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Summer 2003 Issue
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FEATURES
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THE DIRT ON CLEANLINES:

It May Not be Bad for Your Health

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When medical research challenges cultural beliefs held by millions of people, it's guaranteed to attract a lot of attention – especially when it involves a near sacred virtue like cleanliness.

The ink was barely dry on newspapers worldwide regarding new data by French physician and researcher Jean-Francois Bach when he visited UC Davis School of Medicine and Medical Center in October. Bach, whose research supports a controversial theory on the healthfulness of dirt, was in Sacramento to deliver the school's annual Nelson Medical Lecture. The theory explores the impact that improved hygiene and better socioeconomic conditions have had not only on reducing infection rates worldwide but also on the increasing incidence of allergies, asthma and other autoimmune diseases, especially in industrialized countries.

Known as the "hygiene hypothesis," the theory holds that early contact with some germs and dirt arms the maturing immune system against some allergic conditions, much as vaccines protect against disease. It also suggests that the decrease of infections brought about by civilization has undesirable consequences, such as reducing the resiliency of the immune system. The hypothesis has been gaining support in the medical community through both epidemiological and laboratory studies. Bach's most recent research – showing that European farm children are much less likely to have allergies and asthma than their non-farm-raised counterparts – has added more data to the growing body of evidence. Featured in the Sept. 19 issue of the New England Journal of Medicine, the study garnered an editorial and coverage by the international press.

"Dr. Bach's study really brought the hygiene hypothesis home with respect to the dramatic increase of allergies in the western world," said M. Eric Gershwin, chief of the Division of Rheumatology, Allergy and Clinical Immunology at UC Davis School of Medicine and Medical Center. "Asthma rates in California alone have risen from 4 percent to 11 percent of the population in a generation, largely due to environmental factors."

In an interview following his UC Davis visit, Bach said he wasn't surprised by the attention his study received.

"The study attracts attention because of the very interesting interaction between the evolution of our society and the occurrence of major diseases," said Bach, who is chief of the Laboratoire d'Immunologie Biologique at the University of Paris, Necker Medical School.

Bach's study analyzed the environments and asthma rates of more than 800 children in rural areas of Switzerland, Austria and Germany residing in farm and non-farm homes. Among the farm children, 4.1 percent had hay fever and 3.1 percent had allergy-related asthma. The rates were higher among the children not on farms: 10.5 percent had hay fever and 5.9 percent had allergic asthma. The study found that other measures of allergies were also higher in the children not living on farms.

Bach discovered that increased exposure to germs from animal waste coincided with a markedly low rate of asthma and allergies. He found that exposure at a young age to endotoxins – bits of bacterial cell walls from farm animals and other sources – may actually strengthen the immune system.

"This is the first paper that puts together the application of the hygiene hypothesis to allergic and autoimmune diseases, with a comprehensive discussion of postulated mechanisms," Bach said.

According to the hygiene hypothesis, humans have always had close contact with dirt and animals, and this exposure to bacteria stimulated the immune system. A sheltered immune system, some researchers say, tends to be high-strung and will overreact to pollen, particles and dust.

"Throughout history, as soon as we got rid of parasite infestations, allergy rates would rise," said Gershwin. Even today, he said, immigrants from rural areas of Southeast Asia develop allergies when they settle in the United States and are rid of certain kinds of parasites.

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  According to the hygiene hypothesis, humans have always had close contact with dirt and animals, and this exposure to bacteria stimulated the immune system. A sheltered immune system, some researchers say, tends to be high-strung and will overreact to pollen, particles and dust.  
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