Researchers have found a strain of bird flu that can resist Tamiflu, the drug that governments and the World Health Organization are stockpiling in preparation for a widely predicted flu pandemic.
The scientists say health authorities should consider stocking up on more than one anti-flu drug.
The Vietnamese and Japanese researchers, led by Yoshihiro Kawaoka of the University of Tokyo, will publish their findings in Nature on Thursday (20 October).
They isolated the drug-resistant H5N1 virus from a 14-year-old Vietnamese girl.
Most of the virus strains found in her blood were resistant to oseltamivir, the flu drug sold commercially as Tamiflu. This resistance was the result of a single genetic mutation in one of the virus's eight proteins.
"We've been watching for this change [in the virus]," Kawaoka says. "This is the first, but we will see others. There's no question about it."
The researchers acknowledge that their findings are based on a single patient, but say the results suggest, "it might be useful to stockpile zanamivir as well as oseltamivir in the event of an H5N1 influenza pandemic".
It is not the first time flu experts have advised governments to stockpile zanamivir, whose trade name is Relenza (see 'Bird flu: in favour of contingency plans').
In August, Kenneth Tsang and colleagues suggested in The Lancet that H5N1, which has killed 60 people in Asia, would be less likely to become resistant to zanamivir than to oseltamivir.
Kawaoka calls oseltamivir the "first line of defence. It is the drug many countries are stockpiling, and the plan is to rely heavily on it."
It is widely accepted that the global flu pandemic experts have been predicting for more than a year could ensue if H5N1 became able to spread from person to person (see Time to prepare for bird flu pandemic 'running out').
Kawaoka and colleagues say that, as far as they could tell, the Vietnamese patient had no direct contact with poultry. She had, however, taken care of her 21-year-old brother while he was infected with H5N1.
So far, the World Health Organization has not confirmed a single case of H5N1 being transmitted between people.
Last year, researchers suggested that a Thai woman might have been infected with the virus while caring for her infected daughter (see Bird flu deaths raise fears of human spread).
The World Health Organization, however, never confirmed this, or other suspected cases of human-to-human transmission.
Link to full paper in Nature
Reference: Nature 437, 1108 (2005)