A chemical messenger in the blood can tell doctors whether the cause of an asthma attack is a virus or an allergy.

The finding may one day lead to a new treatment for most severe asthma attacks. It would be an important advance. An asthma attack associated with a viral respiratory infection can begin rapidly and be quite severe.

The study found that virus infections are behind nearly 80 percent of severe asthma attacks. In the vast majority of cases, the culprit is the common cold virus, which typically shows symptoms such as fever, cough, and upper airway symptoms such as nasal congestion. Asthma is a disease of the lower airways, making breathing difficult.

Peter G. Gibson, MBBS, of John Hunter Hospital in New Lambton, Australia, and colleagues report the findings in the August issue of the American Journal of Respiratory and Critical Care Medicine.

"In virus-induced asthma there are different mechanisms operating than those described in allergen-induced asthma," Gibson and colleagues write.

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Gibson's team studied 59 asthma patients treated for asthma attacks. They compared them to 15 people with virus infections but no asthma and to 16 healthy people without asthma or virus infections.

The immune-system cells involved in virus-induced asthma, they confirmed, are different from those involved in allergy-induced asthma. That helps explain why drugs that usually help control allergic asthma don't work as well for allergy triggered by viral infections.

It's not always easy to tell what is causing a severe asthma attack. But that may change. Gibson's team found that virus-induced asthma triggers the release of a specific chemical messenger called interleukin 10 or IL-10.

In an editorial accompanying the Gibson study, University of Wisconsin researchers William W. Busse, MD, and James E. Gern, MD, call the findings "a major step forward." They note, however, that the puzzle of virus-induced asthma still misses three big pieces:

—What, exactly, makes a person susceptible to virus-induced asthma?

—How does virus-induced asthma cause difficult-to-treat airway blockage?

—How can virus-induced asthma be treated and prevented?

Gibson and colleagues note that there are many new drugs that block chemical messengers. If IL-10 is the culprit, it may be possible to block its asthma-triggering effects.

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By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Grissell, T.V. American Journal of Respiratory and Critical Care Medicine, August 2005; vol 172: pp 433-439. Busse, W.W. and Gern, J.E. American Journal of Respiratory and Critical Care Medicine,August 2005; vol 172: pp 405-406.