Elephant seal outbreak highlights first global spread of bird flu in mammals

Luciana Gallo of BIOMAR-CONICET and Marcela Uhart of UC Davis sample for highly pathogenic H5N1 among cormorants (by shore) and terns (in greenery) at Punta Leon in Argentina in November 2023. The tern colony later died from the virus. Credit: Martin Brogger.

A new study has revealed that highly pathogenic H5N1 avian influenza (bird flu) is adapting to mammals in ways that could have serious global consequences for humans, wildlife, and livestock.

This was shown by a massive outbreak among elephant seals in Argentina in 2023.

The research, conducted by the University of California, Davis’ School of Veterinary Medicine and Argentina’s National Institute of Agricultural Technology (INTA), found clear evidence of mammal-to-mammal transmission of the virus.

The study, published as a preprint on bioRxiv, is the first to document the global spread of bird flu in mammals.

The virus appeared in several pinniped species across different countries in a short period.

Genomic analysis revealed that the virus is evolving into separate avian and marine mammal clades in South America, an unprecedented development.

There is growing concern that H5N1 viruses adapted to mammal transmission could jump to other species, including humans.

“This increased adaptation to mammals is worrying, especially for marine mammals,” said Marcela Uhart, a veterinarian with the UC Davis Karen C. Drayer Wildlife Health Center and its Latin America Program. “The more the virus adapts to mammals, the greater the risk to humans.”

The current H5N1 variant began causing problems worldwide in 2020. While humans were dealing with the COVID-19 pandemic, H5N1 was killing tens of thousands of seabirds in Europe before spreading to South Africa, North America, and finally, South America in late 2022.

In February 2023, H5N1 was detected in Argentina, primarily affecting poultry in central Argentina.

By August 2023, after a two-month lull in poultry outbreaks, the virus was found in sea lions in Tierra del Fuego. It then spread rapidly northward, first affecting marine mammals and later seabirds.

In October 2023, the study authors surveyed a breeding colony of elephant seals at Punta Delgada in Península Valdés, Argentina.

They recorded an unprecedented mass mortality event, with around 17,000 elephant seals dead. By November, 96% of pups born that season had died. Tests confirmed H5N1 in both the seals and several terns that died simultaneously.

The virus, arriving through migratory birds, spilled over to mammals and evolved into a marine mammal-adapted virus.

Despite this adaptation, it can still infect birds, as shown by the identical virus found in both terns and elephant seals.

“This virus is capable of adapting to mammals, as evidenced by consistent mutations in the mammalian clade,” said virologist and co-leading author Agustina Rimondi of INTA.

The study’s authors emphasize the need for continued monitoring and research to understand the virus’s impact on human health, wildlife conservation, and ecology.

The research team included scientists from UC Davis, the National Institutes of Health, INTA-CONICET, the Wildlife Conservation Society in Argentina, and the Rega Institute in Belgium.

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