Police in Mexico take precautions.

By Kent Atkinson of NZPA

Wellington, April 27 NZPA - Two New Zealanders expert in genetic shifts and mutations of influenza strains say there is a risk that the swine flu virus spreading out of Mexico may acquire resistance to Tamiflu.

Also known by the chemical name oseltamivir, Tamiflu has been stockpiled by the World Health Organisation (WHO), New Zealand's Ministry of Health and governments worldwide for use in the event of an influenza pandemic.

It slows the spread of the flu virus in the body and is a key tool for protecting essential workers such as doctors and nurses during the three months or more that it may take to develop effective vaccines for a pandemic flu strain.

But resistance to the retro-viral drug is increasingly being found in seasonal flu viruses in New Zealand -- four last year and one this year -- in line with similar surveillance in other southern hemisphere countries and the United States.

Dr Sue Huang, head of the WHO National Influenza Centre run by New Zealand's Institute of Environmental Science and Research, told NZPA the centre was monitoring oseltamivir resistance to guide policy on the use of Tamiflu.

Dr Huang said from Geneva, where she is attending a WHO meeting on national influenza centre contingency plans, that it was "extremely important" to monitor oseltamivir resistance because of the reliance being placed on Tamiflu stockpiles in the event of a pandemic.

She warned there was a possibility that as the swine influenza spread among people already exposed to Tamiflu-resistant strains of influenza, the swine flu may evolve resistance to the drug.

"Influenza virus is notoriously unpredictable," she said.

"It is very important to monitor closely in order to provide early warning if (this) situation ... emerged."

New Zealand virologist Richard Webby, director of the WHO collaborating centre for studies on the ecology of influenza viruses in lower animals and birds at St Jude Children's Research Hospital in Memphis, Tennessee, told NZPA the risk of a Tamiflu-resistant swine flu evolving was a "valid possibility".

"Flu viruses do exchange gene segments and it is possible that the swine virus could swap out its neuraminidase (the target for oseltamivir) with a human strain," he said.

Problems would arise if the resulting virus was able to grow well and transmit between humans.

Dr Huang, based at the National Centre for Biosecurity and Infectious Diseases at Lower Hutt, said that the Tamiflu resistant strains of seasonal flu found in New Zealand had all been from the influenza A family. Strains resistant to oseltamivir emerged in the Northern Hemisphere in January last year, and since then five viruses of the A/H1N1 strain had been detected in New Zealand patients, and reported to WHO.

According to the WHO, tests on H1N1 flu patients in 12 countries last year showed 31 percent had the H274Y mutation associated with Tamiflu resistance.

Southern Hemisphere incidence of the mutation in tests on the H1N1 virus ranged from 100 percent in South Africa to 13 percent in Chile.

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