Ege, a researcher at Munich University Children's Hospital in Germany.
"Within the microbial spectrum under investigation, several germs with a potential for asthma prevention were identified," he added.
Results of the study are published in the Feb. 24 issue of the New England Journal of Medicine.
Asthma is a chronic lung disease that causes inflammation of the airways. This inflammation narrows the airways, making it difficult to breathe. Symptoms include wheezing, coughing, chest pain and tightness, and shortness of breath, according to the National Heart, Lung, and Blood Institute (NHLBI).
The exact cause of asthma remains unknown. People with a family history of the disease are more likely to develop asthma, suggesting a possible genetic component to the disease. But environmental exposures, such as encountering certain viruses, are also associated with the development of the condition, according to the NHLBI.
Numerous studies have also found that exposure to a variety of microbes in the environment, such as bacteria or fungi, appear to provide protection against asthma, perhaps by helping the body create a tolerance for allergens.
And, other research has found that children living on farms -- especially those exposed to cows, pigs and hay -- seem to be less likely to have asthma and allergies.
Exactly how exposure to a variety of microbes might protect against asthma is still unclear, researchers noted.
"How these microorganisms reduce the asthma risk remains largely unknown," said Ege. "A combination of environmental microorganisms might stimulate the innate immune system and counterbalance an asthma-prone immune status. Alternatively, the diversity of microbial exposure might prevent specific [disease-causing] microorganisms with an asthma-inducing potential."
The current study assessed the incidence of asthma in children living on farms compared to children without farm exposure in two different groups of youngsters. The researchers also tested dust samples from the children's bedrooms and mattresses to get an idea of what type of microbes the children were regularly exposed to.
The first group of children included 6,843 rural and suburban elementary school children from southern Germany. This study, dubbed PARSIFAL, also included dust samples from 489 children's mattresses. The second study, called GABRIELA, included 9,668 elementary school kids from rural areas of Austria, South Germany and Switzerland. Dust samples were collected from 444 children's bedrooms in this study.
House dust from farm homes had a greater variety of bacteria and fungi, a finding that was statistically related to a lower rate of asthma.
In both study populations, youngsters who lived on farms had lower rates of asthma and allergies, according to the study. In the PARSIFAL group, the risk of asthma was reduced by 51 percent in children living on farms. In GABRIELA, the reduction was 24 percent, according to Ege.
The researchers also found that diversity of microbial exposure lowered the odds of asthma by 38 percent in the PARSIFAL population and by 14 percent in GABRIELA, according to the study.
"Not all microbes are detrimental; some are beneficial for asthma and allergies," explained Ege. But, he cautioned that this doesn't mean that dirt or microbes in general should be considered healthy, or that parents should try to deliberately expose their children to microbes.
"There are enormous numbers of different microorganisms which affect human health by various measures and in opposite directions. There are still dangerous infections, which should be prevented by vaccination or treated with antibiotics," Ege stressed.
One expert who wrote an accompanying editorial in the same journal noted that it was "of critical importance" to figure out how microbe exposure affects the biology of the lungs and immune system.
"This study shows that kids who grow up on farms are less likely to develop asthma, but I don't think there's anything that can be applied to treatment right now," said Dr. James Gern, a professor of pediatrics and medicine at the University of Wisconsin School of Medicine and Public Health in Madison.
"As we learn more about what these protective exposures are," he said, "it's possible we could come up with ways to prevent asthma in the future."
Read more about the causes of asthma from the National Heart, Lung, and Blood Institute.
SOURCES: Markus Ege, M.D., M.P.H., researcher, Munich University Children's Hospital, Germany; James E. Gern, professor, pediatrics and medicine, University of Wisconsin School of Medicine and Public Health, Madison; Feb. 24, 2011, New England Journal of Medicine