The recent rise of bird flu cases in mammals, including foxes, otters, minks, seals, and even grizzly bears, is a cause for concern, according to experts. However, there is no need for immediate panic for humans as the virus would have to mutate to spread between humans significantly.

Since late 2021, Europe has been gripped by its worst-ever bird flu outbreak, with North and South America experiencing severe outbreaks.

This has led to the culling of millions of domestic poultry worldwide, many with the H5N1 strain. The global outbreak is also responsible for the deaths of tens of thousands of wild birds.

Tom Peacock, a virologist at the Imperial College London, told Agence France-Presse (AFP) that it constitutes a "panzootic" – a pandemic among animals, in this case, birds.

"We are not fully sure why it's happening now, but we think this might be driven by a slightly different strain of H5N1, which is spreading very effectively in wild, migratory birds," Peacock said.

Rarely that bird flu jumps over into mammals – and rarer still that humans catch the potentially deadly virus.

On Thursday, the United Kingdom Health Security Agency said that a fox had recently tested positive for H5N1. It joins eight foxes and otters which tested positive in the U.K. last year, all of which had a PB2 mutation.

Peacock said this mutation "allows the virus to replicate better in mammalian cells." But further mutations would be "required for the virus to cause a flu pandemic" in humans, he added.

France announced last week that a cat had been put down after testing positive for H5N1. And last month, the U.S. state of Montana's parks service said three grizzly bears with bird flu had been euthanized.

All of these mammals were suspected of having eaten infected birds.

Paul Wigley, a professor of animal microbial ecosystems at the U.K.'s Bristol University, said that while "there is no transmission within mammalian populations, the risk to humans remains low."

However, two recent larger-scale infections have raised concerns that bird flu has the potential to spread between mammals. One was an outbreak of H5N1 with the PB2 mutation at a Spanish farm in October, leading to the culling of more than 50,000 minks.

Research published in the journal Eurosurveillance last month said its findings "indicate that an onward transmission of the virus to other minks may have taken place in the affected farm."

Transmission between the minks has not been confirmed, with further research ongoing. The mass death of 2,500 endangered seals found along Russia's Caspian Sea coast last month has also raised concern.

A researcher at Russia's Dagestan State University, Alimurad Gadzhiyev, said last week that early samples from the seals "tested positive for bird flu," adding that they were still studying whether the virus caused the die-off.

Peacock warned there have been mixed reports from Russia about the seals, which could have contracted the virus by eating infected seabirds. But if the seals gave each other bird flu, it "would be yet another very concerning development," he added.

"The mink outbreaks, the increased number of infections of scavenger mammals, and the potential seal outbreak would all point to this virus having the potential to cause a pandemic" in humans, he said.

According to experts, if H5N1 did mutate into a strain that could circulate among humans, the current seasonal flu vaccine could be fairly easily updated to include it.