THURSDAY, Feb. 23, 2006 (HealthDay News) -- The most common type of seasonal flu has now become resistant to widely used antiviral medications, experts say.
Writing in the Feb. 23 issue of the New England Journal of Medicine, Dr. Frederick G. Hayden repeated the recent warning from the U.S. Centers for Disease Control and Prevention that H3N2 (influenza A) has become resistant to two members of the M2 ion-channel inhibitor family of drugs, amantadine and rimantadine.
Prior surveillance had shown that the drugs failed against resistant flu strains in about 2 percent of cases. But that statistic jumped dramatically to about 70 percent in Asian countries by the 2004-2005 flu season, and 15 percent in North America and Europe. By this fall -- as flu season began in North America -- the percentage of amantadine- and rimantadine-resistant isolates in the United States had climbed to over 92 percent.
"This is really unprecedented," said Hayden, a professor of internal medicine and pathology in the Division of Infectious Diseases and International Health and the University of Virginia in Charlottesville.
Flu experts stress, however, that seasonal strains are still responsive to two antivirals being stockpiled to combat bird flu -- Relenza (zanamivir) and Tamiflu (oseltamivir). And vaccines aimed at preventing flu in the first place continue to be effective.
Amantadine is available over-the-counter in China, where it lost much of its punch during the epidemic of severe acute respiratory syndrome (SARS), probably because of increased use. It's not entirely clear why the drug became ineffective in North America so quickly, but "that's what viruses do," said Doris Bucher, an associate professor of microbiology and immunology at New York Medical College, in Valhalla, N.Y.
Growing resistance to the two drugs led doctors to write 1.7 million prescriptions for Tamiflu last year, according to the article. However, the drug isn't yet available in quantities to satisfy current needs for seasonal flu and meet government stockpiling efforts in anticipation of a potential bird flu pandemic.
Inhaled Relenza, effective against both kinds of flu, is also available in limited quantities, in part because it has not been widely used and insurance companies won't pay for it when other drugs are cheaper.
Bucher said there is sometimes resistance to Tamiflu, but Relenza generally works in those cases. "I thought it was very interesting that Hayden said a combination of M2 [ion-channel inhibitor drugs] and neuraminidase inhibitors enhanced antiviral effects. They have to do more work on that, but that's good news," she said.
Generally, viruses become less virulent as they become resistant to medications, Bucher added. But that hasn't been the case with the influenza A.
To combat the illness, there has to be better identification of resistance in viruses and individuals, and the supply of drugs has to be increased, Hayden said. Fewer than 40,000 courses of inhaled Relenza are available in the United States.
"We have to increase production capacity and bring other alternatives to approval so we aren't dependent on the single drug, Relenza," said Hayden.
But it's not time to disregard the other medications, he said.
"We don't know when the [avian flu] pandemic may occur or what the virus type may be," he said. "It could be susceptible to the older class of drugs."
For more on influenza and how to prevent it, head to the U.S. Centers for Disease Control and Prevention.