If the risk to humans remains low, the growing number of cases among mammals is considered worrying, according to specialists interviewed by AFP.
Since its emergence in 1996, the H5N1 avian influenza virus has caused mainly seasonal epizootics. But "something happened" in mid-2021 that made it much more infectious, according to Richard Webby, a virologist and director of the World Health Organization's (WHO) avian disease research center.
Since then, epizootics have become annual, extended to new areas and synonymous with mass deaths of wild birds and the slaughter of tens of millions of poultry.
For Webby, this is the largest outbreak of avian influenza ever known. He led research, published this week in the journal Nature Communications, showing that the virus has rapidly evolved as it spreads from Europe to North America.
The researchers also infected a ferret with one of the new strains of bird flu. They found a "huge" and unexpected amount of virus in his brain, a reflection of a more serious illness than with previous strains, he told AFP.
While pointing to a still low risk in humans, he observed that "this virus is not static, it evolves", which "increases the risk that, even by chance", the virus can "acquire genetic traits allowing it to be more of a human virus".
Cases of humans who have contracted the virus, sometimes fatal, usually after close contact with infected birds, are rare.
But the detection of the disease in a growing number of mammals, including new species, is "a really worrying sign," according to Richard Webby.
Last week, Chile announced that nearly 9,000 sea lions, penguins, otters, porpoises and dolphins have died from bird flu on its northern coast since early 2023. Most are believed to have contracted the virus through eating infected birds.
An employee of the Department of Agriculture and Livestock picks up dead cormorants on a beach in Coquimbo, on May 29, 2023 in northern Chile © MARTIN BERNETTI / AFP
"Recent transmissions to mammals need to be closely monitored," WHO chief Tedros Adhanom Ghebreyesus warned in February.
Admittedly, there is no "clear evidence that this virus is easily maintained in mammals", according to Ian Brown, head of virology at the UK Animal and Plant Health Agency.
And, if the virus evolves to become "more efficient in birds", it remains "unsuitable for humans", he told AFP.
Avian viruses bind to different receptors on the host cell compared to human viruses, Webby said, explaining that it would take "two or three minor mutations in a virus protein" for them to become more adapted to humans.
Vaccination of poultry
One way to reduce the number of bird flu cases and reduce the risk to humans would be to vaccinate poultry, Webby said.
Some countries, including China, Egypt and Vietnam, have already organized such vaccination campaigns.
But many others are reluctant because of possible import restrictions and fears that infected birds could fall through the cracks.
One way to reduce the number of bird flu cases and reduce the risk to humans would be to vaccinate poultry © CHAIDEER MAHYUDDIN / AFP/Archives
In April, the U.S. began testing several vaccine candidates for potential use on birds. The France recently indicated that it hopes to start vaccinating poultry as early as this fall.
Vaccinating poultry is not "a silver bullet because the virus is constantly changing," according to Christine Middlemiss, the UK's chief veterinary officer. But traditionally reluctant countries should consider using it more often, she told AFP at an event at the British embassy in Paris last week.
For the director general of the World Organization for Animal Health, Monique Eloit, the issue of poultry vaccination should be "on the table". After all, she recalls, "everyone now knows that a pandemic is not just a fantasy, it could be a reality."
© 2023 AFP