News that Japan is to cull a record number of chickens this year because of bird flu is the latest grim indicator of the gravity of what has been described as the worst-ever outbreak of the disease.
Its latest avian flu case, found at a farm in the south-western prefecture of Miyazaki, brings to more than 10 million the number of poultry slaughtered in the country’s October-to-May peak period for infections.
Highly pathogenic avian influenza, consisting of influenza viruses related to H5N1, has killed many wild birds and there have been several outbreaks among poultry across the world, including in Europe and the US.
More than 57 million birds died of avian flu in the US last year, a record number for the country.
Cases have surged across Europe in recent weeks. In France, 217 outbreaks were detected at farms by December 20, more than double the number from only two weeks before.
Berlin Zoo was forced to temporarily close in November due to the discovery of an infected bird. It reopened on Christmas Eve.
What is causing the disease in birds?
Bird flu was a regular seasonal headache, outbreaks resulting in whole flocks of poultry having to be culled.
What makes the current situation worrying is that there are many more outbreaks and the disease has become a year-round problem.
"The virus continues to tune itself up," says Prof Ian Jones, professor of virology at the University of Reading in the UK.
"In the last few years, it’s been very successful in wild birds, particularly migratory wildfowl. Anywhere on a migratory route has seen a lot more activity.
"It always used to be the case it would start in September or October, and would be over by March, and we had summers free of disease. In the last two summers it’s almost endemic, and the same on the [European] continent and US."
Last year was the UK’s worst for bird flu. In England and Wales, farmed birds must currently be housed indoors, meaning free-range eggs cannot be produced and there is pressure for increases in the prices of eggs and poultry.
Could people be at risk?
There are frequent warnings that if H5N1 mutates and begins spreading between people, it could set off another pandemic.
But although many wild birds have died, and there have been multiple outbreaks in farms, the risk to people is not high, especially as current forms of the virus are thought to be less dangerous to humans than those that circulated in previous decades.
People who work with birds, or children who find and play with the carcass of a bird that died from the condition, may be more likely to be infected with the virus, Prof Jones says. But he adds that there is no evidence it is infecting people more easily.
"It’s never shown any propensity to transmit between people," he says.
Late last year, the UK's Health Security Agency described the risk to people as "very low", because "very close contact" with birds is needed.
"Although none of these strains easily infect people and they aren’t usually spread from person to person, small numbers of people have been infected around the world, leading to a small number of deaths," the organisation said.
According to the agency, in recent years five strains of avian influenza have threatened people: H7N9, H9N2, H5N6, H5N8 and "a type of H5N1 strain more commonly found in Asia".
In less serious human cases it can cause conjunctivitis and mild, flu-like symptoms, but it can lead to severe, potentially fatal complications, including pneumonia.
How long will the current outbreak last?
Prof Jones says he expects that at some point in the future, the type of influenza that is circulating and causing the current outbreak will be replaced by a different form. This could ultimately lead to a lessening of the danger.
However, because wild birds are a "natural reservoir" for the disease, it is, he says, not set to disappear in the foreseeable future and will likely remain a threat to wildlife and farmed birds.
"It shuffles its genetic material and comes up with a new variant, but I don’t see any reason why it would change quickly," he adds.
"But if a new variant arises and it’s more successful, it will naturally displace H5N1. We have to just get used to H5N1 being around most of the time and keep up with the biosecurity measures that prevent it from getting from the wild population to the farmed population."
