Most U.S. flu resistant to standard drugs

NEW YORK (Reuters Health) - About 90 percent of influenza viruses circulating in the US this flu season are resistant to amantadine and rimantadine, two commonly used antivirals, report investigators at the Centers for Disease Control and Prevention in Atlanta.

These drugs should not be used for the treatment or prevention of flu in the United States until susceptibility to them has been re-established, Dr. Rick A. Bright and his associates advise.

According to their report in the Journal of the American Medical Association, the frequency of resistance increased from 1.9 percent in early 2004 to 11 percent during the 2004-2005 season.

Bright's team obtained influenza isolates collected from 26 US states between October 1 and December 31, 2005.

The results showed that 92 percent of 290 influenza A(H3N2) and 25 percent of 8 influenza A(H1N1) had mutations conferring resistance to amantadine and rimantadine. Moreover, all 10 H3N2 isolates submitted from Mexico and all 3 from Canada also contained the drug resistance mutation -- M2.

The authors note that similar increases in resistance to these antivirals have been reported in Asia.

In a related editorial, Dr. David M. Weinstock and Dr. Gianna Zuccotti, from Memorial Sloan-Kettering Cancer Center in New York, comment that resistance to these antivirals was observed in Asia as early as 1997, and increased markedly thereafter, a phenomenon that they attribute to the availability of over-the-counter amantadine and rimantadine.

Moreover, fears of an avian influenza pandemic have led to inappropriate use of these antivirals among humans and to treat poultry and livestock in Asia. They point out that resistance to the newer class of influenza antiviral agents called neuraminidase inhibitors, which include Tamiflu and Relenza, is also beginning to show up in Japan.

The response to increased antiviral resistance "must be global and immediate," Weinstock and Zuccotti advise. Among their recommendations: education for individuals and communities, prevention of the release of over-the-counter antiviral drugs, and recognition that fears of an avian influenza pandemic may be fueling the drastic increase in resistance.

However, there is hope, they add, that if antiviral use is curtailed, susceptible strains may reemerge so that the drugs could regain their utility against epidemic and pandemic influenza.

SOURCE: Journal of the American Medical Association February 2, 2006.