Scientists here are reporting that a strain of the H5N1 avian flu virus is showing resistance to Tamiflu (oseltamivir), the antiviral many health organizations and governments are stockpiling to protect against a potential pandemic.

Tamiflu is proving less effective against the H5N1 strain that surfaced in northern Vietnam earlier this year, Hong Kong scientists reported. The greatest number of human deaths from Avian flu have occurred in Vietnam since the start of the outbreak in 2003.

They also reported that general resistance to the drug is growing in Japan, where Tamiflu is prescribed routinely for common human influenzas. Thus far, H5N1 has been detected only in poultry in Japan.

In light of this bad news, public health experts are now calling on pharmaceutical manufacturers to rev up production of an alternative antiviral Relenza (zanamivir).

“There are now resistant H5N1 strains appearing, and we can’t totally rely on one drug (Tamiflu),” William Chui, Ph.D., an honorary associate professor with the department of pharmacology at Queen Mary Hospital here, told Reuters.

Unlike Tamiflu, which is taken orally, Relenza is inhaled. Chui suggested that drug companies make an intravenous form of Relenza since injected medications can be absorbed more readily in patients with gastrointestinal and acidity problems.

An intravenous form of Relenza could also be given in high doses if necessary and would also kick in faster, a benefit, scientists said, for patients who are very ill and need quick protection.

News of the H5N1 strain’s growing resistance to Tamiflu came just one day after a top World Health Organization public health official warned that a pandemic could kill millions of people. David Nabarro, M.D., executive director of sustainable development and health environments at WHO, said the next flu pandemic will likely be caused by a mutant of the current virus that is currently causing bird flu in Asia. Experts agree that it may be just a matter of time.

To date, H5N1 has appeared in 11 countries, most of them in Asia, and has killed 62 people, with the highest fatalities occurring in Vietnam, Cambodia, and Thailand, according to WHO.

All cases have occurred from animal to human transmission, and at this point there is no confirmed evidence of efficient human-to-human transmission. However, health authorities fear the longer the H5N1 strain continues to persist in the environment, the greater the likelihood it will mutate into a pathogen easily passed between humans.

Most recently, the virus has infected patients, including children, in Indonesia. And now H5N1 is advancing on Europe.

Both domestic and wild fowl in Russia tested positive for the influenza A H5N1 strain, although no human cases have been reported. News of the virus’s presence in Europe sparked the Europe Union to begin stockpiling antiviral drugs, including Tamiflu.

But growing antibiotic resistance to flu drugs is worrisome. Earlier this month in a special online edition of The Lancet, scientists at the Centers for Disease Control and Prevention reported that 12% of influenza A strains worldwide have developed resistance to the most widely used flu medications.

The authors warned that amantadine and rimantadine would eventually lose their effectiveness against influenza. Although their report does not specifically mention the H5N1 strain, the CDC noted that their findings have “broad implications for agencies and governments planning to stockpile these drugs for epidemic and pandemic strains of influenza.”