Dogs that bring outdoor dust into the house might actually be doing a favor for babies in the home. Research suggests that exposure to doggy dust imparts immune protection to infants. A study of mice shows that the benefits derive from microbes in the dust that enter the intestines and improve the microbial mix, steering the immune system toward fighting disease and away from initiating allergic reactions.
The findings present a microbial twist in the hygiene hypothesis, which argues that a less-than-sanitary early life may prime a child’s immune system against overreacting to grass, dust mites and other ordinary substances. Past studies suggested that babies exposed to multiple siblings, daycare, pets or farm living grow up to have less asthma or allergy.
In the new study, researchers find that dust from a house with a dog contained more diverse microbes than dust from a home with no pets. Since human infants ingest at least some dust, the scientists fed one kind of dust or the other to mice that were 6 to 8 weeks old. Although unappetizing, it had the desired effect: Exposure to the dog-house dust greatly toned down reactions in the mice that were exposed to a common trigger, cockroach allergen.
Mice getting the no-dog dust had inflammation in airways. They also had evidence of excess mucus and immune proteins that are common in allergic reactions. But these reactions were virtually absent in mice that had been primed with the dog dust, the scientists report December 16 in the Proceedings of the National Academy of Sciences.
The researchers also found that the doggy-dust microbes affected the bacteria inhabiting the young mouse intestines. “We saw a profound change in the microbiome,” says study coauthor Nicholas Lukacs, an immunologist at the University of Michigan Medical School in Ann Arbor. The dust-primed mice developed colonies of Lactobacillus johnsonii, a protective bacterium that lives in the mammalian gut.
L. johnsonii provided defense against a common infection. Mice exposed to dog dust better fended off a pathogen called respiratory syncytial virus that often hits babies. RSV is the most common cause of pneumonia in U.S. children under age 1, according to the Centers for Disease Control and Prevention.
The study suggests protection against RSV in newborns is possible, says Ian Mitchell, a pediatrician at the Faculty of Medicine at the University of Calgary in Canada. The findings offer “a glimmer of hope that we might find a way to perhaps delay the time to infants acquiring RSV,” he says. RSV infections that strike later in childhood tend to be less severe.
Newborns’ gut microbiomes are still works in progress. The period when exposure to environmental factors such as dust microbes makes a difference in lifelong immunity may be short. Lukacs says researchers have yet to define this window of opportunity.
Introducing L. johnsonii on its own into the mice’s digestive tract also induced some protection against allergy and infection, but less than the dog-house dust induced. Lukacs says that suggests the dust contains components other than L. johnsonii that have value in orienting the gut microbiome.
K.E. Fujimura et al. House dust exposure mediates gut microbiome enrichment of Lactobacillus johnsonii and immune defense against airway allergens and respiratory virus infection. Proceedings of the National Academy of Sciences. Published online December 16, 2013. doi: 10.1073/pnas.1310750111.
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