Threatwatch is your early warning system for global dangers, from nuclear peril to deadly viral outbreaks. Debora MacKenzie highlights the threats to civilisation – and suggests solutions

Exactly 10 years after H5N1 bird flu exploded across south-east Asia, the virus is still widespread, and has been joined by new killer types of bird flu. Human cases of H7N9 flu are surging in south-east China, and a new type of bird flu, H10N8, has claimed its second human victim, in the same region.

Now it seems that all of these viruses stem from a single, mother virus. Targeting it might stop it from spawning new, deadly viruses in the future.

Few people have heard of H9N2, but this virus was crucial in giving rise to the three dangerous bird flu viruses that have emerged so far in China – H5N1, H7N9 and H10N8.

None of these viruses has yet evolved the ability to spread readily in people and potentially trigger a pandemic – although we know H5N1 can, and H7N9 and H10N8 seem similar. But even if those viruses never go rogue, their cousins might, because the real problem is their common ancestor, which endowed them with the genes that make them dangerous.

The enabler

"H9N2 is the enabler, the one to worry about," Robert Webster of St. Jude Children's Research Hospital in Memphis, Tennessee, told New Scientist. Bird flu is usually a gut infection in ducks, but H9N2 has evolved into a benign respiratory virus in chickens that has spread across Eurasia. When multiple flu viruses infect the same host, they can swap genes. They may be named for their various H and N surface proteins, but H5N1, H7N9 and H10N8 all got some or all of their "internal" genes from H9N2.

Those genes – for the enzymes that replicate the viral genome, for example, or a protein that confuses a host's immune system – can make these viruses dangerous, says Webster. Any of them might become pandemic if they acquire the right mutations to spread in people – or hybridise with a normal human flu.

Closing Asia's ubiquitous live poultry markets would be the key to controlling N9N2, says Webster, as this is where H9N2 and its spin-off viruses spread, mingle and evolve – and where humans catch them.

That is just what China is trying to do. As millions celebrate Lunar New Year this week with home-slaughtered poultry, Shanghai and three other cities have shut their live markets and officials are urging people to eat pre-slaughtered, frozen birds. The continued threat from China's bird flu may depend on whether that catches on. "It's the Year of the Horse in China," says Webster. "I hope they can get the stable door closed before the horse has bolted."

Back with a vengeance

H7N9 emerged in south-east China last spring, infecting 136 people, a third of whom died. In response, Chinese flu scientists called for live poultry markets to be shut last April. Some were – but they were re-opened when flu cases dropped in the following months.

The virus returned with a vengeance at the end of last year, however. So far131 more infections have been reported since October last year. Caitlin Rivers of Virginia Tech in Blacksburg, Virginia, has modelled the spread of H7N9, and says she is convinced that poultry markets are a primary driver of the outbreak.

Now the affected area is growing and south-east China is reporting some half dozen new cases per day, so the call to close markets is strengthening. China's National Health and Family Planning Commission announced this week that live markets should close if they harbour H7N9, after the country's Ministry of Agriculture confirmed that the virus is mainly found in these markets, rather than on farms.

Killer in the family

But H7N9 is not the only problem. The latest killer in the family, H10N8, took the life of a 73-year-old woman in December last year, and now a 55-year-old woman is critically ill with it after visiting a live poultry market. "We know very little about H10 viruses," says Richard Webby, also at St. Jude, and there may be other new viruses that have not been detected yet.

Meanwhile, H5N1 still circulates in Chinese poultry – silently, because it can spread in vaccinated birds without causing obvious disease. A Canadian woman died of H5N1 flu this month after flying home from a visit to Beijing, even though the city has not reported a case of H5N1 in years.

Shanghai and three cities in Zhejiang Province, the hardest hit by the current outbreak of H7N9, have now temporarily shut their live markets. Officials from one of these cities, Hangzhou, say they want to make the closure permanent, and switch to frozen, centrally slaughtered poultry. Zhang Yonghui, head of the Guangdong Centre for Disease Prevention and Control, told China's official news service Xinhua this week that the government should switch the country to such industrial chicken slaughter.

That will not be easy. Hopes that the temporary market closures of last spring would become permanent were dashed when consumers demanded their re-opening. "I can understand the difficulty," says Webster. "Culturally, the Chinese must eat freshly slaughtered poultry on New Year's Day," and they tend to prefer it at any time.

But the risks, he says, have become too high. The world now waits to see what the future holds for China's chickens.