Explainer: what is going on with bird flu?

The world is in the grip of a multi-year panzootic – the animal equivalent of a pandemic.

It’s caused by a lineage of the highly pathogenic avian influenza A (H5N1) virus, or bird flu, clade It first emerged in Europe in late 2020 and now effects wild birds and poultry in every continent – except Australia.

It has also made the jump to mammals in what are known as spillover events. On 25 March US officials reported H5N1 had been detected in cattle for the first time, which has since spread to cows in 36 herds in 9 states.

The US Centers for Disease Control and Prevention issued a health alert on April 1 after a Texas farm worker tested positive for the virus, which he contracted after being exposed to infected dairy cattle.

So, should we be worried about it in Australia?

Where did it start?

Strains of avian influenza are either classified as “high pathogenicity” (HPAI) or “low pathogenicity” (LPAI) based on their clinical effect.

Most avian influenza viruses are of low pathogenicity and commonly infect wild birds such as waterfowl (Anseriformes) and shorebirds, terns and gulls (Charadriiformes). But when viruses of the A (H5) and A (H7) subtype get into chickens they have the potential to mutate to become highly pathogenic.

In 1997 the HPAI H5N1 virus first emerged in poultry in Hong Kong, and again in China in 2003, and has predominantly remained a disease in chickens since.

“Wild birds very, very rarely carry HP virus around because, obviously, being highly pathogenic, it’s got a significant disease impact to the infected birds,” says Dr Frank Wong, a senior research scientist at the CSIRO’s Australian Centre for Disease Preparedness (ACDP) and the World Organization for Animal Health (WOAH) laboratory reference expert for avian influenza.

“This particular lineage [clade] has been quite adaptable and able to infect many species of birds with unpredictable and varying impacts,” says Wong.

This means when the virus infects wild migratory waterfowl it might be asymptomatic, and because the birds don’t feel sick they are able to carry it over long distances.

“But when the virus infects other birds, it could also have quite a devastating impact in that the birds fall sick very rapidly and then you also get mass mortalities,” says Wong.

Wong says there is significant concern about the virus impact on poultry production, but also its impact and implications to wildlife populations.

“H5N1 now presents an existential threat to the world’s biodiversity,” Dr Chris Walzer, Wildlife Conservation Society Executive Director of Health, told Cosmos in March.

“It has infected over 150 wild and domestic avian species around the globe as well as a dozens of mammalian species.”

Of the current outbreak in the US in dairy cattle, Wong says there is not a lot of evidence to suggest it’s a respiratory infection, instead the virus is infecting the mammary glands.

“Given how dairy farms operate, obviously if there’s high virus growth in the milk there is potential for quite substantial environmental contamination. And also, the milking or farming equipment being contaminated and then spreading from herd to herd.”

How worried should we be of transmission to humans?

US authorities say that pasteurisation is effective at inactivating HPAI and testing has not detected any live, infectious virus in the commercial milk supply. 

Instances of humans contracting the virus from infected animals are rare. According to the World Health Organization, from 2003 to 1 April 2024 there have been a total of 889 cases and 463 deaths – a 52% mortality rate – reported from 23 countries.

Of these, 13 cases have been associated with clade in 7 countries since January 2022 but no instances of human-to-human transmission have been identified.

Wong says that most of these infected cases in humans have been epidemiologically linked to confirm that they were close contacts of infected animals.

“Most of those cases were from those situations where people were dealing with an outbreak in either domestic or wild birds.”

Wong adds: “The advisory in the US and also in other countries, including Australia, is that when people are dealing with mortality events or a bird flu event, that only authorised persons are dealing with the sick and dead animals and those authorised people should be wearing PPE.”

Will clade reach Australia?

There is no way to prevent the strain from entering Australia through migratory wild birds.

Thus far, Australia has been able to avoid the virus because it is located outside of the migratory flyways of the wild birds that predominantly carry it.

“But given that this virus has now infected a very large number of different species to different impacts, there is that potential that there are bridging species that could bring the virus into Australia,” says Wong.

“For example, Southeast Asia already has this virus … it’s pretty much close to our doorstep.”

In December 2023, Australia’s Chief Vet published an updated incursion risk assessment on the likelihood of entry and establishment of HPAI in Australia via wild birds found that there is: “a high risk, with moderate uncertainty.”

The authors write: “It is expected that the likelihood and associated risk of incursions into Australia will continue to remain at these levels for some time and potentially increase rather than diminish given continued virus evolution.”

So, what is being done?

Relevant agencies are keeping up to date with regional and global circulation of the virus.

There is also a campaign to raise public awareness that unusual events should not be dealt with by members of the public. Instead, they are to be reported immediately so the animals can be properly diagnosed in a nominated state laboratory or the ACDP national reference laboratory.

Wong says that animal health labs have been coordinating with ACDP to ensure their diagnostic tests are able to pick up this lineage with good sensitivity.

“There is also a wild bird surveillance program that occurs in Australia, coordinated by Wildlife Health Australia with wild bird faecal sampling to survey for avian influenzas,” he says.

“Anything that is positive from active surveillance that is detected by the state animal health labs is sent to us as the national reference lab for confirmation and also for full characterisation.

“When we do that, we’re able to tell whether those viruses are local avian influenza viruses that occur naturally in our wild birds, or whether a sample that is picked up as positive was due to an introduced virus.”

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