In the peer-reviewed study published in Nature Communications, researchers from the St. Jude Children’s Research Hospital in Memphis, Tennessee, found that the new strains of the H5N1 avian influenza virus changed rapidly and became more severe as they spread through Europe and the Americas in the past two years.
In recent months, increasing numbers of mammals have been found to be infected with the virus, with mass die-offs of seals and sea lions reported in Russia and the Americas and dozens of foxes, skunks, dolphins, raccoons, cats and other mammals found to be infected as well.
“We haven’t seen a virus quite like this one,” said corresponding author Richard Webby of St. Jude's Department of Infectious Diseases in a press release. “In 24 years of tracing this particular H5N1 flu lineage, we haven’t seen this ability to cause disease but also be maintained in these wild bird populations.”
The scientists found that the virus gained a different version of a viral protein called neuraminidase, which increased its ability to transmit between birds, before arriving in Canada and then spreading to the rest of the Americas.
After reaching Canada, the virus quickly mutated again, mixing with other flu viruses and becoming more adapted to the bird population, including species that used to not be as affected by the flu.
The virus also became more severe since it spread to the Americas, withferrets infected with samples of the virus collected from an infected eagle in February 2022 experiencing rapid weight loss, lethargy and severe neurologic symptoms such as paralysis and impaired muscle control.
“Some of these are really nasty viruses,” added Webby. “There’s a huge amount of the virus in the brain of infected animals. That’s the hallmark of what we saw with these flu strains — increased pathogenicity associated with high virus load in the brain. That’s not the first time we’ve seen H5 viruses in the brain, but these are probably some of the most virulent we’ve looked at over 24 years of following these viruses.”
The researchers also infected ferrets with samples of the virus collected later in the outbreak. The researchers noted that the virus samples with more gene segments acquired from North American lineages seemed to cause more severe disease.
Webby stressed that this situation is surprising as the viruses' ability to cause severe disease changed with just a few reassosrtment events.
In a recent interview with AFP, Ian Brown, virology head at the UK's Animal and Plant Health Agency, stated that it would take "two or three minor changes in one protein of the viruses" for the bird flu to become more adapted to humans.
The outbreak has been widely affecting both birds and mammals. In the US alone, over 58 million poultry have been affected and thousands of infected wild birds have been found in almost every state. Nearly 200 cases have been detected in mammals across the US as well. In Europe, tens of thousands of wild and domestic birds were found to be infected in over 24 countries, with many sea birds affected.
Human cases have been detected in the UK, US, Cambodia, Ecuador and Chile.
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