Australia was the only continent free of H5N1 bird flu until last week, when a brown skua with the virus was found in Cape Le Grand national park in Western Australia, about 700km from Perth. Within days, authorities identified at least 16 other dead birds.
Bird flu can rarely spread to humans in close contact with infected birds or animals and has the potential to cause severe illness. More than 100 human H5N1 cases have been reported around the world since late 2020, mostly in the US and Cambodia.
The risk in Australia is low. H5N1 is still a bird virus and can’t spread easily to humans or from human to human.
How H5N1 bird flu came to Australia
The H5N1 clade 2.3.4.4b arose in 2020 and spread rapidly around the world, infecting bird species that previously weren’t major carriers. This includes charadriiformes – the group of sea birds skuas belong to.
Once it spread to Antarctica and then to Heard Island, 4,000km from Perth, it was a matter of time before it reached Australia.
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H5N1 is typically carried by wild birds, which may then infect poultry, cattle or other animals. In poultry, the virus is highly pathogenic, meaning it will kill most infected birds. This is why the standard response is to cull infected birds.
At this stage, H5N1 has not affected other Australian wildlife, humans or farming. Australia has strong biosecurity and experts hope our farms can be protected.
We’ve had outbreaks of H7 bird flu – but not the H5N1 strain
We have had increasing outbreaks of H7 bird flu affecting poultry farms in Victoria and New South Wales. This is also a strain of bird flu but it has had a less severe impact on poultry globally.
The H7 outbreaks were controlled by culling infected birds.
How can humans catch bird flu?
Human infections may occur in people with close contact with sick or dead birds or animals. Infected birds will typically be dead or severely ill.
Cattle, unlike birds, do not look severely ill. In the US, the infection was noticed when their milk became yellow and thick.
Historically, about half of people who acquire H5N1 from birds or animals die of the infection. But the current H5N1 strain has had a lower fatality rate in the US, killing two of 71 infected people.
What happens when humans get bird flu?
Signs and symptoms in humans include eye redness (conjunctivitis), feeling feverish and flu-like symptoms: sore throat, cough, running nose, body aches, headaches and fatigue.
Less-common symptoms may present including diarrhoea, nausea or vomiting.
What about poultry and eggs?
Chicken, eggs and other poultry products are safe to eat in Australia. There are no outbreaks in poultry farms, nor is the virus widespread in Australian wildlife.
If the virus became widespread, cooking would kill it but handling contaminated poultry products would pose a risk.
Australia has had success in rapidly controlling H7 bird flu outbreaks in commercial poultry farms. But back-yard poultry and eggs may pose a greater risk as these are not subject to the same stringent controls.
People with chickens who notice dead or sick birds should not touch them, and should notify authorities.
What about other animals?
Cats on farms overseas have been infected and develop severe neurological illness – staggering and having seizures.
The US has suffered widespread infection in dairy cattle and poultry, with most cases from clade 2.3.4.4b, the strain that has spread globally. Human cases have mostly been in farm workers.
The virus struck US dairy cattle in 2024 and spread through the country by the trade and movement of infected cattle across state lines, as well as contaminated milking equipment and feed. In the US, cattle feed may contain poultry byproducts but this is not allowed in Australia.
Some studies showed fragments of H5N1 could be found in about 20% to 30% of commercial milk samples. Pasteurisation should kill the virus, so only raw milk would pose a risk.
While the virus concentrates in mammary glands and milk, it is not typically found in beef.
What might happen next?
If the virus continues to spread in Australia, it may affect our poultry and dairy farms. Infected chickens would need to be culled.
In France, the poultry industry was so severely affected it turned to vaccines. These are only partially effective and didn’t stop the outbreaks but vaccines are being used now because the scale of the problem is so great.
Shortages of eggs and chicken may occur in the event of severe outbreaks.
Why doesn’t bird flu spread between humans?
We have different virus receptors in our upper respiratory tract to those in birds, which is why H5N1 cannot easily take hold or transmit in humans.
There have been mutations in the virus that signal some adaptation to mammals, including humans, dogs, cats, dairy cattle, foxes, minks and marine animals. Mutations that suggest human adaptation of the virus have been detected in several animal species.
The more opportunities the virus gets to be in contact with humans, the greater the risk of a mutation arising from reassortment of genetic material from bird flu and seasonal flu.
The virus can reassort all by itself in a pig, which has human- and bird-type receptors.
If the virus mutates further to bind easily to human receptors or to evade our immune system, a pandemic may occur.
While there are no human vaccines for the latest bird flu strain, there are H5 vaccines which may provide partial protection. Finland became the first country to offer such a vaccine to farm workers.
Meanwhile, the flu vaccine protects against seasonal flu. Reducing its burden may help reduce the opportunities for the H5N1 to mutate.
C Raina MacIntyre is a professor of global biosecurity and head of the biosecurity program at the University of New South Wales’ Kirby Institute. Pan Zhang is a PhD student in the biosecurity program. This article first appeared in the Conversation
