The H5N1 bird flu virus, first detected in 1996, has evolved and spread globally, now killing wild birds in Australia. Experts say the risk to humans is low but fear an impending decimation of seal populations after tens of thousands of deaths in South America and Antarctica.
Origins and evolution
In 1996, geese in poultry farms in Guangdong province, southern China, started to get sick with a virus causing bleeding and neurological dysfunction, then death. The strain likely arrived as a relatively harmless virus in migrating wild birds but morphed into something highly pathogenic, killing about 40% of infected geese.
Dr Frank Wong, a virologist and avian influenza expert at the CSIRO’s Australian Centre for Disease Preparedness, says, “Right from the beginning there was great concern about the emergence of this virus.”
Avian influenza viruses are classified by two types of proteins on their surface: hemagglutinin (16 sub-types) and neuraminidase (nine). Only H5 and H7 have evolved into deadly strains. Wong says until 1996, an H5 virus had never been seen spilling into domestic geese to become highly pathogenic in domestic birds. “That was an immediate flag,” he says.
Spread and mutation
From the H5N1 virus detected in 1996, variants appeared in Hong Kong markets and farms in 1997. In 2002, almost a million poultry birds in Hong Kong were culled after H5N1 detections.
Prof Marcel Klaassen, a bird flu expert and ecologist at Deakin University, says, “All the ducks and geese were free ranging and in the 1990s we get these outbreaks of HPAI (highly pathogenic avian influenza). This H5N1 Guangdong goose virus was isolated but they couldn’t control it.”
It circulated in Hong Kong and east Asia until 2005, when it was found in wildlife. By then, the virus had evolved to survive in some wild birds, allowing it to spread to Europe and Africa. Wild ducks have been described as the “Trojan horses” of the H5N1 story, carrying the virus without being affected.
The virus re-emerged in Europe in 2021, and the current lineage has swept across Europe, North and South America, Antarctica, and now Australia. Since 1996, H5N1 has infected more than 400 bird species and more than 100 mammal species, including seals, sea lions, and dairy cattle in the US.
Wong explains, “Influenza viruses and how they impact different hosts is really a numbers game. The more types of host animals that it is allowed to infect without there being some control, the more chances it has to pick up new traits. It’s most successful trait is adaptability.”
Avian influenza viruses evolve quickly due to errors in gene copying. Most mutations are not successful, but some may give an advantage in the right environment. Dense poultry farming acts as an incubator for highly pathogenic strains, as a virus that kills its host in a domestic flock can still spread through the population before death.
Klaassen goes further, saying, “This is a man-made virus. Did these viruses evolve in wildlife? No, they did not. Low pathogenic viruses come in and, with high densities of birds and large numbers, you have an ideal environment for a virus to evolve. All of the high pathogenicity influenza viruses evolve in poultry.”
Impact on mammals
Since 1997, there have been about 500 human deaths from H5N1, almost all among people working closely with infected poultry. Health authorities say the risk to humans is low, with no recorded cases of human-to-human transmission.
However, H5N1 has infected mammal species, particularly pinnipeds like seals and sea lions that share spaces with infected wild waterbirds. Dr Rachael Gray, a veterinary scientist at the University of Sydney, witnessed the devastation in Antarctica in late 2023: “At one beach we saw a line of dead elephant seals – a whole tideline of dead animals.”
Gray says, “It was devastating. In birds you see respiratory symptoms – nasal discharges. But what we are seeing more of are these neurological signs. In pinnipeds, they can’t walk properly. They have seizures and you see this clouding of their cornea. It is distressing to see. When it sweeps through a colony, you are talking two to four days [of symptoms before death] in birds and a little longer for mammals. The virus has become well adapted at getting into mammal cells.”
Tens of thousands of pinnipeds have died in South America and Antarctica, and Gray fears for Australia’s three resident pinnipeds: the New Zealand fur seal, the Australian fur seal, and the endangered Australian sea lion.
“On the first incursion, it will be awful,” she says. “Some might develop immunity, and hopefully that gets passed on to their offspring, but I don’t think we know enough yet [on how the virus response will evolve]. The virus can survive for extended periods of time on different surfaces, particularly in cool conditions. That gives the virus a better chance to survive [in Australia] because it has arrived in our winter.”
H5N1 arrived in Australia in migratory seabirds from the sub-Antarctic, with genetic testing showing the virus matched the type that killed thousands of elephant seal pups at Heard Island. Gray adds, “Every year we got through without it we thought, ‘maybe Australia’s geographic isolation will help us’. But that was a naive hope and now, maybe, I have naive hope that it won’t hit our sea lions and fur seals. History is telling me the opposite.”
