The COVID-19 pandemic may have crushed seasonal flu to historically low levels, but another type of flu — avian influenza or bird flu — has showed no signs of slowing.
In the past month, severe bird flu has popped up in poultry farms across Europe and Asia, with Japan confirming its third outbreak for the winter season so far.
The outbreaks follow Japan's worst winter for bird flu yet. More than 3 million chickens were destroyed in 2020-21.
Closer to home, six Victorian farms, including an emu farm, culled hundreds of thousands of birdsin 2020 and early 2021 after multiple outbreaks involving three different strains of the virus.
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While bird flu viruses do generally stick to infecting birds, they occasionally make the potentially deadly leap to other animals, including humans.
In May, for instance, the first confirmed human case of a rare bird flu subtype was reported in China.
Ricardo Soares Magalhães, an infectious disease epidemiologist at the University of Queensland, says this recent spate of new bird flu strains that can hop to humans is unusual.
"Usually, you'd see these viruses just affecting the poultry population, and very few human cases, or none at all," Dr Soares Magalhães said.
"But most of the different examples we've had in the last year-and-a-half have been viruses that had some human transmission."
And despite the COVID-19 pandemic being driven by a coronavirus, epidemiologists are "very wary" when it comes to emerging flu strains, he added, with influenza still at the top of pandemic-potential diseases.
What is bird flu and where does it come from?
Bird flu is caused by a handful of influenza viruses, just like the seasonal flu that circulates each winter.
But while seasonal flu infectionsrise in cool weather, drop off in springand spreadeasily in human populations, bird flu — with the odd exception — is transmitted only between animals or from animals to humans.
It mostly circulates in wild birds, and spreads when migratory waterfowl fly between their summer and winter homes.
Larger birds, such as ducks and geese, tend to ferry bird flu viruses around the world, said Frank Wong, a CSIRO Australian Animal Health Laboratory microbiologist and World Organisation for Animal Health reference expert for highly pathogenic and low pathogenic avian influenza.
(Generally, if smaller migratory birds such as shorebirds are infected with bird flu, they're more likely to delay their migration, or not set out on their journey at all.)
And now, it's peak autumn waterfowl migration time in the northern hemisphere, which is why European and Asian countries are seeing an uptick in bird flu outbreaks, Dr Wong added.
"When the birds congregate [to feed and breed] … mixing of birds also results in mixing of viruses, including influenza viruses. Then when the birds fly south or westwards for the winter, they carry those viruses with them.
"If those wild birds interact with domestic birds, the viruses they're carrying might spill over and cause outbreaks in domestic poultry."
Free-range farming may increase spillover odds too.
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Most bird flu viruses out there are low pathogenic strains, causing little to no disease to the wild birds that carry them.
But the problem is they can quickly become highly pathogenic, incredibly contagious and lethal.
Once a highly pathogenic bird flu virus has made its way into a poultry farm, it can spread rapidly and devastate entire flocks.
What's the bird flu situation in Australia?
Australia is in a pretty good place when it comes to bird flu.
There have been only eight outbreaks of the disease in Australia since the 1970s, withthe biggest happening in Victoria last year.
Unlike Europe and Asia, Australia has no large waterfowl seasonal migrations from abroad, which bring in new viruses each year, Dr Wong said.
"Australian wild ducks are different species to migratory ducks and geese up in the northern hemisphere.
"Our endemic species of ducks are what we call nomadic. They don't travel according to the seasons — they mainly stay within the Australo-Papuan region — and they move according to drought and rain cycles."
Australia also has stringent controls around how poultry is shuttled into and around the country.
And the National Avian Influenza Wild Birds surveillance programme analyses bird poo and the like to keep tabs on the low-pathogenic H7 strains circulating in the wild, Dr Wong said.
Among other biosecurity measures, local regulations state that poultry farms cannot be located near lakes or other bodies of water, Dr Soares Magalhães said: "Just being a few kilometres away can be a risk factor."
That's because the virus doesn't always need direct contact or faeces to spread between birds.
"Because it's a respiratory virus, it can be aerosolised at very large distances. So having those water bodies nearby will attract wild birds, and that means you will have a greater chance of transmission through the air."
Still, these measures aren't completely watertight. And if bird flu is detected in an Australian poultry farm, the policy is clear.
"In Australia, we don't want these viruses around, so regardless of if it's a low pathogenic strain or a high pathogenic strain, depopulation is the way to go," Dr Soares Magalhães said.
Depopulation — or culling — may seem an extreme measure, but the disease can quickly causedebilitation and death, especially if it's a highly pathogenic strain.
"The best strategy from an animal welfare perspective is to depopulate the flock," Dr Soares Magalhães said.
"That happens on a radius of 3 kilometres around the affected zone, and then we impose a surveillance zone out to another 7km."
So that's birds. What about humans?
For a human to get bird flu, they need direct contact with infected birds, or contaminated feathers or faeces. It can't be passed on by eating eggs and cooked meat.
So far, there's been very little human-to-human bird flu transmission, but that doesn't mean new strains won't gain that ability, Dr Soares Magalhães said.
"COVID [which probably originated in bats] is a good example of that."
Pigs can be infected with more than one flu virus, and if that happens, they can act as a virus mixing vessel of sorts to produce new viruses.
Influenza's genetic code, which dictates qualities such as the animals itinfects and its contagiousness, is stored as a strand of RNA. If two (or more) influenza viruses meet in a pig's body, they can swap sections of that RNA strand.
Most of the time, these mutations die out. Occasionally, they might spawn a particularly pathogenic strain.
"The H5N1 virus is a good example of a virus that emerged through the interaction of a poultry virus with a swine virus, and has elements of a human virus," Dr Soares Magalhães said.
"But there are some exceptions to that rule.
"H7N9 is a virus that did not need a pig at all. It came straight from chickens to human beings."
Which are the viruses to watch?
Of the different birdflu viruses, H5 and H7 subtypes have the propensity to mutate from low to high pathogenic strains.
The H5N1 subtype, for instance, was first detected in a human in Hong Kong in the 1990s and, in 2003, kicked off a major bird flu outbreak, killing at least 280 people.
There have been more than 860 confirmed cases of H5N1 in humans to date, 456 of whom died.
"The current lineage of highly pathogenic H5 that's causing outbreaks in the northern hemisphere has adapted to be more amenable for infection in many different wild bird species, including ducks and geese," Dr Wong said.
"This has allowed it to have this rapid seasonal spread, when the conditions are right."
Then there's H7N9, whichwas first reported in humans in China in 2013. It has been reported in more than 1,500 people since and can cause severe disease.
What makesthis virus trickier to contain is that it doesn't produce many symptoms in birds.
"That virus is actually a little more insidious," Dr Wong said.
"When a low-pathogenic H7N9 circulates in chickens or ducks, it's harder to spot because the chickens or ducks may not show signs of disease.
"And the right interplay of genes that virus carried [allowed] multiple spillovers into humans."
Just recently, a third H type was found in humans. China reported that a H10N3 virus hospitalised a 41-year-old man.
When it was detected in birds, epidemiologists weren't overly concerned about it spilling over into humans, because there's no history of H10 viruses infecting us, Dr Soares Magalhães said.
"But then there was a human infection as a result of that virus, and the mechanism of transmission was similar to H7N9, whereby there was no indication of pig involvement."
So how worried should we be about a bird flu pandemic?
The COVID-19 pandemic has seen more funding made available for surveillance programs for diseases such as bird flu.
But even before the pandemic, regions where most bird flu strains first popped up — such as China — really stepped up their poultry farm biosecurity strategies, Dr Soares Magalhães said.
"China will perhaps be the location where new viruses emerge, but they will stop it very quickly."
So it's very much a watch and wait scenario, but we might not have to wait too long.
Dr Soares Magalhães'sspatial epidemiology group is helping a World Health Organization program rank countries in South-East Asia according to their capabilities to control diseases that, like bird flu, can jump from animals to humans.
But the part of the world he has an eye on is further afield.
"It's very likely that new viruseswill start to emerge;not in the traditional countries where that has happened, but in[what was known as] the Eastern Bloc," Dr Soares Magalhães said.
Even though the biosecurity of farms in the region has been scaled up in the past couple of decades, it "still tends to be suboptimal",he said.
Neighbouring Poland is currently grappling with multiple outbreaks of highly pathogenic H5N1 bird flu.
"Poland isthe largest poultry producer in Europe, so they have the largest at-risk population.
"No wonder Poland is bearing the brunt of this."
Climate change could encourage new strains of bird flu to emerge too. As the world warms, migratory birds may spend winter elsewhere, and mix with different bird populations — and viruses.
"Every single year, we will have wildlife-originated influenza viruses in the poultry population, and I'm sure there will be a time when a virus similar to H5N1 will pop up," Dr Soares Magalhães said.
"There's a lot of naive poultry populations out there, and this gives a lot of opportunities for these new emerging viruses to really be devastating.
"Everyone is expecting a big resurgence in the next few years."
