TUESDAY, Nov. 12, 2024 (HealthDay News) -- The H5N1 strain of avian flu is now passing easily among South America's elephant seals and drastically cutting herd populations, a new report finds.

"It is likely that more than half of the reproductive population died due to the virus. It will take decades before the numbers are back to the 2022 population size," said Valeria Falabella, director of coastal and marine conservation at Argentina's Wildlife Conservation Society.

The sea mammals' population may slowly rebound, but in 2023 mutant bird flu killed an estimated 17,000 elephant seals, including about 97% of their pups, researchers reported Nov. 11 in Nature Communications.

Called avian influenza because it originated in birds, H5N1 has over the past few years mutated so it can thrive in a wide range of mammals, including seals, dolphins, cows and, most recently, pigs.

Humans are mammals too, of course, so health experts fear it might someday prove easily transmissible between people. 

American dairy and poultry farm workers have been catching the illness via close proximity with cows and birds, but so far human-to-human transmission has not been confirmed.

The story for Argentina's elephant seal population has already been tragic, according to a team co-led by Marcela Uhart. She directs the Latin America Program at the University of California Davis' veterinary school. 

In 2023, H5N1 spread "efficiently" through breeding colonies of elephant seals along Argentina's Península Valdés, Uhart's team found. The same deadly phenomenon was happening to seal populations in five other South American countries.

Lab investigations determined that the avian flu virus in South America has already evolved into distinct classes that affect either birds or mammals, a step the research team called "unprecedented."

After first appearing in birds in Argentina in February of 2023, a "highly pathogenic" strain of H5N1 was spotted in infected sea lions at Argentina's southern tip by August of the same year.  It quickly moved northward and beginning in October started infecting and decimating populations of elephant seals breeding in Península Valdés. 

The death toll was devastating, although in 2024 much smaller groups of elephant seals have returned to Península Valdés once again. So far, none of the seals have tested positive for H5N1.

“It’s beautiful to walk the beaches now and hear elephant seals again,” said Uhart in a Wildlife Conservation Society news release. “At the same time, we’re walking among piles of carcasses and bones, and seeing very few elephant seal harems, so it’s still disturbing.”

The team noted that the same strain of H5N1 virus was also found among terns that were in the same locale in Península Valdés.

“This virus is capable of adapting to marine mammal species, as we can see from the mutations that are consistently found in the viruses belonging to this clade [virus subtype]," said virologist and co-lead author Agustina Rimondi of Argentina's National Institute of Agricultural Technology. "Very importantly, our study also shows that H5 marine mammal viruses are able to jump back to birds, highlighting the need for increased surveillance and research cooperation in the region.” 

In the meantime, the 2023 outbreak was a catastrophic setback for elephant seals, study co-author Falabella said.

“We were totally appalled by the dramatic impact of the epidemic of avian influenza on this population,” she said.

More information

Find out more about H5N1 avian flu at the U.S. Centers for Disease Control and Prevention.

SOURCE: Wildlife Conservation Society, news release, Nov. 11, 2024