A Vietnamese girl who came down with the H5N1 avian flu strain appears to have shown resistance to the flu treatment Tamiflu.

That's raising worries that Canada's first line of attack, in the case that bird flu leads to a human pandemic, may not be effective.

A team of researchers, led by influenza expert Yoshihiro Kawaoka of the University of Wisconsin, reports that the 14-year old girl was given the drug in late February yet continued to shed resistant viruses after treatment.

Tamiflu is the brand name for oseltamivir and is thought to be one of only a few available flu treatments effective against the deadly H5N1 strain.

For that reason, many countries have been rushing to stockpile the drug in advance of what many epidemiologists say is an inevitable influenza pandemic. Many are worried that H5N1 could be the strain that leads to a human pandemic

Canada has plans to stockpile just under 16 million pills of Tamiflu, enough to treat 1.6 million people, for the first wave of a mild to moderate pandemic.

But this new study, though based on only a single patient, suggests that oseltamivir may not be effective against the bird flu. The viruses the girl shed were, though, fully susceptible to another drug, zanamivir, sold by GlaxoSmithKline under the brand name Relenza.

Tamiflu still attacks "the vast majority of the viruses out there," said Kawaoka, but he suggests his team's findings underscore the need for stockpiling both drugs, which are part of a class called neuraminidase inhibitors.

The study was to be published in next week's issue of Nature, but was released early by the journal because of its importance.

The other disturbing aspect of the girl's illness is that she appears to be yet another case of human-to-human transmission of H5N1 bird flu. The girl, who survived the illness, likely acquired the infection while nursing her brother, a confirmed H5N1 case.

There have been only a handful of documented cases of human-to-human transmission.

Questions have been raised before about the efficacy of Tamiflu. The World Health Organization reported in May that a virus isolated from another Vietnamese patient showed partial resistance to the drug.

And studies of the drug's effectiveness on human strains of the flu have shown that resistance does occasionally occur. For instance, a study of Japanese children treated with the drug found about 16 per cent developed resistance.

Bird flu has killed more than 60 people in Asia since 2003, most of them poultry farmers who had close contact with their birds.

Health experts have long feared that H5N1 could soon mutate and acquire the ability to spread easily from person to person.

The World Health Organization says that the risk of transmission of bird flu to humans remains low and is therefore not worried that the virus appears to have spread from Asia to Turkey and possibly to Romania.

No human cases of the disease have been detected in Europe.