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Hygiene Hypothesis: Are We Too "Clean" for Our Own Good?

Increased hygiene and a lack of exposure to various microorganisms may be affecting the immune systems of many populations - particularly in highly developed countries like the US - to the degree that individuals are losing their bodily ability to fight off certain diseases.

That's the essence of the "hygiene hypothesis," a fairly new school of thought that argues that rising incidence of asthma, inflammatory bowel disease, multiple sclerosis and perhaps several other diseases may be, at least in part, the result of lifestyle and environmental changes that have made us too "clean" for our own good.

"Medicine has a lot of history behind it related to why certain diseases are so widespread and certain diseases are not widespread," said Subra Kugathasan, MD, Medical College of Wisconsin Associate Professor of Pediatrics (Gastroenterology), who has made a study of developments in hygiene hypothesis research.

"The immune system is there for a reason, said Dr. Kugathasan. "It's there to recognize 'the bad guys.' The immune system allows your body to kill those bad guys and allows you to survive. In order to harden the immune system, the immune system requests some kind of stimuli all the time."

"The hygiene hypothesis suggests that the more hygienic one becomes, the more susceptible one is to various autoimmune diseases. The autoimmune diseases, the diseases that result from all the activation of your immune system, are increasing. The hygiene hypothesis - and we don't yet have a proof of it - acknowledges that the maturation of the immune system needs some kind of hardening, some kind of resistance. Put another way, you cannot really build up good muscles without doing exercise."

From Pet Dander to Pig Worms
The common belief that has driven medicine, as well as public perception and hygiene practices, is that when we get sick it is because of something we ate, or inhaled, or were exposed to in other ways. The hygiene hypothesis points in a different direction, proposing that in many diseases it is a lack of exposure to the "bad guys" that causes harm.

While the evidence was by no means clear-cut, one study indicated that in some cases contact with certain pet dander in the home actually decreases a child's risk of wheezing from asthma later in life. Other studies show that children who lived on farms when they were very young have reduced incidence of asthma, which has led several researchers to conclude that organisms in cattle dust and manure may be the stimuli that their immune systems needed to fight off asthma.

In another study, conducted by University of Iowa Division of Gastroenterology director Dr. Joel Weinstock, intestinal worms were shown to have a very dramatic effect on mice in offering protection from inflammatory bowel disease. This was followed up using whipworms from pigs, Trichura suis, in a small number of humans. The worms were selected because they are "safe," as many pig farmers come in contact with them every day, they do not enter the human bloodstream, and they cannot live in the human intestine for more than a week.

All of the six patients who were given the worm treatment for their bowel disease eventually went from chronic illness to complete remission with no diarrhea, no abdominal pain and no joint problems. In very general terms, this small-scale test of the hygiene hypothesis worked because microorganisms from the worms positively affect the body's immune response to bacteria and viruses.

"Think about countries in Africa like Gambia, a country that has been studied very well," said Dr. Kugathasan. "Ninety to ninety-nine percent of people in Gambia have intestinal worms at some point in their lives. But the chronic immune diseases like asthma, Crohn's disease, or multiple sclerosis are not heard of, never even mentioned in their life. They don't know anything about such diseases in those countries. While one may argue that maybe their population is genetically not predisposed to these diseases, other factors appear to be in play."

"What has happened now, with globalization and human migration, people move to areas that are very, very clean. Within one generation we have moved into a different environment. What we have been finding out is that in the second generation of Asian, Latin American and African children, where the first generation had been exposed to those kinds of parasites and early childhood infections, the second generation that has moved to 'cleaner' countries has not been exposed. The incidence of Crohn's disease, multiple sclerosis, and chronic asthma is as common in the second generation from the third world as in those with European or North American backgrounds, and in some cases even higher."

Playing in the Dirt
Dr. Kugathasan and others interested in hygiene hypothesis have not proposed that "playing in the dirt," or making society less hygienic in general, are useful goals in medicine. But they do propose that taking the impact of reduced immunological strength into account for certain diseases could be beneficial.

For example, researchers who are looking into the impact of microorganisms produced by cattle on asthma in children maintain that the more they learn about how cattle exposure relates to asthma, the closer they will come to developing an effective preventive treatment.

"Over the years, what's happened with modern medicine is that we have become more aware of the disease process, so we are avoiding diseases by learning more about how they spread," said Dr. Kugathasan. "We are becoming much cleaner and learning how to prevent many diseases by immunization. And we are isolating ourselves by not going into epidemic areas. Now we don't even allow kids to play in the yard barefoot. Children playing in the dirt barefoot are exposed to a lot of microorganisms and worms and everything else, and that's not happening the way it used to."

"So the hygiene hypothesis doesn't only apply to Crohn's disease and inflammatory bowel disease," Dr. Kugathasan says. "It applies to many other conditions. This doesn't mean children should roll around in the dirt or necessarily change medical practice in the US. But to keep the immune system working properly, you need controlled stimulus or else it doesn't know how to recognize the bad guys. Treatment is meant to suppress the system, while the hygiene hypothesis suggests that it doesn't always hurt in the long run to give stimulus the other way around."

It's important that a child go through normal childhood illness, for example, notes Dr. Kugathasan. "When we visit the doctor to suppress a lot of things like colds, rather than, in effect, letting nature run its course, we're making immediate treatment the priority rather than long-term prevention, using the analogy of immunological 'muscle-building.' We know that antibiotics wipe out normal cells, too, but you don't want to destroy what medical science has accomplished. Maybe there's no going back, but it's important that we take what the hygiene hypothesis is telling us into account when treating our children."

Dan Ullrich
HealthLink Contributing Writer

Article Created: 2004-09-24
Article Updated: 2004-09-24

MCW Health News presents up-to-date information on patient care and medical research by the physicians of the Medical College of Wisconsin.

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