Friday, January 30, 2009

On pets and the hygiene hypothesis

Over the past few days, the animal and science blogs have been full of stories about the "hygiene hypothesis" with the release of several new studies. For those who haven't heard of it, the hygiene hypothesis is the idea that the developing immune system needs to be exposed to antigens (in the form of dirt, bacteria, viruses, parasites, pet dander, foods, etc) in order to properly learn how to respond to such insults, and that kids who are raised in very hygienic environments without exposure to such antigens end up with immune systems that don't work right. The uneducated immune system, in the search of work to do and antigens to destroy, attacks the normal cells of the body and becomes hyperreactive to normal everyday antigens like wheat or dust mites. Proponents say that this is why we're seeing an increase in the prevalence of allergies, asthma, and autoimmune disorders like type 1 diabetes.

The hypothesis has been in the news lately because of a recent study out of Tufts, which found that exposing research animals to various internal parasites could prevent and reverse autoimmune disease, like type 1 diabetes in mice. In order to survive in the host for any length of time, internal parasites release various substances that modulate the host's immune system. That explains why infecting a person with worms can have an effect on inflammatory or autoimmune disorders.

The authors suggest that a baby's desire to explore the world with his mouth is an evolutionary adaptation to train the immune system and expose himself to bacteria, viruses, and internal parasites that will help to regulate the immune response. The authors also suggest that you should have "two dogs and a cat" as your child is growing up so that he can be exposed to their internal parasites, which is... er... a little arbitrary and kind of wrong. First, why would it take two dogs but only one cat? Second, it seems unethical to leave your pets untreated for worms so that you can expose your kids (on multiple levels)- deworm your poor pets!

Besides that little suggestion, the study is really interesting and lends some more credibility to the hygiene hypothesis! We already knew that raising kids with dogs and cats provides protection against the development of allergies and asthma, so this is one more study to show that interaction with the dust, dander, and dirt of nature is good for humans.

Another study was recently released, written by an alum of the U of MN vet school (yay!). In response to the common warning that you shouldn't let your dog lick you on the face, she looked into whether dog owners who do let their dogs give them full-on mouth kisses share more strains of E. coli (a normal GI bacteria for both dogs and humans) with their pets than dog owners who don't allow smooches. The results?
There was no evidence that owners who sleep with their dog or allow face licking were more likely to have shared strains of E. coli, according to the study, which was expected to be published in an upcoming issue of the American Journal of Veterinary Research.
That's good news for me, since, let's face it, I'm a dog-kisser (and cats, and the occassional velvet-nosed horse)- nothing crazy, of course, but I think being comfortable with a dog licking your face is some sort of pre-requisite for getting into vet school.

So, what do these studies teach us?
  • Eating dirt is not necessarily bad, and might actually be essential for appropriate immune system development.
  • If you live with a dog, don't freak out if he gets the occassional smooch in.
  • If you live with kids and dogs, well............

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