Key to lower allergy and asthma risk: Dirt, dander & germs

Early exposure to bacteria and certain allergens may have a protective effect by shaping children’s immune responses — a finding that researchers say may help develop preventive strategies for allergies and wheezing, both precursors to asthma.

Infants who grew up in homes with mouse and cat dander and cockroach droppings in the first year of life had lower rates of wheezing at age 3, compared with children not exposed to these allergens soon after birth.

The protective effect, moreover, was additive, the researchers found, with infants exposed to all three allergens having lower risk than those exposed to one, two or none of the allergens. Specifically, wheezing was three times as common among children who grew up without exposure to such allergens (51 percent), compared with children who spent their first year of life in houses where all three allergens were present (17 percent).

Infants exposed to a diverse range of bacterial species in house dust during the first year of life appear to be less likely to develop asthma in early childhood, according to a study published on June 6, 2014, in the Journal of Allergy and Clinical Immunology.

Early exposure to bacteria and certain allergens may help shape children's immune responses - leading to a lower allergy and asthma risk.

The first year critical

Through the first three years of life, cumulative exposure to allergy-provoking substances from cats, mice, cockroaches and dust mites — but not from dogs — was associated with more wheezing and allergic reaction in the new study.

This was an expected result, based on earlier research. But this association was reversed when the researchers analyzed exposures for just the first year of life, when greater exposure to certain allergens, those from cockroaches and mice, was associated with less risk of wheezing and allergy at three years.

Some 41 percent of allergy-free and wheeze-free children had grown up in such allergen and bacteria-rich homes. By contrast, only 8 percent of children who suffered from both allergy and wheezing had been exposed to these substances in their first year of life.

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These results indicate that immune responses might be shaped by exposures during the first year of life differently than they are by later exposures. “These findings suggest that concomitant exposure to high levels of certain allergens and bacteria in early life may be beneficial,” the researchers wrote in the journal paper.

“Our study shows that the timing of initial exposure may be critical,” says study author Robert Wood, MD, chief of the Division of Allergy and Immunology at the Johns Hopkins Children’s Center. “What this tells us is that not only are many of our immune responses shaped in the first year of life, but also that certain bacteria and allergens play an important role in stimulating and training the immune system to behave a certain way.”



When researchers studied the effects of cumulative exposure to both bacteria and mouse, cockroach and cat allergens, they noticed another striking difference. Children free of wheezing and allergies at age 3 had grown up with the highest levels of household allergens, and were the most likely to live in houses with the richest array of bacterial species.

UC San Francisco researcher Susan Lynch, PhD, a lead author for the multi-institutional study, also found that exposure during the first year of life to household dust containing higher levels of two specific groups of bacteria that are abundant in the human gut — Bacteriodes and Firmicutes — was associated with less asthma risk in the analysis of data from 104 inner-city babies in four cities.

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Lynch, an associate professor of medicine with the Division of Gastroenterology at UCSF, said there is no obvious mechanism explaining the association, but the evidence supports earlier research that strongly pointed to the influence of microbial species in shaping immune responses.

A benefit from cockroaches for lower allergy and asthma risk?

In an editorial accompanying the study, Dennis R Ownby, MD, professor of pediatrics at Georgia Regents University stated, “If it is true that cockroaches and mice are important sources of both major allergens and potentially allergy-suppressive bacteria, we have an interesting yin and yang, which may explain some of the inconsistencies and apparent contradictions of previous reports regarding allergen exposures and urban asthma.”

The study was conducted among 467 inner-city newborns from Baltimore, Boston, New York and St. Louis whose health was tracked over three years. The investigators visited homes to measure the levels and types of allergens (from cat, cockroach, dog, dust mite and mouse) present in the infants’ surroundings and tested them for allergies and wheezing via periodic blood and skin-prick tests, physical exams and parental surveys. In addition, the researchers collected and analyzed the bacterial content of dust collected from the homes of 104 of the 467 infants in the study.

If the study results are borne out in follow-up research in other populations, it might warrant testing of new strategies, Lynch said, including, “microbial supplementation to inoculate children in early life with appropriate microbes to help protect them against wheezing and allergy.”

Lynch’s own work and research by several others in the field has led her to become convinced that “the composition and function of the gut microbiome strongly influence immune reactions and present a novel avenue for development of therapeutics for both allergic asthma and a range of other diseases.”

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The hygiene hypothesis

According to Boushey, “Strict avoidance of allergens to lower asthma risk has been unsuccessful. Maybe permitting allergen exposures, with increased exposure to the sources of certain microbes, might be more successful in reducing asthma risk.”

Boushey noted that the research team’s new research associating increased bacterial diversity and abundance with lower risk appears to support the “hygiene hypothesis,” which suggests that the increase in the prevalence of allergies and asthma in modern, westernized countries might be an unintended consequence of children being exposed to fewer bacteria in cleaner indoor environments.

Previous research has shown that children who grow up on farms have lower allergy and asthma rates, a phenomenon attributed to their regular exposure to microorganisms present in farm soil. Other studies, however, have found increased asthma risk among inner-city dwellers exposed to high levels of roach and mouse allergens and pollutants. The new study confirms that children who live in such homes do have higher overall allergy and asthma rates but adds a surprising twist: Those who encounter such substances before their first birthdays seem to benefit rather than suffer from them.

Importantly, the protective effects of both allergen and bacterial exposure were not seen if a child’s first encounter with these substances occurred after age 1, the research found.

“If confirmed by other studies, these findings might even have us think of returning to the patterns of exposure of the 1940s, when families were larger, food was less processed and sterilized, and children spent a lot of their time outdoors,” he said.



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