Seasonal flu resistant to two common antivirals

Seasonal flu resistant to two common antivirals

Do not use amantadine and rimantadine

Clinicians should not prescribe amantadine and rimantadine because the influenza A (H3N2) virus currently circulating in the United States has developed almost complete resistance to the antiviral drugs, the Centers for Disease Control and Prevention reports.

The CDC has tested 120 virus isolates in the United States and found that 109 (91%) were resistant to amantadine and rimantadine. That represents a sharp increase from last year when only 11% of isolates tested were resistant. Only 1.9% were resistant to the antivirals the year before that.

"We don’t know what accounts for the unexpected increase in resistance," said Julie Gerberding, MD, MPH, director of the CDC. "One theory is that the virus strain circulating this year may have undergone a spontaneous mutation. We know flu viruses constantly evolve, so this could have happened. Another theory is that it is related to the use of these drugs in over the counter settings in other parts of the world. This has been one of the factors that has been associated with rapid appearance of drug resistance in a number of other categories of infectious diseases and could be playing a role here as well."

However, all H3 and H1 influenza viruses tested to date are susceptible to the other commonly used antivirals (oseltamivir and zanamivir). During this period the CDC recommends oseltamivir (Tamiflu) and zanamivir (Relenza) be prescribed if an antiviral medication is needed for the treatment or prevention of influenza.

"We have those drugs available and we are getting word out to physicians that if they are going to treat or prevent flu with drugs that they should be using [them]," she said. "We don’t think this is going to effect a large number of patients, because not many patients typically are treated with amantadine and rimantadine. But there are some situations where these drugs are used — prophylaxis in nursing homes for example [and] treatments in some clinical settings."

Holding an unusual weekend press conference to rapidly get the word out in mid-January, Gerberding stressed that the situation is not related to H5N1 avian flu. "I do want to emphasize we are talking about seasonal flu," she said. "This has nothing to do with avian flu or flu pandemics."

Influenza activity is starting to pick up across the country as 18 states are reporting either widespread or regional activity. Individuals who have not been vaccinated against influenza still can get vaccinated especially since influenza will continue to circulate for several more months. This year, more than 80 million doses of vaccine have been distributed but remaining supplies vary from state to state so individuals may have to check with more than one provider to receive vaccine. Also, the CDC will have available 3.5 million doses from its stockpile to sell through manufacturers as soon as possible. In addition there are doses of vaccine presently available for sale by manufacturers and distributors.

Testing of influenza isolates for resistance to antivirals will continue throughout the 2005-2006 influenza season and recommendations will be updated as needed. In addition, CDC is working with state and local health departments and clinicians across the country to monitor drug resistance in influenza viruses.