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An unprecedented mass die-off of elephant seals on the shores of Patagonia, Argentina, suggests that a highly contagious strain of bird flu is now being transmitted between mammals, and poses a growing threat to the world’s biodiversity, say researchers.

Some 96 per cent of elephant seal pups (Mirounga leonina) at three breeding sites where the H5N1 strain of avian flu was detected in the Valdes Peninsula region in southern Argentina died in October 2023, according to at the Wildlife Conservation Society and his colleagues.

Extrapolating the mortality rate at the three sites, the researchers estimate that 17,400 baby elephant seals were killed by the virus in Patagonia, the highest figure since scientists began studying their numbers closely three decades ago.

“The mortality of pups was extraordinary, catastrophic and probably also included a large number of adults,” says Campagna. “It may take decades for these seal populations to get back to where they were.”

H5N1 was first detected in China in 1996 and had been largely confined to domesticated birds. But since 2021, it has spread more quickly in wild populations throughout the Americas and Europe.

The virus has infected at least andspecies — more than any previous strain of avian flu.

More than 500,000 birds in South America — the most affected region — have died from the disease and a growing number of mammal species are being affected. Thewas killed by the virus earlier this month.

Researchers largely thought mammals were only catching the virus from contact with infected birds, such as by eating them or eating farm feed contaminated with infected bird faeces. However, there is g that H5N1 may now be spreading from mammal to mammal.

Some studies published last year suggested that H5N1 may be circulating betweennd between wild

At the sites where elephant seals were found dead, they had little to no interaction with infected bird populations, Campagna says. It is also highly unlikely that such a large number of mammals could have been infected in less than a month without the infection being transmitted among them. “We now have to conclude that this was mammal to mammal transmission,” he says.

Newborn elephant seal pups suckle their mothers to feed, so there is little chance of eating infected birds, says at the University of California, Davis, who is studying the deaths of elephant seals.

The disease also spread rapidly in sea lions all along the Atlantic coast in August last year, killing thousands in areas where few birds were affected, suggesting the sea lions could transmit the virus, says Uhart. “This is all highly suggestive of some sort of transmission between mammals,” she says.

Mammal to mammal transmission increases the risk to the world’s mammals and also the chance of the virus eventually adapting to infect humans more easily, she adds. Researchers are studying genetic samples from the elephant seals to confirm whether this is occurring.

The virus has also been detected in brown skuas in South Georgia island, 2700 kilometres east of Argentina’s southern tip, where there have also been many deaths in kelp gulls, elephant seals and fur seals. There are concerns it will reach the Antarctic continent in the coming months where it could devastate vulnerable species such as penguins.

H5N1 must be genetically studied in other affected populations quickly to understand its mutations in mammals and the possible risk to humans, says Uhart.

Although mammal to mammal transmission in the elephant seals is likely, further studies are required to determine whether the virus’s evolution has made it more likely to infect humans, says at Erasmus MC in Rotterdam, the Netherlands.

“Viruses adapted to marine mammals may have some traits that increase the likelihood for transmission to humans,” he says. “We have seen this with H10N7 in Europe in 2014 and with H5N1 bird flu in isolated cases of seal infection. At the same time, however, the virus may acquire other genetic changes that promote seal infection that make it less likely to spread to humans.”