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Exposure to allergens from cats, dogs, cockroaches and mice lowered the risk of developing asthma in urban children, according to a national study led by a UW-Madison doctor.

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Cockroaches, mice, cats and dogs can make asthma worse, but exposure to the pests and pets during infancy can lower the risk of developing the breathing disease, according to a national study led by a UW-Madison doctor.

“To our surprise, cockroaches and mice — the very things we thought were going to be problematic — seem to be associated with less asthma at age 7,” said Dr. James Gern, a pediatric allergist at the university, who headed up the study published Tuesday in the Journal of Allergy and Clinical Immunology.

The findings add to a concept called the “hygiene hypothesis.”

A lack of exposure to allergens and germs in early childhood may suppress the development of the immune system, causing it to go haywire when it is later exposed.

The idea has led allergists to shift advice on peanuts, saying children at increased risk of developing a peanut allergy should consume moderate amounts of peanut protein starting at ages 4-11 months, instead of avoiding the food.

The new findings about asthma could encourage parents with babies to worry less about pets and dust at home, said Dr. Todd Mahr, a pediatric allergist on the American Lung Association in Wisconsin’s leadership board.

“Having the cat and the dog be around your child is not a bad thing, and maybe if you’re not the cleanest housekeeper, that’s not something to beat yourself up about,” said Mahr, who works at Gundersen Health System in La Crosse.

About 8 percent of U.S. children have asthma, in which airways become sore and swollen. They experience coughing, wheezing and difficulty breathing.

In the new research, Gern and others with the Urban Environment and Childhood Asthma study, funded by the National Institutes of Health, looked at 442 children from birth to age 7 in poor neighborhoods of Baltimore, Boston, New York City and St. Louis.

Each child had at least one parent with asthma or allergies, giving the children a higher risk. By age 7, 130 of them, or 29 percent, developed asthma.

Kids in homes with the least amount of roach, mouse, cat and dog allergens in house dust had nearly a 40 percent chance of developing asthma.

Those with the most allergens in dust had less than a 10 percent chance.

The difference was most pronounced for cockroach allergens and least for dog allergens.

Certain types of bacteria found in the dust also seemed to protect children from asthma, the study found.

Having a mother who smoked during pregnancy, or who said she was stressed or depressed, increased the risk of children developing asthma.

Those and other factors appear to make poor, urban environments worse overall for acquiring the condition.

Another study this month, by Gern and others at UW-Madison and Marshfield Clinic, looked at more than 500 rural children, half of them on dairy farms. The farm children had about a third less hay fever and two-thirds fewer skin rashes.

A study of Madison-area children a decade ago found less asthma among those who had dogs, Gern said.

“Whether it’s a dog or a farm setting or, somewhat surprisingly, some of the pests in urban areas, they seem to be having positive effects on the risk of allergic disease and respiratory illnesses, including asthma,” he said.

If specific bacteria or allergens are identified and proven to reduce the risks, drugs or probiotics might be developed to ward off asthma, Gern said.

For now, the prescription is general. “It makes sense to get kids outdoors and have them exposed to lots of different things when they’re young,” he said.

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David Wahlberg is the health and medicine reporter for the Wisconsin State Journal.