The graceful black swan is a familiar sight in waterways across Australia, but it could disappear if avian flu ever takes hold here.
That's the grim conclusion of an international study that has sequenced the iconic bird's genome — and that of its white cousin — for the first time.
"Black swans have a similar genetic basis [to mute swans], but there are distinct differences," said Kirsty Short, study co-author and virologist at the University of Queensland.
The study, published today in the journal Genome Biology, found Australian black swans (Cygnus atratus) lack some immune genes that combat viral infections found in other species of swans and waterfowl.
"What it tells us is that if avian flu ever got into Australia, black swans would be really, really vulnerable," Dr Short said.
"And that's a really scary proposition because ... this virus is going gangbusters [overseas]."
She said the finding highlighted the ongoing need to deal with global outbreaks as well as the importance of surveillance and biosecurity.
"Ultimately, you can't control where birds migrate, can you?"
Black and white
There are a number of different subtypes of avian influenzas.
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. Both originally appeared in poultry and spread to wild birds and back again.
Unlike other species of swans and wild birds such as Mallard ducks, black swans are known to be particularly susceptible to these highly pathogenic avian influenzas (HPAI), better known as "bird flu".
While their paler cousins and water fowl may suffer only mild symptoms such as listlessness, black swans die so quickly there are often no signs, said Anjana Karawita of the Australian Centre for Disease Preparedness.
"They usually die within two to three days," Dr Karawita said.
In 2020, five black swans at Jerusalem Biblical Zoo died after being infected with H5N8 strain, but mute swans (Cygnus olor) and water fowl were not affected at all.
Until the genomes were sequenced, no-one knew why black swans were so susceptible to avian flu.
Michelle Wille of the University of Melbourne said the study was one of the first to look at the immune system of wild birds.
"As the virus sweeps through sub populations [of birds], we discover species that are highly susceptible," said Dr Wille, who was not involved in the study.
"This is one of the very few studies that is actually being proactive instead of reactive."
Along with sequencing the two swan genomes — which also revealed the gene responsible for the different coloured plumage — the team led by Dr Karawita looked at how genes in cells responded to the virus.
This analysis, known as a transcriptome, revealed that cells taken from black swans mounted an uncontrolled inflammatory reaction when they were infected, in much the same way as someone with severe COVID-19.
Dr Short said the finding that black swans were missing key genes suggested they were probably not just susceptible to avian flu, but a whole lot of different viruses.
"I suspect that the isolation of Australia has probably played a role in keeping out these viruses and therefore keeping the black swan safe."
'It's on our doorstep'
Over the years there have been small homegrown instances of HPAI viruses in Australia.
The most recent was an outbreak of H7N7 in farms Victoria in 2020, which evolved in poultry from a low pathogenic virus found in wild birds in Australia.
Thousands of chickens and turkeys had be culled before the outbreak was contained.
The bird flu virus of greatest concern is H5N1, which has not arrived in Australia yet.
Since H5N1 first appeared in the late 1990s, it has evolved "like crazy", Dr Short said.
Over the past two years, a devastating lineage of the virus known as 2.3.4.4b has spread around the globe.
"For the first time ever it's entered South America and it's just decimating the bird populations there," Dr Short said.
It's also widespread in Europe, the UK, North America and Asia.
"It's on our doorstep."
The overseas outbreak has been devastating, said Dr Wille, who is involved with the surveillance of avian influenza in Australian wild bird populations.
"We have no understanding of how many how many wild birds have died [overseas] from this outbreak because most agencies are not counting.
"There are just so many dead birds and there is no indication that it is going to decrease any time soon."
Dr Wille said screening of birds in Australia had not detected any sign of the virus here yet.
"I think the risk will be low probably until September when the migratory birds come back."
Waterfowl, which are thought to have had a role in the spread of the H5N1 virus in Europe, Asia and Africa, don't migrate to Australia.
While smaller wading birds are not considered common hosts of HPAI viruses, Dr Wille said the risk was unclear.
"We need to remain vigilant because even if only one shorebird migrates with HPAI onboard and arrives in Australia, this could trigger outbreaks."
She said current surveillance programs focused on species such as ducks, which are less affected but more likely to spread the disease.
"This virus arriving in Australia would be catastrophic, so we should do all we can to monitor the situation for early detection and rapid response."
Avian flu is not just an issue for poultry and wild bird flocks, but it could potentially be a threat to human health as well, Dr Short said.
"If COVID has taught us anything it's that we need to be aware of these emerging form of threats and especially viruses that can spill over into humans as well.
"So we really need constant vigilance against avian flu."
