A mountain lion, bear and skunk are some of the latest victims of bird flu virus spreading around the world.
The highly pathogenic avian influenza (HPAI) known as H5N1 has scientists worried.
Since 2021, the virus has gone gangbusters and made its way to every continent except Australia and Antarctica.
"We know the virus is extremely widespread in the Northern Hemisphere and is now starting to move into South America, which it's never done before," said Ian Barr, deputy director of the World Health Organization (WHO) Collaborating Centre for Reference and Research on Influenza.
The situation has become so dire that countries in the European Union and the US Biden Administration are now considering testing avian vaccines.
Until now, many countries including Australia have been reluctant to vaccinate poultry because of the risk of bird flu spreading from vaccinated but asymptomatic poultry into other animals, as well as export trade implications.
A spokesperson from the Australian Department of Agriculture, Fisheries and Forestry (DAFF) said the government was monitoring the international situation and supported vaccination when effective — and regularly updated — vaccines become available.
"We encourage countries currently experiencing significant losses of poultry due to HPAI outbreaks to take up vaccination," they said.
While routine vaccination for bird flu is currently banned in Australia, that could shift in the future.
"If Australia were also suffering or facing such losses, we would be encouraging our own industry to adopt vaccination," the spokesperson said.
So why have things become so grim, so fast? And what is Australia doing to keep it at bay?
Let's start with the virus itself.
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Evolution of H5N1
Bird flu is caused by a handful of viruses. Most of these are low-pathogenic viruses that circulate all the time in wild birds and cause little to no disease.
Only certain strains of two subtypes — H5 and H7 — cause severe disease and death. These highly pathogenic subtypes first evolved in poultry then spread out to wild birds, then back again.
H5N1 first appeared on the scene in 1996 but it only started to go global in 2003, according to Marcel Klaassen of Deakin University who has been monitoring avian influenza in wild birds over the past two decades.
Then in 2014 a new lineage appeared in Korea and before long, it jumped from Europe into North America.
"Then I thought, 'Oh, man, now the shit has really hit the fan,'" Professor Klaassen said.
Over the following four years there were more waves of bird flu, but nothing like the current wave that started in October 2021, driven by a new variant known as 22.214.171.124b.
Professor Barr said there is now evidence this variant is hooking up with other bird flu viruses in North America, making it even more virulent.
"It's concerning that there's so much virus out there that has spread into so many species," Professor Barr said.
How bad is bad?
The death toll is staggering.
Around half a billion poultry birds around the world — more than 58 million in the US alone in the past year — have died of H5N1 bird flu.
And it's not just poultry.
As headlines about turkey shortages in the lead up to Thanksgiving rang out in the US last year, veterinarians in Spain detected the virus in a mink farm that killed nearly 52,000 animals.
Meanwhile, the official numbers only tell part of the story for wild birds, Professor Klaassen said.
"The number of wild birds recorded thus far is 100,000, but I think it's grossly underestimated.
"It's in the millions, rather than in the hundreds of thousands reported in the databases."
Colonies of gannets have disappeared in Scotland, and at last count the disease has ripped through about 236 different wild bird species as diverse as bald eagles, vultures, pelicans and penguins.
"We have no idea how our wild birds are really impacted by this current lineage in the Northern Hemisphere," Professor Klaassen said, adding that COVID and the war in Ukraine had prevented monitoring of populations in Siberia.
Why are we worried about the mink farm?
The mink farm outbreak raised fears the virus had evolved to spread between mammals for the first time.
"If it is true, it's concerning because we haven't seen mammal-to-mammal transmission before," Professor Barr said.
But, he said, it was still too early to tell if there had been mammal-to-mammal transmission, or if the minks, which were kept in open sheds, had come in contact with infected birds.
"I don't think you can jump to the conclusion, because there was an outbreak at the mink farm, that it was transmissible, even though it did affect a lot of animals."
Mammals were known to be infected by H5N1 well before the mink farm outbreak.
"When this virus first broke nearly 20 years ago, it killed zoo animals like tigers when they were fed carcasses of birds that were infected," Professor Barr said.
Since January 2021, there have been 186 outbreaks of H5N1 in mammals affecting 17 species including foxes, otters and seals, in addition to bears, mountain lions and skunks.
Despite the name "flu", highly pathogenic strains of avian influenza cause more than a respiratory illness, often affecting the central nervous system and brain of mammals.
The concern is if H5N1 becomes prevalent in mammals, it could mutate and spread more widely to humans.
But so far the risk to humans from H5N1 is low, according to the WHO, which has registered a total of 868 cases and 457 deaths since 2003.
"Considering the number of wild birds and poultry that have been infected and disposed of, sometimes with not all the care you might like, the number of infections has been remarkably low in humans," Professor Barr said.
"It's concerning, but it's not panic stations at this time."
Currently, another lineage of HPAI known as H7N9 is more of a problem for humans.
What are we doing in Australia?
If H5N1 entered Australia, it would decimate poultry and wild birds just as it has overseas, and there are fears some species, such the Australian black swan, could be wiped out.
Australia has had homegrown outbreaks of another type of highly pathogenic virus known as H7N7, which was caused by the evolution of low pathogenic viruses carried by local wild birds into a deadlier form.
The latest in 2020 caused the culling of poultry on several free-range farms in Victoria.
But Professor Klaassen said H5N1 would be harder to contain if it got into wild birds.
Luckily, for now, Australia has a geographic advantage.
Avian influenza in Europe, Asia and North America is mainly spread by large species of waterfowl that don't migrate to Australia.
But there is a small chance H5N1 could be brought to Australia by migrating shorebirds.
While a sick bird might struggle to find its way here from Siberia, the increasing amount of virus circulating in the Northern Hemisphere has upped the ante in the past 12 months.
Professor Klaassen and his colleague Michelle Wille of the University of Melbourne have been tasked with coming up with a risk assessment for Australia.
"We need to remain vigilant because even if only one shorebird migrates with HPAI onboard and arrives in Australia, this could trigger outbreaks," Dr Wille said.
Their research on low pathogenic viruses identifies where and how wild birds could spread a highly pathogenic virus in Australia, and which species to target for surveillance.
"We have essentially given ourselves a bit of a head start so that if things kick off, we don’t need to scratch our heads about how to best do surveillance," she said.
So far, the news is good. There have been no cases of H5N1 influenza found in migratory shorebirds that flew in before summer.
"That is remarkable that we didn't find this specific strain," Professor Klaassen said.
"And on top of that, there have not been any mass mortalities here."
But this could change when the shorebirds return next spring after the breeding season in the Northern Hemisphere.
Surveillance is also undertaken in commercial poultry flocks, and the disease is notifiable.
"Commercial poultry producers should continue to have appropriate biosecurity measures in place to avoid wild birds coming into contact with their flocks," the DAFF spokesperson said.
"Strict border control measures and import conditions are also enforced to stop the disease from entering Australia via imported birds and poultry products."
How effective is vaccination?
"The Western world has always shied away from vaccination because they want to eliminate any evidence of disease in the population and don't want these diseases circulating around," Professor Barr said.
"But if this virus is going to hang around and become endemic in some countries, I don't see any alternative, unless you have extremely good biosecurity in your animal husbandry."
Professor Klaassen agreed, saying, "it's not looking good unless you do something dramatic like vaccination" but added that it must be done the right way.
Research he conducted in Bangladesh shows vaccines wiped out signs of bird flu on poultry farms, but the virus continues to be spread by asymptomatic poultry, and circulate at high levels in live bird markets and wild birds.
"You can easily protect your poultry, but if you don't also protect your wild birds ... they become the victim."
Dr Wille agreed, saying that vaccination reduced the amount of culling in the poultry industry and reduced spillover into other species.
"If we have learnt anything from the H7N9 outbreaks, it will likely decrease the number of human (and mammalian) cases," she said.
"But vaccines need to be effective, protective and administered properly, and post-vaccination surveillance needs to be done."