The virus causing record cases of avian influenza in birds across the world is changing rapidly, experts have warned, as calls increase for countries to vaccinate their poultry. While emphasising that the risk to humans remains low, the experts who spoke to AFP said that the surging number of bird flu cases in mammals was a cause for concern. Since first emerging in 1996, the H5N1 avian influenza virus had previously been confined to mostly seasonal outbreaks. But “something happened” in mid-2021 that made the group of viruses much more infectious, according to Richard Webby, the head of a World Health Organization collaborating centre studying influenza in animals. Since then, outbreaks have lasted all year round, spreading to new areas and leading to mass deaths among wild birds and tens of millions of poultry being culled. Webby, who is a researcher at St Jude Children’s Research Hospital in the US city of Memphis, told AFP it was “absolutely” the largest outbreak of avian influenza the world had seen. He led research, published this week in the journal Nature Communications, showing how the virus rapidly evolved as it spread from Europe into North America. The study said the virus increased in virulence, which means it causes more dangerous disease, when in arrived in North America. The researchers also infected a ferret with one of the new strains of bird flu. The found an unexpectedly “huge” amount of the virus in its brain, Webby said, indicating it had caused more serious disease than previous strains. Emphasizing that the risk in humans was still low, he said that “this virus is not being static, it’s changing”. “That does increase the potential that even just by chance” the virus could “pick up genetic traits that allow it to be more of a human virus,” he said. In rare cases, humans have contracted the sometimes deadly virus, usually after coming in close contact with infected birds.