A dog in the house is more than just good company. There’s increasing evidence that exposure to dogs and livestock early in life can lessen the chances of infants later developing allergies and asthma. Now, researchers have traced this beneficial health effect to a microbe living in the gut. Their study, in mice, suggests that supplementing an infant’s diet with the right mix of bacteria might help prevent allergies—even without a pet pooch.

"This paper elegantly illustrates how an environmental exposure protects against an allergic response by mediating the gut [bacteria]," says John Penders, a molecular epidemiologist at the Maastricht University Medical Center in the Netherlands, who was not involved with the work. "Studies like this provide new leads” about how one might manipulate the microbes in the gut to prevent or treat allergies.

More than a decade ago, U.S. researchers reviewing the health records of children with pets—dogs, and, to a lesser extent, cats—discovered that the kids were less likely to develop allergies and asthma than other children were. Other epidemiology studies in Europe have supported this connection, not just with pets, but with livestock as well. In 2010, Susan Lynch, a microbiologist at the University of California, San Francisco, showed that dogs who partly live outdoors shuttled environmental microbes into the house, some of which were also found in the human gut. She and others had already discovered that gut microbes affected immune responses, and so she wondered if the allergy protection provided by pooches happened via gut bacteria.

Lynch and her colleagues collected dust from a house with no animals and from a house with an indoor/outdoor dog. They fed that dust mixed with water to young mice and subsequently challenged the immune systems of the animals by giving them ground-up cockroaches or egg protein, two substances known to elicit allergic reactions in both rodents and people.

Mice receiving dust from the dog’s house weathered the challenge with little to no allergic reaction, but the other mice developed the mouse equivalent of a runny nose and revved up immune activity in their airways, the researchers report online today in the Proceedings of the National Academy of Sciences. In the dog dust-exposed mice, there were fewer allergy-associated immune cells and those that were present produced fewer immune system molecules that tend to lead to a strong reaction.

Lynch’s team surveyed the kinds of bacteria in the mouse guts before and after exposure to the dust. Mice with the dog’s dust—and a less allergenic immune system—had an unusually large amount of a microbe called Lactobacillus johnsonii, the team reports. When it fed that bacterium to mice, those mice had a dampened allergic reaction, even without being exposed to the dog’s dust. Those mice also got less sick when infected with a virus that in humans can cause infants to later become asthmatic. “Our studies suggest that [this bacteria] is a critical mediator of airway protection against environmental insults,” Lynch says.

The new work adds another piece of evidence to the long-debated hygiene hypothesis, which holds that a modern, cleaner lifestyle may make us more susceptible to allergies, asthma, and autoimmune disorders. "There are a lot of studies which show exposure to pets and/or livestock reduces prevalence of allergic disorders, so this is an exciting and provocative step in understanding the mechanism behind that," says Suzanne Havstad, a biostatistician at Henry Ford Hospital in Detroit, Michigan, who was not involved with the work.

While it’s possible dust from the dog’s household directly transfers extra L. johnsonii into a person’s gut, Lynch suspects that other bacteria in the environment get carried into the house on the dogs, become airborne, and are swallowed. Once in the gut, they force a change in that microbial community that favors an increase in L. johnsonii already present.

Before anyone starts thinking about a bacteria-laced dietary supplement for their kids or adopting a dog just to fight allergies, much more work, including clinical studies, would need to be done, Lynch notes. "One should be very careful about transferring results from mouse models to humans,” adds Markus Ege, an epidemiologist at the University of Munich in Germany. “The experimental setting in mice is very artificial.”

Still, Penders says, “[t]he potential of Lactobacillus johnsonii as a probiotic in the prevention of allergic diseases is definitely something that should be further explored.”