By Angus Dalton
Australia is the last continent free from a virulent strain of bird flu that has killed an unprecedented number of animals, led to the culling of half a billion poultry birds and sparked “enormous concern” within the World Health Organisation about a leap into humans, after a Texan man fell ill with the virus.
Speculation that the problem variant, called clade 2.3.4.4b, has spread between cows in the US has sparked fear the disease is getting closer to posing a serious pandemic threat to humans.
A highly pathogenic form of bird flu can emerge when it mutates within high-density poultry flocks.Credit: Bloomberg
The risk of the virus gaining the ability to spread between people is very low, but the scenario could be devastating. Based on 889 human infections since 2003, the disease has a 52 per cent mortality rate.
“For some reason, in the last five years [avian flu] has become supercharged,” Professor Michael Ward, chair of Veterinary Public Health at the University of Sydney, said. “It’s exponentially increased in terms of these spillover events and the dynamics of it have certainly shifted up a gear.”
A devastating ‘panzootic’
Spillover events refer to when a virus leaps between species.
A mild form of bird flu circulates naturally in waterfowl, mostly ducks and geese. But when this low pathogenic form of the virus infects domestic poultry flocks it can mutate into a deadly, highly pathogenic version of the virus and spill back over into wild birds.
A bald eagle that tested positive to avian influenza receives treatment at rehabilitation centre in Milwaukee.Credit: AP
Clade 2.3.4.4b probably emerged in this way in 2021 and now the world is in the grip of a bird flu panzootic – an animal pandemic.
Scientists have reported horrific scenes of South American beaches littered with the bodies of 17,400 elephant seal pups. The virus has killed polar and grizzly bears, decimated Peruvian pelican populations and infected bottlenose dolphins, lions and skunks.
The virus has ripped through the world’s largest northern gannet colony in the UK, killing 70 per cent of the population and turning survivors’ eyes from blue-white to black. Scientists don’t know why.
In northern gannets, the birds that survive avian influenza are left with blacked-out eyes.Credit: Getty
And, for the first time, bird flu has spread to Antarctica, where it could have a catastrophic effect on penguins and seals.
“These viruses seem to now be jumping into mammals and back into birds, which we haven’t really seen before,” Ward said. “The more often that happens, the more likely it is that at some point, these viruses are going to start picking up undesirable characteristics that then would allow them to transmit from human to human. The cattle example in the US is sort of going down that path.”
First infection in cows leads to latest human case
On March 25, US officials announced the virus had infected a herd of dairy cows in Texas, the first cases of avian flu in cattle. A month later, at least 34 herds across nine states were sick with the flu.
New genomic analysis shows the strain was circulating undetected in cows since early January and may have gained at least one adaption linked to viral spread in mammals.
Amid the cow outbreak, a dairy farmworker fell ill with the virus which, unusually, manifested as conjunctivitis. The strain was not one that could pass on to another person and the man recovered.
But the incident prompted WHO’s chief scientist, Dr Jeremy Farrar, to say he held enormous concern about a scenario in which the virus “evolves and develops the ability to infect humans and then, critically, the ability to go from human to human”.
Avian flu jumped into cattle for the first time in Texas earlier this year and may have been spreading from cow to cow.Credit: AP
Infection in pigs could be particularly dangerous because they’re “mixing vessels” that can catch strains of human and bird flu, Ward said.
In a coinfected pig, “one virus could pick out characteristics from the other,” Ward said. “That sort of situation can set up one of these super viruses that has that ability to transmit much more easily.”
Why does Australia remain virus-free?
Despite the concern, humans are still a “dead end” host for bird flu. The virus has never achieved sustained transmission between humans and it remains a bird-adapted disease, said Dr Frank Wong, a bird flu expert for WHO from the CSIRO’s Australian Centre for Disease Preparedness.
“There’s still no conclusive evidence that cattle were spreading it to each other through a respiratory infection,” Wong said. “When you sample the cattle, the viral load is mainly in mammary glands, the udders and in milk, and not in the respiratory tract of cows.”
The virus may have spread between cows through unsterilised milking equipment or feed, not in an airborne manner like COVID-19, limiting its potential to take off as a human pandemic.
Australia remains a hold-out for the disease, Wong said, because of strict biosecurity laws and the fact the continent isn’t a major stopover for the species of migrating ducks and geese that carry the virus. But a Department of Agriculture, Fisheries and Forestry report recently upgraded the risk of highly pathogenic bird flu arriving in Australia to high. The report said migratory shorebirds, shearwaters and nomadic waterfowl were species that could bring the virus to Australia.
The first sign of the disease here, Wong said, would probably come from a mass die-off of wild birds or from a suddenly ill backyard chicken flock. Detection of the virus in poultry would trigger a swift “stamping out” measure, or culling of affected flocks, which is how authorities stamped out eight previous outbreaks of the disease.
DAFF investigates mass bird die-offs and also conducts general surveillance of wild bird faeces to test for the virus’s presence. Wong’s lab checks if positive samples are of the exotic, problem variant or the low pathogenic strain that naturally circulates.
The clade behind the latest worldwide outbreak has never been detected in Australia.
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