Either scenario calls for closer monitoring of seals, said David Stallknecht, an expert on wildlife diseases and influenza at the University of Georgia, who was not involved in the research.

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“We need to just keep our eyes on them,” he said. “The easiest way to tell if this persists in seals is to keep testing them.”

The current version of H5N1 has become unusually widespread in wild birds and has spilled over repeatedly into mammals, including bobcats, raccoons and foxes. Scientists believe that most wild mammals are contracting the virus directly from birds.

But a bird flu outbreak on a Spanish mink farm last autumn suggested that the virus could spread efficiently among some mammalian species. And a mass die-off of sea lions in Peru has raised concerns that marine mammals might be spreading the virus to one another, too.

Seals are known to be susceptible to bird flu, and other versions of the virus have previously caused outbreaks in the animals.

The new study is a collaboration between researchers at several academic institutions and wildlife organisations, including Marine Mammals of Maine and New England Wildlife Centres, as well as federal scientists.

The researchers collected samples from 1079 wild birds and 132 gray seals and harbor seals stranded along the North Atlantic coast from January 20 to July 31, 2022. “That gave us a really powerful ability to see what is happening in the birds and the seals in the same time in the same region,” Puryear said.

There were two waves of flu in wild birds, the researchers found. The first, which peaked in March 2022, primarily affected raptors, while the second, which began in June, hit gulls and sea ducks known as eiders.

No seals tested positive for bird flu during the first wave of bird infections. But during the summer stranding event, 19 of 41 seals tested positive.

The researchers found two slightly different versions of the virus in the seals. One matched what was circulating in terns, while the other resembled what was circulating in a broader array of birds, including gulls and eiders. The finding suggests that the virus spilled over at least twice.

Because these seals do not typically eat birds, the scientists suspect that the animals are picking up the virus from the environment, perhaps through contact with bird droppings.

Viral samples from the seals also had mutations that were rare or absent in birds. Three seal samples had mutations that have been shown to improve viral replication or increase virulence in mammals.

Such mutations are not unique. In another recent study, a team of Canadian scientists found the same mutations in some viral samples taken from bird flu-infected foxes. “When there’s a bird-to-mammalian spillover event, they seem to be acquired to pretty quickly,” Sawatzki said.

The presence of these mutations is not, in and of itself, a reason to “sound the alarm,” Stallknecht said. But continued surveillance is necessary not only to safeguard human health but also to protect wild animals from a virus that has already proved devastating.

“These emerging diseases need to be looked at on a bigger scale than just ‘pandemic potential’,” he said, “because they affect a lot of other species on the globe.”

This article originally appeared in The New York Times.

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