Scientists Investigate Bird Flu Outbreak in Seals

Last summer, the highly contagious strain of bird flu that had spread to North American birds spread to marine mammals, prompting an increase in seal strandings along the Maine coast. In June and July, more than 150 dead or sick seals washed ashore.

Now, a study provides new insights into the outbreak. Of the 41 stranded seals tested for the virus, nearly half were infected with it, scientists reported Wednesday in the journal Emerging Infectious Diseases. It is likely that wild birds introduced the virus to seals at least twice, the researchers concluded. In several seals, the virus showed mutations associated with mammalian adaptation.

The risk to humans remains low and the seal outbreak has declined rapidly, the scientists said.

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“It was a no-win event, as far as we can tell,” said Kaitlin Sawatzki, postdoctoral researcher at Tufts University’s Cummings School of Veterinary Medicine and author of the new paper. “The virus that entered these seals did not persist.”

But the report comes amid growing concerns that the virus, which has already caused the biggest bird flu outbreak in the country’s history, could adapt to spread more efficiently among mammals, triggering potentially a new pandemic.

It remains unclear whether the seals were spreading the virus to each other or picking it up primarily from birds. But the number of seals affected suggests either the virus spreads easily among marine mammals or the barrier of transmission from birds to seals is low.

“We really don’t know if it spreads bird-to-seal, bird-to-seal, bird-to-seal 100 times or if it gets into a few seals and then spreads,” said Wendy Puryear, an expert in virus to Tufts vet. school and author of the new journal. “Both are possible,” she added. “Neither of them are great.

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

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

The current version of H5N1 has spread uncharacteristically among wild birds and has spread several times among mammals, including bobcats, raccoons and foxes. Scientists believe that most wild mammals contract the virus directly from birds.

But an outbreak of bird flu in a Spanish mink farm last fall suggested the virus could spread effectively among certain mammalian species. And a mass die-off of sea lions in Peru has raised fears that marine mammals could also spread the virus to each other.

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

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

Researchers collected samples from 1,079 wild birds and 132 gray seals and harbor seals stranded along the North Atlantic coast from January 20 to July 31, 2022. “It gave us a really powerful ability to see what happens in birds and seals. same time in the same area,” Puryear said.

There have been two waves of influenza in wild birds, the researchers found. The first, which peaked in March 2022, mostly affected raptors, while the second, which began in June, affected gulls and sea ducks called eiders.

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

The researchers found two slightly different versions of the virus in the seals. One matched what circulated in terns, while the other resembled what circulated in a wider range of birds, including gulls and eiders. The finding suggests the virus has spread at least twice.

Because these seals don’t usually eat birds, scientists suspect the animals pick up the virus from the environment, possibly through contact with bird droppings.

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

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

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

“These emerging diseases need to be looked at on a larger scale than just ‘pandemic potential’,” he said, “because they affect many other species around the world.”

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