The ongoing outbreak of deadly avian flu has already killed millions of birds and has become an even greater cause for concern as it spreads to mammalian species.

Toronto-based infectious disease specialist Dr. “This is an infection that has epidemic and pandemic potential,” Isaac Bogoch told the CBC. “I don’t know if people realize how big of a deal it is.”

The H5N1 avian influenza virus is not exactly new. But earlier, it mostly affected birds on poultry farms. In 2020, however, according to the Centers for Disease Control and Prevention, gene-swapping between poultry and wild bird viruses resulted in a “wild bird-adapted” version of the virus. This made it very easy for migrating wild birds to spread the virus to each other and to domestic birds along their route.

As of 2022, H5N1 has killed more than 58 million domestic birds, such as chickens, ducks, and turkeys, in the United States alone. When a deadly virus hits a poultry or egg farm — some of which have more than a million birds on the premises — the facility typically culls the entire flock to prevent further spread.

During the same period, about 6,000 cases have been reported in wild birds in the US

Scientists have found a variety of wild mammals infected with the virus, including bears, foxes, otters and seals. Since October 2021, there have been five confirmed human cases worldwide and one death, according to the BBC.

Ian Brown, director of scientific services at the UK’s Animal and Plant Health Agency, told the BBC he was “keenly aware of the risks” of avian flu becoming a pandemic in humans.

“This is a global proliferation concern,” he said. “We need to see new strategies, those international partnerships globally, to overcome this disease. If we don’t solve the problem around the world, we’re going to continue to face that threat.”

In October, a major outbreak occurred at a mink fur farm in Spain. Researchers describing the outbreak in a paper published last month believe that wild birds initially transmitted H5N1 to mink farms, but once it spread from mink to mink.

“This outbreak signals the real potential for the emergence of mammal-to-mammal transmission,” Michelle Wiley, a wild bird virus researcher at the University of Sydney, told the CBC.

Not a single worker in the field wearing protective gear appears to have been infected. But some scientists worry that minks could be a stepping stone for the virus to reach humans.

“It’s incredibly relevant,” Tom Peacock, a virologist at Imperial College London, told Science magazine. “This is a clear mechanism for the initiation of H5 epidemics.”

Journalist Zeynep Tufeki, who has covered the Covid-19 pandemic extensively, wrote in a New York Times opinion piece published this week that “an even deadlier pandemic could soon be here.” She also spoke with Peacock, who noted that minks’ respiratory systems make them a particularly good host species for viruses that can infect humans.

In her op-ed, Tufeki called for several precautionary measures, including expanding testing capabilities and ramping up vaccine development and production. She also called for the closure of mink farms – something some countries have already done due to animal cruelty concerns and the fact that the farms were also hotbeds for COVID-19.