Asthma Epidemic (and Hygiene Hypothesis)

In todays NY Times health section, the topic of asthma is discussed.

In any case, about 20 million people in the United States have asthma today, including at least 6 million children, and 5,000 people a year die from it. Children in the inner cities seem to be especially hard hit, with exposure to cockroaches and diesel fumes suspected as the culprits. But the cause is not known for sure.

The article discusses some reasons for the increased prevalence of asthma. Diet, obesity, exposure to allergins and air pollution and my favorite topic the hygiene hypothesis

One theory that has received attention recently is the “hygiene hypothesis,” the idea that children today are raised in homes that are too clean and that asthma is somehow caused by the lack of exposure to infections and bits of microbes early in life. Under this theory, germs are supposed to help the immune system develop normally, and without them the system may overreact to other substances in the environment, producing allergies and asthma.

There is some evidence to support the idea. Studies find that children raised on farms are less prone than others to asthma, maybe because they are exposed to plenty of microbes in barns and stables. But the connection is still not fully understood, and some viral infections clearly make asthma worse.

The entire NYT article is based on paper recently published in the New England Journal of Medicine entitled: The Asthma Epidemic by Waltraud Eder, M.D., Markus J. Ege, M.D., M.P.H., and Erika von Mutius, M.D. (2006, 355(21):2226-2235).

This is figure 4 from the journal article. It shows the various ways in which different types of exposures interact with different economic living conditions, resulting in variations in the prevalence of asthma.


Figure 4. Effect of the Interaction between Various Types of Exposures and Various Genetic Backgrounds in a Range of Racial and Ethnic Groups on the Prevalence of Asthma through Pathways Involving Atopy, Airway Inflammation, Airway Hyperresponsiveness (AHR), or Other, Unknown Factors.

Unfortunately, both articles give short shrift to the involvement of helminth infections in the hygiene hypothesis.

Chronic infestation with helminths may also confer protection, but short-lived episodes of infestation may exacerbate atopic disorders.

This article: Allergy, Parasites, and the Hygiene Hypothesis published in "Science" provides the reasoning behind helminth infections and the hygiene hypothesis. Full text is not freely available but since it's in "Science" you should be able to find it in your local library.

An interesting article entitled Eating Dirt by Gerald N. Callahan can be found here at the CDC.

A few hygiene hypothesis links:

http://www.pbs.org/wgbh/evolution/library/10/4/l_104_07.html

http://www.respiratoryreviews.com/jan03/rr_jan03_hygiene.html

http://www.ucsfhealth.org/childrens/health_library/reuters/2005/
09/20050923elin024.html

http://www.sciencenews.org/pages/sn_arc99/8_14_99/bob2.htm

http://www.mult-sclerosis.org/news/Jul2000/TheHygieneHypothesis.html