Study Finds Some Resistance to Flu Drugs

The problem comes from overusing the antivirals Tamiflu and Relenza, experts say

TUESDAY, April 3, 2007 (HealthDay News) -- Japanese researchers are reporting signs of resistance to two popular antiviral drugs commonly used to fight the flu strain known as influenza B.

The two drugs, zanamivir (Relenza) and oseltamivir (Tamiflu), have been effective against the flu and are widely used, particularly in Japan, according to the report in the April 4 issue of the Journal of the American Medical Association.

"We are concerned about emerging resistance patterns through overuse of these drugs," said Dr. Marc Siegel, a clinical associate professor of medicine at New York University School of Medicine. "This study is another caution to the overuse of a drug based on fear."

These drugs are designed to be used by people who have just gotten the flu or are in close contact with someone with the flu, Siegel said. "These drugs should not be given when you hear on the news that miles away someone has the flu," he said.

There has been evidence of influenza A virus that is resistant to Tamiflu, but information on influenza B viruses has been limited. Influenza B viruses are the cause of annual flu outbreaks, the researchers said.

In the study, Dr. Shuji Hatakeyama, of the University of Tokyo, and colleagues looked at the cases of influenza B caused by reduced sensitivity to these drugs in Japan.

In 2004-2005, there was a major outbreak of an influenza B in Japan, and Hatakeyama's team collected influenza B isolates from 74 children before and after Tamiflu therapy and from 348 untreated flu patients.

The researchers found a variant of the virus that had a reduced drug sensitivity in 1.4 percent of the children who had received Tamiflu. In addition they found that 1.7 percent of the flu viruses from untreated patients had reduced sensitivity to Relenza, Tamiflu, or both.

"Continued surveillance for the emergence or spread of neuraminidase inhibitor-resistant influenza viruses is critically important," the authors wrote. "Further evaluation of the biological properties of neuraminidase inhibitor-resistant influenza viruses is needed to fully assess their pathogenicity in humans," they concluded.

Siegel added: "The overuse of Tamiflu based on fear of influenza can breed resistance. You want to save your drugs for proper use, so you don't breed resistance."

More information

For more information on flu, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Marc Siegel, M.D., clinical associate professor of medicine, New York University School of Medicine, New York City; April 4, 2007, Journal of the American Medical Association
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