by Last summer, the highly contagious strain of avian flu that had spread through North American birds made its way into marine mammals, prompting a spike 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 epidemic. Of the 41 beached seals tested for the virus, nearly half were infected, 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 carried mutations associated with mammalian adaptation.
The risk to humans remains low and the seal epidemic has declined rapidly, the scientists said.
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“It was a dead-end event, as far as we know,” said Kaitlin Sawatzki, a postdoctoral researcher at the Cummings School of Veterinary Medicine at Tufts University and author of the new paper. “The virus that got into those seals didn’t persist.”
But the report comes amid growing concerns that the virus, which has already caused the largest avian flu outbreak in the nation’s history, may adapt to spread more efficiently among mammals, potentially sparking a new pandemic.
It’s not clear whether the seals were spreading the virus to each other or if they picked it up primarily from birds. But the number of affected seals suggests that the virus spreads easily among marine mammals or that the barrier for bird-to-seal transmission is low.
“We don’t really know if it’s going bird-to-seal, bird-to-seal, bird-to-seal 100 times or if it’s going into a couple of seals and then spreading,” said Wendy Puryear, a virus expert at the Tufts Veterinary. school and an author of the new document. “Both are possible,” she added. “Neither of them are great.”
Both scenarios require 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.
“We just have to 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 prevalent in wild birds and has spread repeatedly to mammals, including bobcats, raccoons and foxes. Scientists believe that most wild mammals contract the virus directly from birds.
But an outbreak of avian flu at a Spanish mink farm last fall suggested the virus could be spreading efficiently among some mammalian species. And a mass die-off of sea lions in Peru has raised concerns that marine mammals may also be spreading the virus to one another.
Seals are known to be susceptible to bird flu, and other versions of the virus have already caused outbreaks in animals.
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.
The 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. “This has given us a really powerful ability to see what’s going on in birds and seals.” same time in the same region,” Puryear said.
There have been two waves of flu in wild birds, the researchers found. The first, which peaked in March 2022, mainly affected birds of prey, while the second, which began in June, affected gulls and sea ducks known as eiders.
No seals tested positive for avian flu during the first wave of avian infections. But during the summer stranding event, 19 out of 41 seals tested positive.
Researchers have found two slightly different versions of the virus in seals. One matched what was circulating in terns, while the other resembled what was circulating in a wider range of birds, including gulls and eiders. The finding suggests that the virus has spread at least twice.
Because these seals don’t typically eat birds, scientists suspect the animals pick up the virus from the environment, perhaps through contact with bird droppings.
Viral samples from seals also had mutations that were rare or absent in birds. Three seal samples carried mutations that have been shown to enhance viral replication or increase virulence in mammals.
These mutations are not unique. In another recent study, a team of Canadian scientists found the same mutations in some viral samples taken from foxes infected with bird flu. “When there is a bird-to-mammal spillover event, they seem to get acquired pretty quickly,” Sawatzki said.
The presence of these mutations isn’t, by itself, a reason to “sound the alarm,” Stallknecht said. But continued surveillance is needed 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 examined on a scale larger than just ‘pandemic potential,'” he said, “because they affect many other species around the world.”
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