Flu: there’s good news and bad news. On the one hand, there have been no new human cases of H7N9 bird flu in China since 8 May. But we can’t celebrate yet. Virologists report today that H7N9 spreads through the air between ferrets, just like the experimental H5N1 bird flu that caused an outcry in 2012. The outcry came after the virus was shown to be able to mutate to spread easily among us while remaining deadly. It now appears that H7N9 bird flu in China can already do that.
Meanwhile, New Scientist can reveal that China is considering closing live poultry markets permanently in an attempt to stop spread of the virus.
H7N9 flu has caused 131 known cases in humans, 32 of whom died, since it emerged in February this year. Tests continue to show that while the virus is a very rare infection of birds in live poultry markets, it is circulating and evolving. But the infection is so rare that those birds can’t be passing the virus to enough other birds to keep it circulating. This means it must also be carried, less rarely, by some other animal – so far unknown.
Worryingly, the virus already possesses several of the mutations that Dutch researchers found would make another bird flu, H5N1, transmit readily in ferrets, the best test animal for human flu. It was not certain that the mutations had the same effect in H7N9, but now researchers have shown ferrets do transmit the virus.
Spread before symptoms
Yi Guan and colleagues at the University of Hong Kong in Pokfulam and colleagues found that H7N9 gives ferrets classic flu symptoms – but they spread the virus before symptoms develop. This means, as with most flu, that it would be impossible to control the spread of H7N9 by isolating people as soon as they show symptoms.
Moreover, healthy ferrets housed in cages with infected ferrets all came down with flu. Three ferrets in a nearby cage also caught the virus, showing it was airborne – though only one became ill. This could be, says co-author Richard Webby of St Jude Children’s Research Hospital in Memphis, Tennessee, because the virus does not transmit efficiently this way, possibly giving the animals only a small dose. This may be why transmission between humans has so far been limited.
The key now, the team says, is to stop H7N9 becoming more common in poultry, as this would “greatly increase” its opportunities to evolve more transmissibility in mammals. “It is essential to stop the virus spreading at this phase as we do not know when the viruses will acquire [greater] human-to-human transmission,” says Guan. “We need to know where the viruses are hiding; which is the major host species that replicates the virus.”
The Chinese government has slaughtered birds and closed poultry markets in major cities throughout the nine eastern provinces affected by the virus. That, and the onset of warmer weather that inhibits survival of the virus in the environment, are probably behind the slowdown in human cases, said Keiji Fukuda, head of flu at the World Health Organization, at the World Health Assembly in Geneva, Switzerland, this week. But, he warns, “it is unlikely that the virus has simply disappeared”.
“We have to continue to work hard to stop this outbreak”, especially by finding the virus’s hosts, says Ron Fouchier at the Erasmus Medical Centre in Rotterdam, the Netherlands, who discovered the mutations that make bird flu transmit in ferrets.
Hua Wang of the Jiangsu Provincial Center for Disease Control and Prevention in Nanjing, China, confirmed this week that markets are a crucial piece of the puzzle, reporting that a human case of the virus in Nanjing was virtually identical to a sample from a cage in the poultry market where she worked (New England Journal of Medicine, DOI: 10.1056/NEJMc1306100).
“The Chinese government is thinking about shutting live markets permanently as a pilot project” in cities like Nanjing and Suzhou, Wang says. Then, if human infections stay low, this could be extended.
But the country’s poultry industry is already down $6.5 billion in lost sales due to H7N9, according to an official estimate. So “there is great pressure to open the markets again”, says Wang.
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