Infants are much less likely to suffer from allergies or wheezing if they are exposed to household bacteria and allergens from rodents, roaches and cats during their first year of life, the study found.
The results stunned researchers, who had been following up on earlier studies that found an increased risk of asthma among inner-city dwellers exposed to high levels of roach, mouse and pet droppings and allergens.
“What we found was somewhat surprising and somewhat contradictory to our original predictions,” said study co-author Dr. Robert Wood, chief of the Division of Allergy and Immunology at the Johns Hopkins Children’s Center in Baltimore. “It turned out to be completely opposite — the more of those three allergens you were exposed to, the less likely you were to go on to have wheezing or allergy.”
About 41 percent of allergy-free and wheeze-free children in the study grew up in homes that were rich with allergens and bacteria. 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.
The study was published June 6 in the Journal of Allergy and Clinical Immunology.
The findings support the “hygiene hypothesis,” which holds that children in overly clean houses are more apt to suffer allergies because their bodies don’t have the opportunity to develop appropriate responses to allergens, said Dr. Todd Mahr, an allergist-immunologist in La Crosse, Wis., and chair of the American Academy of Pediatrics’ Section on Allergy & Immunology.
Prior research has shown that children who grow up on farms have lower allergy and asthma rates, possibly due to their regular exposure to bacteria and microbes, the researchers noted in background material.
“The environment appears to play a role, and if you have too clean of an environment the child’s immune system is not going to be stimulated,” Mahr explained.