Dear Science,
Recently, a friend who is in public health told me about the hygiene hypothesis: One in five Americans has an allergy, while the number in third world countries tends to be far lower—possibly due to our excessive cleanliness. Should I be taking a daily dose of mud with my multivitamin? Does the hygiene hypothesis have any scientific merit, or is it just the next fad for people who don’t vaccinate their kids?
Baking Dirt Pies
The hygiene hypothesis has a neat symmetry to it: Our immune systems, bored by our crisply clean modern environment, are battling false enemies—random junk floating around (allergies) and our own bodies (autoimmune diseases). The correlation that you noted—places with tons of infectious diseases have comparatively low rates of allergies and autoimmune disease—holds true even as people move around the planet. Southeast Asians in Southeast Asia are much less likely to have allergies than Southeast Asians living in the United States.
Animal studies provide hints of how this might work—there is evidence of classes of immune cells left fallow too long suddenly lashing out against something, anything. It all makes a bit of sense. Human beings have one of the strongest and most violent immune systems of any living thing. Blame it on selection bias. We’re the survivors of filthy cities, or the descendants of people who lived for generations in filthy cities. Smallpox, syphilis, plague, measles, mumps, rubella, pertussis, tuberculosis, worms of all shapes and sizes, malaria—if you’re alive today, some miserable ancestor of yours from merely one or two hundred years ago had to survive an onslaught of these. This is no small feat.
With all this said, the hygiene hypothesis still isn’t completely proven. For humans at least, it remains a correlation rather than demonstrated cause and effect. We still don’t quite know which sorts of infections, if any, would be the right ones. Being continuously infected with worms, viruses, bacteria, and tuberculosis isn’t so great for health, either—allergy-free or not. Scientists right now are trying inoculations with gentle bacteria (probiotics), purified parts of these pathogens, and even gentle stimulators of the fallow immune cells, all as a way to get the possible benefit of being exposed to disease without the downsides of having, say, malaria swimming in your blood. As you can imagine, these are slow and careful studies, done on otherwise healthy people. Nobody wants to make people ill in the effort to demonstrate some protection from controlled exposure to filth.
So, for now, let your kids play in dirt. But vaccinate them. What is vaccination, other than safe, controlled exposure to some of these pathogens? If you believe this hygiene hypothesis argument, vaccination matters even more.
Cleanly Yours,
Science
Send your science questions to
dearscience@thestranger.com.
Thank you for addressing this No-Vaccination madness! Please get your kids vaccinated.
Enter snarky, neo-cynical-hippie, survivalistic comment from CTMCMULL…
Fuck dirt. I’m eating a hookworm.
You don’t have to LET them.
If there’s ONE unhygienic item in the room, turn your back for ONE second, and any child worth his salt is trying to put it in his mouth…
🙂
Some things never change.
Do not bait the troll.
I’ve heard a version of this theory, but it’s entirely based around the early development of our immune systems and has nothing to do with our adult bodies getting, er… bored or whatever. Nothing to do with nationality, either.
The basic premise is that kids who grow up on farms or around filthy animals have a lot fewer allergies than kids who grow up in ultra-sanitized, air-purified, animal-free environments because their rapidly growing immune systems have been exposed to such a wide variety of germs at that important early stage of development. The theory says that there’s an important evolutionary reason for why toddlers stick everything they possibly can into their mouths (to sample their surroundings and develop immunities) – and that kids who don’t get exposed to certain biological agents until later on have missed out on their best shot at ever developing an effective immunity.
Are we talking about the same theory, here?
Oh, and parents: my non-expert, but reasonably sane advice is to vaccinate your kids and buy a (naturally filthy) pet for them to play with.
@5, I believe you’re mostly right about this being a mostly developmental issue. (sorry about all the “mostlies”). But there is still quite a lot of development and remodelling of the immune system in adults. It’s why we get autoimmune diseases in middle-age, probably, rather than because of something that happened in childhood. So, exercise those immune systems all you grownups. Eat rasperries and cherry tomatoes fresh out of the garden, without rinsing, and cuddle and kiss your pets, before you go for a winded jog in the woods (breathing in the dusty complex environment very deeply). By-the-by, the effective pet ratio is that cats are probably the most effective antigenic stimulants, and dogs are somewhat less so. Two cats should probably provide all the stimulus you need. They look clean, cats, but they are germy fluffy bags of antigens.
“So, for now, let your kids play in dirt.”
And play in the dirt with them. If you have a yard, do some gardening and make the kids help. Getting a little dirt under your fingernails can be extremely therapeutic, whether or not it has anything to do with allergies.
I am an ob/gyn in buenos aires, argentina, working in a very large maternity hospital that mostly serves migrant population from Bolivia Perú and Paraguay. I am constantly exposed to virus and bacteria, and as far as tuberculosis goes, I have a positive dermic test to it, going to prove that I’ve been exposed, infected but never sick and probably won’t ever be. (and that’s mainly because I’ve been vaccinated against it at birth and again at six, a vaccine that hasn’t been administered in the USA for years) An american doctor who comes in direct contact with TB acts like it’s the plague.
Often international medical organizations such as doctors withouth borders and mediciens du monde pick up third world doctors for their missions, not only because they are used to adverse conditions and work better in low tech environments, but because they are inmune to most shit that goes around.