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The H5N1 Avian Influenza

Humans infected with bird flu appear to have more of the virus in their throat and nose than people with standard human influenza strains, a conference is due to hear today.

The findings may help explain why avian influenza A (H5N1) has such a high death rate in humans, more than 50% mortality.

Dr Menno de Jong, head of the virology department at the University of Oxford's clinical research unit at the Hospital for Tropical Diseases in Ho Chi Minh City, studied 17 patients with bird flu.

He found the virus is often associated with disseminated infection in blood and faeces, and with higher levels of viral replication in the nasopharynx compared with contemporary Vietnamese influenza cases.

High viral levels, disseminated infection and an intense inflammatory response also seem associated with poor outcomes, he is due to tell the Australasian Society for Infectious Diseases annual scientific meeting in Wellington, New Zealand today.

"Our main findings are that influenza H5N1 seems to be characterised by high virus levels in the respiratory tract, evidence suggesting disseminated infection [virus detection in blood and rectum] and [likely as a result of this] an intense inflammatory response," de Jong says.

High levels of viral replication are likely to play a role in determining a patient's outcome by direct effects of the virus or by the inflammatory response to the virus, he says.

"The reason for the high mortality probably is not high replication rates per se, but high replication rates of an extremely virulent virus," he says.

Antiviral drugs should be started early to prevent as much inflammatory response as possible, he says.

Two of the people in his study developed resistance to the antiviral drug oseltamivir and died, as reported in the New England Journal of Medicine last December.

Resistance implies suboptimal suppression of replication and strategies to minimise it include ensuring adequate levels of the drug in the body by increasing the dose or giving it intravenously, or combining it with other antiviral drugs, he says.

De Jong says avian-type cell receptors being mostly in the lower respiratory tract could explain why bird flu does not spread among humans, as reported in the journal Nature recently.

This may explain why viral load seems higher in the throat than nose, and why all infected developed pneumonia, he says.

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