Elephant seal colony lost 95% of its pups to bird flu

A year after an unprecedented outbreak of H5N1 avian influenza swept through elephant seal breeding grounds on Península Valdés in Argentina, the beaches are quieter, and the normally bustling colonies have drastically diminished. 

The bird flu outbreak, which killed over 17,000 elephant seals, including almost 97% of the pups, has left a scar on this vulnerable population, with only about a third of the seals typically expected here returning.

“It’s beautiful to walk the beaches now and hear elephant seals again,” said Marcela Uhart, director of the Latin America Program at the UC Davis Karen C. Drayer Wildlife Health Center. 

“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.”

Mammal-to-mammal transmission of bird flu

A study published in Nature Communications, co-led by scientists from UC Davis and Argentina’s National Institute of Agricultural Technology (INTA), sheds light on how H5N1 spread among marine mammals during the 2023 outbreak. 

The research provides critical evidence that H5N1 was able to transmit from one marine mammal to another, efficiently infecting the seals in what became a devastating event across southern South America. 

Evolution of the bird flu virus

Genomic analysis revealed that, upon its arrival in South America, the virus branched into separate strains: one adapted to avian hosts and another to marine mammals – an unprecedented shift.

“We’re showing the evolution of H5N1 viruses that belong to genotype B3.2 over time since their introduction in South America in late 2022,” said co-lead author Agustina Rimondi, a scientist at INTA and the Robert Koch Institute. 

“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.” 

“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.”

Elephant seal population hit hard by bird flu

To gauge the full extent of the impact, the Wildlife Conservation Society (WCS) in Argentina has been monitoring the seal population as part of a long-term conservation effort. The losses are staggering.

“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 WCS Argentina director of coastal and marine conservation Valeria Falabella.

The effects of the outbreak have undone decades of conservation progress, Falabella added, as the virus claimed many of the species’ most fertile females and dominant males. This loss of reproductive individuals will have long-term consequences for the population’s recovery.

Monitoring efforts and lingering questions

Tracking H5N1 among elephant seals has become an essential task for scientists. WCS Argentina’s monitoring program is working in tandem with the UC Davis team, which documents and tests any animals that die, while INTA conducts the testing and genomic sequencing of samples. 

As of now, no elephant seals have tested positive for bird flu in the new breeding season, though scientists are left with numerous questions from last year’s outbreak. 

Experts still do not know how the virus was primarily transmitted among seals – whether through aerosols, saliva, feces, or another means – and whether surviving seals may have developed protective antibodies.

Outside of Argentina, the virus has continued to wreak havoc across bird populations and livestock. 

In the United States, H5N1 has spread beyond birds, reaching dairy cows and recently even swine, according to a U.S. Department of Agriculture report from October. To date, there is no known human-to-human transmission of the virus.

Global spread and impact of bird flu

The H5N1 variant clade 2.3.4.4b began its spread in 2020, around the same time as the COVID-19 pandemic.

First, the outbreak killed tens of thousands of seabirds in Europe before reaching South Africa, North America, and eventually South America in late 2022. H5N1 was detected in Argentina in early 2023, primarily affecting inland poultry farms. 

However, by mid-2023, the virus had moved to sea lions near Tierra del Fuego and then quickly progressed northward, bringing deadly consequences for both marine mammals and seabirds.

Mass mortality of elephant seals

By October 2023, elephant seal colonies at Punta Delgada on Península Valdés witnessed mass mortality, which researchers found was due to HPAI [highly pathogenic avian influenza] H5N1. 

The outbreak had a cascading impact, with other species, like terns, dying in the same region and timeframe. 

The study authors emphasize the critical need for continuous monitoring and deeper research into H5N1’s adaptability across species. Its capacity to infect new hosts could pose broader implications for human health, biodiversity, and ecosystems.

According to the scientists, the virus’ increased flexibility to adapt to new hosts could have significant global consequences for human health, wildlife conservation, and ecosystems

This stark outlook highlights the importance of an integrated approach to managing and understanding zoonotic viruses like H5N1 that bridge ecosystems and blur the line between wildlife and human health.

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