Written by Charlotte Harrison, Science Writer.
A paper published in Nature Communications shows that the rapid spread of the H5N1 avian influenza A virus — otherwise known as bird flu — was facilitated by the genetic mixing of H5N1 with other viruses circulating in wild birds.
Moreover, the authors showed that the rapidly spreading H5N1 lineage caused brain disease in experimental animals, but there was little evidence of transmission to humans.
The H5N1 clade 18.104.22.168b spread among wild birds and domestic poultry across Asia, Europe, Africa and North America in 2021. The researchers showed that the western movement of this clade was quickly followed by genetic re-assortment with viruses circulating in wild birds in North America. The reassortant viruses had different combinations of ribonucleoprotein genes, such as polymerase basic 1 (PB1), PB2 and nucleoprotein genes.
In-vitro experiments showed that the phenotype of the reassortant viruses was different to that of the non-rearranged virus, including higher replication rates and pH of activation. When tested in ferrets and mice, many of the reassortant viruses caused severe disease that involved neurological symptoms, and high levels of virus were detected in the brain of the animals.
Moreover, the reassortant viruses were likely responsible for clade 22.214.171.124b becoming further adapted to the bird population, infecting many different types of birds.
“We haven’t seen a virus quite like this one,” said author Richard Webby 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.”
Some positive news
Reassuringly though, the reassortant viruses appeared to be better adapted to transmit between birds rather than between mammals, meaning the risk to humans is low.
The authors also showed that antiviral therapies with different mechanisms of action — namely oseltamivir, zanamivir and baloxavir marboxil — were active against the reassortant viruses in cell-based assays.
Nevertheless, the authors note that their research indicates that people should avoid contact with wild birds. “Someone would have to work pretty hard to infect themselves with this virus. But if they do happen to be infected, there’s a real chance of getting a severe disease from it,” said Webby.