If you wait long enough, conventional scientific wisdom often reverses itself. For a long time, the conventional wisdom was that if you wanted to remain healthy, you needed to avoid exposing yourself to germs or microbes generally. Doctors and pharmaceutical companies have been happy to pump antiobiotics into our systems, sell us antibacterial soaps and sprays, and encourage the view that if cleanliness is next to godliness, it’s even closer to healthiness. Sterilize yourself and your home! What could possibly go wrong?
In 1989 the hygiene hypothesis was proposed. The hypothesis was that a lack of early childhood exposure to microbes of all sorts may increase susceptibility to diseases later in life by suppressing the natural development of the immune system. Later, a group of US researchers found that children who grew up with dogs had less chance of developing asthma or allergies. Further studies elsewhere, including Europe, have shown that exposure to daycare, multiple siblings, and livestock confers similar benefits.
And just last week I read in Science News about a recent study (“Dog dust may benefit infant immune systems“) that suggests that the microbial mix (aka “dog dust”) that dogs bring into a home may impart immune protection to infants. Specifically, they found that dust from a home with a dog contains a more diverse microbial mix than do those (sad and sorry) homes bereft of dogs. The scientists fed the dog-enriched dust to one group of mice and the dog-deprived dust to another group of mice that were 6 to 8 weeks old. They found that “exposure to the dog-house dust greatly toned down immune reactions” when that group of mice was later exposed to common allergens. When the dog-deprived group was exposed to common allergens, they developed inflammation and excess mucus.
The researchers also found that the intestinal microbiome of the mice exposed to that magical elixir of dog dust was “profoundly” affected. This is especially interesting in light of the growing evidence that our microbiomes—that ecological community of microorganisms that shares our body space—play an essential role in maintaining our health, and, yes, in occasionally making us sick. To take just one example, autism may be linked to problems in the bacterial community of the intestine. I’ve read speculation that autism is related to the “hygiene hypothesis.” Autism appears to be more prevalent in first-world countries, and in middle and upper class households, where antibiotics and more hygienic conditions may in turn result in a less robust bacterial community in an infant’s intestines. (I’ve not read up on this recently, so don’t take my word for it.)
The larger significance of these findings can hardly be overstated. The scientific community is beginning to detail just how much a part of nature we are, and how much the rest of nature is a part of us. Consider that for each of your cells, there are 10 microbial cells in your body; for each of your genes, there are 400 microbial genes. In an important sense, we are not organisms but metaorganisms; thus, try though we may, we cannot isolate ourselves from nature without harming ourselves.
Think about it from an evolutionary perspective. For nearly all of the last 200,000 years, modern humans have been adapting and thriving in the world thanks in part to a deep symbiosis with our microbiome (not to mention the larger ecosystem). In this light, it’s hardly surprising that our very recent attempts to wipe out our microbes are making us sick. What a host of unintended consequences we bring when we kick out the dog and cat, sterilize our homes, gobble antibiotics indiscriminately, and lather up with our antibaterial, antifungal, anti-life soaps.
So breath deep the dog and be well. And if you’re a parent or would-be parent who thinks it’s a good idea to get a dog, well, here’s yet another reason why you’re right. Oh, and one other thing: don’t bother bathing the dog before letting kid and dog play together.