By Angus Dalton
Examine, a free weekly newsletter covering science with a sceptical, evidence-based eye, is sent every Tuesday. You’re reading an excerpt – Sign up to get the whole newsletter in your inbox.
A virulent strain of bird flu has claimed its first human victim with the death of a patient in Louisiana, US health officials have reported.
The aggressive strain or “clade” has devastated global wildlife since 2021 and poses the greatest pandemic threat from avian influenza since the virus was first detected 28 years ago. However, the overall risk of the disease gaining the ability to spread between humans remains low.
Particles of H5N1 bird flu flagged in yellow.Credit: CDC
Researchers sequenced samples swabbed from the nose and throat of the Louisiana patient, who caught the virus from backyard chickens.
The results, released in late December by the Centres for Disease Control and Prevention (CDC), were troubling: the virus had mutated to better latch onto receptors in our upper airways, a tweak that may allow it to become more infectious between people.
The patient who died was older than 65 and had underlying health problems. But in November, a 13-year-old girl in British Columbia, Canada, was also hospitalised with the virus in a critical condition.
The teenager was hooked up to supplemental oxygen for six weeks as she battled acute respiratory distress syndrome – where fluid on the lungs makes it difficult to breathe.
The girl also had conjunctivitis, a hallmark of the 60-odd other human cases of bird flu infection that struck North America last year. Almost all the infections have been mild and were caused by contact with sick cows or chickens.
But the source of the Canadian teenager’s severe infection is a mystery – although genomic analysis showed both she and the Louisiana patient were infected with a strain of the virus associated with wild birds and poultry rather than cows.
Experts have said these cases underscore the urgency of staying on top of human cases and the virus’s evolutionary dance as it spills between a network of wild and domestic host animals.
However, an Australian bird flu guru says the US death hasn’t shifted the needle on pandemic risk.
How did we get here?
Highly pathogenic avian influenza arose in 1997. Human infections are very rare because birds have very different cell receptors to us. But when infections do take hold in people, more than half have turned deadly; 52 per cent of about 900 historical cases were fatal.
In 2021, the 2.3. 4.4b clade of the highly pathogenic bird flu H5N1 ripped through global wildlife populations, laying waste to flocks of Chilean pelicans, entire breeding grounds of elephant seals and a critically endangered Bengal tiger.
Workers dispose of dead cranes at the Hula Lake conservation area in northern Israel. Bird flu has killed thousands of migratory birds.Credit: AP
Australia remains the last bastion free from the scourge of the deadly clade, although experts believe its eventual arrival on our shores is inevitable.
In March last year, US scientists detected the virus in Texan dairy cows. Farmworkers (and cats who lapped up the infected cow milk) soon fell ill with the virus.
The infections prompted WHO’s chief scientist, Dr Jeremy Farrar, to warn that he held enormous concerns about the virus evolving the ability to spread from human to human, rather than one-off infections from sick animals.
The virus would have to mutate significantly to gain that ability. It would need to evolve to thrive at a much lower temperature because the inside of a chicken is 8 degrees hotter than a human. At least five genetic tweaks would need to arise at once to make the virus airborne. The virus would also need to bind better to human cells.
But Dr Frank Wong, a bird flu expert for WHO from the CSIRO’s Australian Centre for Disease Preparedness, says even the detection of those genetic adaptions hasn’t raised the risk profile of bird flu for humans.
The risk is serious – but we’re prepared
Viral samples from the backyard flock that infected the Louisiana patient didn’t contain the same genetic tweaks as the virus that killed the patient.
That suggests the virus mutated within the patient’s body after infection. Scientists suspect the virus only mutated in the Canadian patient after infection, too.
Bird flu has torn through hundreds of cattle herds in the US.Credit: AP
That’s good news. It means the mutated, human-adapted virus is probably not circulating in the wild.
“It would be more of a concern if these mutations were present in birds or in animals before it hopped into humans,” Wong says.
He says news of a patient’s death is “unfortunate, but not unexpected, given that the US has had increasing spillovers between animals like birds or infected dairy cattle”. Humans and mammals remain a “dead-end” host for the virus.
“There’s still no evidence of infected humans passing it onto their close contacts. The virus is still not able to be transmitted by respiratory means.”
A bird flu pandemic would be much more severe than COVID-19, going by the historic mortality rate (more than 50 per cent of 900 human cases have been fatal compared to an estimated 3.6 per cent for the ancestral strain of COVID) but we’re also far more prepared and vigilant. Scientists have been monitoring the virus since 1997.
Australia has a stockpile of bird flu vaccines and antivirals ready to roll, boosted by $22.1 million from the government last year.
“Sufficient doses are available to support an initial response to a pandemic by vaccinating frontline workers and priority populations,” a spokesperson for the Department of Health and Aged Care said. While acknowledging the US death, they noted the current risk to the Australian population remains very low.
But the Louisiana case has illustrated how readily the virus can warp to cause severe disease.
UNSW infectious disease expert Professor Raina MacIntyre last year pointed out that the sheer scale of the domineering H5N1 clade, which has infected hundreds of millions of birds and mammals, means the chance of pandemic-causing mutations has never been greater.
California recently declared a state of emergency as bird flu tears through 650 of the state’s dairy herds.
“We are a little bit concerned with the situation in the US where it has been quite challenging to control the infections in dairy cattle,” says Wong. “The longer that we allow the virus to carry on in a new host like cattle, we increase the risk of it picking up mutations that we don’t want.”
Enjoyed this article? Sign up to get the whole newsletter in your inbox.