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Indoor sheds might protect geese from H5N8 Roland Weihrauch/EPA/Alamy

Bird flu is back, and it’s got nastier – for birds, at least. The H5N8 virus has spread into Europe and is killing wild birds as well as invading poultry farms – a major worry for farmers in the run-up to the festive season. So far the virus doesn’t seem to infect humans, but it is evolving.

The current strain is descended from the H5N1 virus, which started killing poultry in China in 1996, and then people too. H5N1 exploded across east Asia in 2004 with the poultry trade, and then spread into Europe and Africa in 2006, thanks to migrating birds. Since then, the virus has lurked mainly in poultry, especially flu-vaccinated chickens in Asia that can carry the virus while being immune to it. So far, 452 people have died after catching it from poultry.

But viruses like H5N1 have also been moving with migrating dabbling ducks like mallards, which are usually immune to it. Birds from all over Eurasia mingle in north-central Asia during the summer, swap viruses, then disperse back to Africa, Asia and Europe for the winter. This has recently allowed H5N1 to hybridise with other kinds of flu. “We do not know what is driving the plethora of H5s,” although changes in climate and migration may be involved, says Julio Pinto at the UN Food and Agriculture Organization in Rome.


Turkey culls

As Europe’s poultry farmers fatten up their geese and turkeys for Christmas, one hybrid of H5N1 is now giving them sleepless nights.

H5N8 appeared in China in 2014, before spreading with migrating ducks into Japan, Korea and across Russia into north-western Europe, including the UK. It also reached Canada and the US, devastating poultry farms until it was stamped out. It now seems to be gone from North America, says David Swayne at the US National Poultry Research Center in Athens, Georgia.

It appeared to cause few deaths in wild birds in 2014, and failed to reappear in Eurasia the following winter. But in June, H5N8 caused a mass die-off of wild birds in the Uvs-Nuur basin between Russia and Mongolia, a protected biodiversity hotspot.

This time round H5N8 has spread west along northern and southern migration routes into India, the Middle East and Europe, as birds have escaped colder weather in recent weeks. The virus is expected to spread further as lakes freeze and ducks keep searching for open water.

Dozens of farms in Denmark, Switzerland and Germany are now infected. Free-range poultry such as geese have now been moved indoors, away from wild birds. Turkeys are extremely susceptible to the virus, and 9000 turkeys were killed last week on an infected farm in Hungary, a country that produces many turkeys and geese for Christmas.

Killing wild birds

Unlike the 2014 strain, the virus is also killing wild birds, including swans, gulls, grebes and tufted ducks. Ron Fouchier at the Erasmus Medical Center in Rotterdam, the Netherlands, says H5N8 has picked up new genes from flu strains in wild birds, which could be making it deadly to more species. The Friedrich Löffler Institute in Insel Riems, Germany, is now testing different species for susceptibility to the virus.

While humans have so far escaped infection, the World Health Organization says the risk “cannot be excluded”. “You can’t be complacent about these viruses,” says Ab Osterhaus, head of the newly launched Research Center for Emerging Infections and Zoonoses in Hannover, Germany.

Osterhaus’s team found that the 2014 H5N8 strain could infect ferrets, the mammal used to model human flu. And he points to the seemingly harmless H7N7 bird flu outbreak in the Netherlands in 2003 that infected hundreds and killed a vet.

“We need to learn much more about the ecology of these viruses,” he says. “They might just die out in wild birds if they don’t sometimes spill over into big poultry populations.” Farmers fattening turkeys and geese for year-end feasts are hoping this week that they won’t be the ones to suffer the next spill-over.

Read more: Five easy mutations to make bird flu a lethal pandemic