Children infected with a common stomach bacterium are less likely to have asthma than other kids, according to a study that will appear in the Aug. 15 Journal of Infectious Diseases.

The bug in question, Helicobacter pylori, is a microbe with a history like no other. A longtime resident of the human stomach, H. pylori went largely undetected until Australian scientists discovered it in 1979 and went on to show that it can cause stomach ulcers. Further work has linked it to stomach cancer. It’s now treated with antibiotics whenever detected.

Because H. pylori had been hitchhiking in humans for so long — possibly 50,000 years or more — microbiologist Martin Blaser of New York University became interested in the possible consequences of knocking it out.

He suspected that widespread antibiotic use has been suppressing H. pylori infections in industrialized countries over the past half century. During that same time, asthma has increased markedly.

Blaser and his colleague Yu Chen analyzed a database of health information obtained from people who enrolled in a national health study in either 1999 or 2000. The researchers focused on children, identifying 4,787 who didn’t have an H. pylori infection upon entering the study and 2,625 others who did. Questionnaires completed by study participants (or their parents) showed that children ages 3 to 13 with H. pylori were less than half as likely to have had asthma as were kids without an H. pylori infection.

Children with H. pylori were even less likely to have had, in the previous year, a bout of allergic rhinitis, which is marked by a runny nose, itchy eyes and inflamed nasal passages. And they were less apt to suffer from wheezing, the researchers report.

Blaser cautions that the association does not prove that an H. pylori infection prevents asthma, a chronic condition in which lung passages can become inflamed by contact with an allergen, smoke, pet dander or any number of other substances. Like allergies, asthma is an overreaction of the immune system to an innocuous substance.

Nevertheless, it’s possible that an H. pylori infection might somehow quell the immune system, Blaser says, or more likely induce the production of compounds that do. Or, H. pylori might just be a marker of something else that protects against asthma, he says.

It’s unclear how H. pylori spreads, but children living in messier households could have more H. pylori infections, says gastroenterologist David Graham of the Baylor College of Medicine and at the Michael E. DeBakey Veterans Affairs Medical Center, both in Houston. Thus, its apparent antiasthma effect might actually result from poor hygiene, he says.

Under a school of thought called the hygiene hypothesis, children who grow up in squeaky clean environments have more asthma and allergies than do kids raised in contact with farm animals or in other less sanitary conditions. The idea is that the immune systems of children in messy environments get regular challenges and thus mature properly.

Even if H. pylori did prevent asthma, the infection is not worth having, Graham says. “One would not allow king cobras to live in one’s house just because they might eat rats,” he says. “H. pylori is a king cobra equivalent in terms of the harm done to humans.”

Patients would be better served if doctors could find a way to condition the immune system to achieve the effect of a dirty environment without the negative consequences, Graham says.

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