Good news for having a dog that's a bad housekeeper

The author's dog

You may have heard that children who grow up in households with animals are less likely to suffer from allergies. But why is this the case? Kei Fujimura from the University of California, San Francisco and his colleagues may have the answer. It’s all about the microbes. Oh and dust. Dust is important too.

The human body is only 10% human by genome. The rest of us is made of microbes that inhabit every part of our bodies, inside and out. The specific composition of those microbes, known as the ‘microbiome’ is proving to be critically important for a whole host of health issues, including allergies. 

Allergies result when the immune system attacks normally harmless items, like pollen. People who aren’t allergic suffer no adverse health affects from breathing in pollen; there’s no reason some people’s immune systems need to protect them from such non-threats. Yet they do and with a vengeance. People can and do die from environmental allergies. 

Where do microbes come in? Our gut microbes can influence whether our immune systems will go into hyperdrive when they encounter harmless bits of fluff. And those microbes are in part determined by what kinds of dust we’ve been exposed to as infants.

Houses with a dog have four times more dust than houses without a dog. As a dog owner, I’d say this is a conservative ratio. Young mice fed dust from the dog homes (yum!) had fewer allergic symptoms later on than mice fed dog-free dust. This was true even when the total amount of dust was the same and when the potential allergen was not dog-related. The dog dust simply protected mice from having allergies.

The researchers found that exposure to the dog dust had altered the microbial composition of the mouse guts. Many of the enriched microbes included bacteria (like Lactobacillus johnsonii) known to down-regulate the allergic response. In fact, feeding the mice L. johnsonii by itself had much the same effect as feeding them the dog dust.

Those helpful microbes could have been ingested with the dust, or something else in the dog dust could have favored the proliferation of resident L. johnsonii. Either way, it seems clear that it was the change in the gut microbes that was protecting the mice from allergies.

It's possible that doctors may one day treat allergies by dosing patients with L. johnsonii. Until then, I'm sticking with having a dog and not cleaning my house.


Fujimura KE, Demoor T, Rauch M, Faruqi AA, Jang S, C Johnson C, Boushey HA, Zoratti E, Ownby D, Lukacs NW, & Lynch SV (2013). House dust exposure mediates gut microbiome Lactobacillus enrichment and airway immune defense against allergens and virus infection. Proceedings of the National Academy of Sciences of the United States of America PMID: 24344318.