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The Hygiene Hypothesis

Here in North America we’re obsessed by cleanliness. We shower daily, sluice down our kitchens with anti-bacterial soap, try to keep our children from playing in the mud.

Through good hygiene, we’ve eradicated or reduced the incidence of many diseases—but some scientists are now beginning to think we may have gone too far.

Nearly 700 new antibacterial products, from soaps and lotions to toys, mattresses and even computer touch screens, hit the market between 1992 and 1998. But some of us are sicker than ever. The number of people complaining of allergies doubled in the 1990s. The number of people with asthma has increased by at least half in the last 20 years. Other disorders of the immune system, such as Crohn’s disease, multiple sclerosis and Type I diabetes, are also affecting more people than ever before.

An increasing number of studies are lending support to the “hygiene hypothesis,” the notion that we’ve made our environment too hygienic for our own health.

For instance, British epidemiologist David Strachan looked at the records of 17,000 British children and found that the more older siblings they had, the less likely they were to come down with hay fever—a disease which, despite its name, is far more common in the city than the country. Strachan wondered if the older children were bringing home more viral infections to their younger siblings, priming their immune systems so they could better tolerate pollen.

Other studies have shown that children from small families who go to day care before age one have fewer allergies than children who stay home; that children who get oral antibiotics before age two tend to have more allergies than children who don’t; and that children who have daily baths and wash their hands more than five times a day have 25 percent more asthma-like symptoms.

Here’s how the “hygiene hypothesis” works: our immune system has two kinds of lymphocytes (white blood cells), Th1 and Th2. Th1 lymphocytes respond primarily to bacteria and viruses, while Th2 lymphocytes primarily respond to parasites–and promote allergy symptoms such as releasing histamines and floods of mucus (to wash the parasites away).

Normally, Th1 and Th2 lymphocytes exist in balance. Bu maybe a baby whose developing immune system isn’t confronted with enough viruses and bacteria, or not the right kind, ends up with an underdeveloped Th1 system and an overdeveloped Th2 system. The Th2 system might begin treating even harmless things like pollen like dangerous parasites.

Maybe by overemphasizing cleanliness, we’re priming children to develop auto-immune disorders late. In particular, some scientist are beginning to think it’s important for children to have contact with soil, which is laden with mycobacteria. The higher prevalence of asthma in the inner city, they suggest, could be related directly to the scarcity of soil.

If the hygiene hypothesis is true, what can we do? Lessening our fanatical attention to hygiene might help, but could also lead to more colds and flu. Instead, we may have to look at dosing ourselves with the bacteria and parasites we’ve been trying to avoid.

Could this work? Well, a study of Japanese children who were given a tuberculosis vaccine made from a weakened form of TB bacteria (a mycobacteria, related to soil bacteria) discovered they had a much lower rate of allergies and asthma than kids who didn’t receive the vaccine. Studies are underway to see if inhaling or injecting mycobacteria can prevent asthma attacks, and preliminary results are promising.

In an even stranger study, patients with Crohn’s disease, a chronic inflammation of the intestines, have been given drinks laced with the eggs of parasitic worms. Crohn’s disease was first identified in the 1930s, and it is far more common in developed, urbanized areas than in poor, undeveloped areas. Until the 1930s, most kids in North America had intestinal worms; most kids in underdeveloped countries still do. Statistical studies indicate that any country that gets rid of worms begins to see more cases of inflammatory bowel disease.

So Dr. Joel Weinstock, a parasitologist, has recruited volunteers with Crohn’s disease to swallow the eggs of a pathogen-free species of pig whipworm (which can’t grow to full size in a human). All the patients, who were resistant to all other forms of treatment, went into remission. Some of the patients have now been in remission for up to a year with doses of worm eggs every three weeks.

Who knows? In the future, we may tell our kids to eat their dirt and swallow their worms—all in the name of better health.

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    • Ogawa on January 5, 2013 at 5:02 pm
    • Reply

    Hello – can you tell me where I can find more info about the study of Japanese children related to the Hygiene Hypothesis, please? Very much appreciated, thank you in advance.

    1. Sorry, I’m afraid I don’t have any more information on the topic than what is in this column.

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