Avian Flu Resistance to Tamiflu (oseltamivir) Antiviral

October 4, 2005

In Hong Kong, researchers have reported that a strain of H5N1 avian flu virus is showing resistance to the antiviral Tamiflu (oseltamivir), which has been stockpiled in case of a pandemic, according to a report by MedPage Today on 30 September.

Resistance to the H5N1 strain of avian flu has been reported in Northern Vietnam, which has reported the greatest number of human deaths from avian flu since its outbreak in 2003. General resistance has also been reported in Japan, where Tamiflu is commonly prescribed for influenza, and where H5N1 has so far been detected only in poultry.

"There are now resistant H5N1 strains appearing, and we can't totally rely on one drug (Tamiflu)," William Chui, Ph.D., honorary associate professor with the department of pharmacology at Queen Mary Hospital in Hong Kong, told Reuters.

The threat is significant - news that the H5N1 avian flu strain is becoming resistant to Tamiflu followed a warning by a top World Health Organization (WHO) public health official that a pandemic could kill millions of people. David Nabarro, MD, executive director of sustainable development and health environments at WHO, stated that a mutant of the current avian flu virus is likely to cause the next flu pandemic, and that its appearance is purely a matter of time.

Alternative to Tamiflu

Public health experts concerned about the Tamiflu resistance are calling on pharmaceutical manufacturers to increase production of Relenza (zanamivir), an alternative antiviral drug. Relenza is administered with an inhaler (Tamiflu is taken orally).

Dr. Chiu suggested that pharmaceutical companies develop an intravenous form of Relenza, since injected drug are more easily absorbed in people with gastrointestinal and acidity problems, according to the MedPage Today Report.

The Current Epidemic

Incidence of the H5N1 avian flu virus has been reported in 11 countries, most in Asia. To date, it has killed 62 people, with the highest fatalities occurring in Vietnam, Cambodia, and Thailand, according to WHO.

Current evidence suggests that all avian flu cases to date have occurred from animal to human transmission, and there is no confirmed evidence to suggest that human-to-human transmission has occurred. However, a report on the WHO website notes that, as the H5N1 strain persists in the environment, it is likely to mutate into a pathogen that can easily pass between humans:

"WHO warns that this animal virus (H5N1) could change into a form which spreads easily from person to person. As people would have no natural immunity, a new influenza virus could cause widespread death, illness, social and economic disruption."

The most recent spread of the virus has been to adults and children in Indonesia. Reports also say that the H5N1 strain is now moving into Europe: tests in Russia have shown the influenza A H5N1 strain has infected poultry, although no human cases have been reported. According to MedPage Today, as a result, the European Union has been stockpiling antiviral drugs, including Tamiflu.

Source: Avian Flu Virus Showing Resistance to Tamiflu, MedPage Today, 30 September 2005.

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