Cockroaches might not seem to have much in common with farm animals. But a new study suggests urban newborns who share their homes with cockroaches, mice and cats might get the same kind of protection from allergies and asthma that farm children seem to get from the animals in their barns.

The finding, published in the Journal of Allergy and Clinical Immunology, is no reason to stop exterminating cockroaches, researchers say. But it is the latest evidence for the “hygiene hypothesis” – the idea allergies might be increasing because many children today grow up in relatively sterile environments. Immune systems that don’t have to fight off many germs end up doing battle with harmless pollens, dust mites and animal dander instead, the theory goes.

Up until now, evidence from urban environments argued against the theory, says study co-author John Wood, chief of allergy and immunology at Johns Hopkins Children’s Center, Baltimore. Previous studies showed low-income urban children in homes with cockroaches and mice – as well as cigarette smoke, high pollution levels and other irritants – had increased asthma risks.

The new four-city study of 467 children shows the same increased risk overall but with a twist: Children exposed to mouse and cat dander, cockroach droppings and certain bacteria before age 1 seemed to benefit. They were the least likely to develop allergies or wheezing – a possible asthma symptom – by age 3.

In other words, it may be all about timing.

“This was a very surprising finding to us,” Wood says, but in line with studies suggesting farm animals and pets offer some early protection. The possibly beneficial bacteria, found in house dust samples, might come from the pests and pets in the homes, he says.

Dirt rich in bacteria and animal dander was found in homes of 41 percent of infants who made it to age 3 without allergies or wheezing vs. 7 percent who did not.

That does not prove the bacteria and dander made the difference, but “this is the very first study to show the equivalent of the farm effect in an inner-city community,” says Mark Holbreich, an allergist in Indianapolis who was not involved in the new study. He has found low levels of allergy among Amish farm children.

“We do believe it is very early exposure” to germs from animals, he says. “We even think there may be some effect prenatally.”

The study is “very exciting,” says Clifford Bassett, a New York City allergist who was not involved in the research. The evolving message, he says, is “don’t oversanitize your world.”

Still, Wood says, he’s not suggesting an open-door policy for mice and cockroaches or an end to house-cleaning. He’s also not suggesting anyone without a cat get one, and he says dogs didn’t seem to help in his study.

But, he says, “20 years ago, we told people they had to remove pets from their home to prevent allergies. We know that’s wrong now.”

The study doesn’t address how babies might get the beneficial effects of dander and bacteria without living with cats, cows, mice or cockroaches. Early doses of beneficial bacteria – probiotics – might help, but research is needed, Wood says.

Wood and colleagues from several institutions also are following the children in the study to see what happens as they age.

He says it remains important for children who already have allergies and asthma to reduce exposure to substances that trigger symptoms. That can include pet dander.

© 2014 USA TODAY. All rights reserved.