Public health authorities are reporting three cases of the pandemic H1N1 influenza in which the virus was resistant to a key antiviral drug.
Until now, all tested strains of the novel H1N1 flu have been susceptible to oseltamivir, known by the brand name Tamiflu, an antiviral drug that can be used to prevent or treat influenza.
But the three cases of a drug resistant virus -- one each in Hong Kong, Denmark, and Japan -- suggest that resistance could arise.
Although most experts say they are not alarmed so far, they note that if widespread resistance to oseltamivir arises, it could take an important weapon out of the hands of doctors.
"Tamiflu has been the main stockpiled drug to be used for treatment and prevention of pandemic influenza, and obviously rapidly developing resistance would be a problem if it were to occur," said Dr. Christopher Ohl, associate professor of medicine at Wake Forest University School of Medicine's Section on Infectious Diseases in Winston-Salem, N.C. "A Tamiflu-resistant virus would not respond to Tamiflu treatment or preventive administration."
What is worrying some is that one of the cases of resistance occurred in a woman in Hong Kong who had not been treated with the drug. In the other two cases, the patients had been given the drug as a preventive measure, according to Dr. Keiji Fukuda of WHO.
Fukuda said the agency is monitoring the situation closely, but no other cases have yet turned up. Moreover, he said, there's no evidence that a resistant strain is beginning to circulate.
"Right now this looks like spontaneous mutation in these patients," Fukuda said at a press conference Tuesday. He said all three patients had the same mutation, and all three had "uncomplicated" disease from which they made full recoveries.
He added there's also no evidence of a resistant strain in any of the contacts of the three patients.
However, drug resistance is an issue with other strains of flu. The U.S. Centers for Disease Control and Prevention said that nearly all of the tested strains of seasonal H1N1 flu this season were resistant to oseltamivir, although there was less resistance among circulating strains of H3N2 influenza A or influenza B.
For that reason, the CDC in June recommended that physicians treat all suspected flu cases, whether seasonal or pandemic, with the other approved member of the same class of drugs, zanamivir (Relenza), wherever possible.
Until the three cases of the pandemic H1N1 virus emerged, there had been no sign of oseltamivir resistance in that strain. Fukuda said there is still no evidence that a resistant strain is being transmitted.
Even so, "The emergence of drug-resistant viruses is not unexpected," said David Topham of the University of Rochester, "especially in light of the increased use of influenza antivirals in the context of this recent pandemic."
Topham cautioned, "Overuse of the antivirals will almost certainly hasten the emergence of resistant strains, which will leave us fewer options for treating those who truly are in need."