H7N9 a major threat

The new H7N9 influenza virus identified in China poses a high risk to humans, researchers are now warning.

National Institute for Viral Disease Control and Prevention scientist Jiangfang Zhou and his colleagues have discovered that the newly-emerged flu strain, which has so far caused 132 confirmed human infections, 39 of them fatal, and was detected for the first time in February of this year, can bind equally well to the respiratory tissues of people and birds.

This makes transmission and further human adaptation significantly more likely. And because, further tests have confirmed, vaccination against - or even infection with - seasonal human flu strains confers no protection against H7N9, this means that the potential to trigger a pandemic with a significaint mortality rate is very high because the majority of the world population is vulnerable to infection.

To reach their conclusions, the Chinese group compared how tightly H7N9 flu can bind onto a chemical called sialic acid, which is present on airway tissue.

In humans, sialic acid is linked to the sugar galactose in what's called a 2,6 configuration, and human strains of flu recognise and selectively lock onto this molecular arrangement. In birds the configuration is different and called 2,3.

This difference in shape accounts for the species barrier that prevents bird-adapted strains of flu from spreading rapidly amongst humans. But H7N9 appears, the Chinese study has shown, to bind equally well to both 2,6 and 2,3 configurations of sialic acid, making a jump into humans much more likely.

The new flu strain also grew very efficiently on human lung and trachea (windpipe) samples, targeting in particuar cells called type II pneumocytes, which secrete the surfactant that keeps the airways open.

Destruction of these cells, which are also a target of H5N1 flu, probably contributes to the respiratory compromise seen in patients infected with H7N9.

Tests on the infected tissues also showed that levels of inflammatory signalling chemicals called cytokines were equivalent to infection with H5N1, again likely contributing to the severity of the infection in humans.

Finally, the team also assessed the response of the virus to the currently-available arsenal of antiviral agents. It is resistant to amantadine, one class of anti-influenza drug, but currently broadly sensitive to zanamivir and oseltamivir (Tamiflu), although the researchers caution that two reports of Tamiflu-resistant strains have already been reported in human cases.

"Although no efficient human-to-human transmission has occurred," say the team in their paper in this week's Nature, "biological features we demonstrated, such as dual receptor binding preference and high growth ability, provide the H7N9 virus with higher transmissibility from avian to human."

Soberingly, they go on to observe that "the threats of the H7N9 virus with pandemic potential should not be underestimated and intensive surveillance must be undertaken..."