HPAI H5N1 found in California seals

California State Parks closes beach to visitors

Seven weaned elephant seal pups that looked sick on February 19 and 20 in California’s Año Nuevo State Park tested positive for highly pathogenic avian influenza (HPAI) H5N1, the U.S. Department of Agriculture’s National Veterinary Services Laboratory confirmed late Tuesday, February 24. Thirty seals have died, but test results have not yet confirmed that they died of H5N1

The outbreak marks the first cases of HPAI H5N1 in marine mammals in California and the first detection in northern elephant seals. 

“This is exceptionally rapid detection of an outbreak in free-ranging marine mammals,” said Professor Christine Johnson, director of the Institute for Pandemic Insights at the University of California, Davis’ Weill School of Veterinary Medicine. “We have most likely identified the very first cases here because of coordinated teams that have been on high alert with active surveillance for this disease for some time.” 

On Monday, February 23, California State Parks closed Año Nuevo State Park to public access for the elephant seal viewing area of the Año Nuevo Coast Natural Preserve. With the confirmation that H5N1 was responsible, the tours have been canceled for the rest of the season.  

State and federal agencies work together

UC Davis scientists have been testing samples from marine birds and mammals at the park and other regions along the coast since 2024. The NSF Center for Pandemic Insights at UC Davis and UC Santa Cruz have been working together to increase surveillance at this site over the past two months in anticipation of a possible outbreak in seals.

The scientific team is now working closely with NOAA Fisheries, the California Department of Fish and Wildlife and the West Coast Marine Mammal Stranding Network to continue to monitor marine mammals along the coast.

On Feb. 19 and 20, Beltran’s team noticed seals at Año Nuevo Reserve with abnormal respiratory and neurological signs, including weakness and tremors. The Reserve’s elephant seals are intensively studied, with 380,000 observations collected over the years. One of the dead weaners was the pup of a marked mother who was herself born on that beach. The pup was entered into the research program when she was 15 days old. She was weaned when her mother left the beach, and two days later the team observed her convulsing on the beach at 9:30 am. By afternoon, she was dead.

“It’s tough to watch animals we have followed and watched for years get sick,” Beltran said. “We know their family lineages." She praised her team’s professionalism in continuing to collect information.

That individual information will help to assess the long-term effects of how many pups survive, whether females are affected, and future births, on the population.

They collected samples from sick and dead elephant seals and took them to UC Davis for testing at the California Animal Health and Food Safety Laboratory System. Initial screening revealed the samples were positive for avian influenza, which the USDA’s NVSL lab now confirms is HPAI H5N1.

“Given the catastrophic impacts observed in related species, we were concerned about the possibility of the virus infecting northern elephant seals for the first time, so we ramped up monitoring to detect any early signs of abnormalities,” said Roxanne Beltran, a professor in Ecology and Evolutionary Biology at UC Santa Cruz. Beltran’s lab leads UC Santa Cruz’s northern elephant seal research program at Año Nuevo.

“We are cautiously optimistic, as most of the adult females had already departed the beach for their routine migrations before the outbreak began, and most seals on the colony seem healthy,” Beltran said. 

Virus threatens wildlife worldwide

As reported by Sharon Guynup in Mongabay in 2024, H5N1 is a highly contagious, deadly strain of avian influenza. It evolved from a mild form of avian influenza in the 1990s into a deadly pathogen from infected industrial poultry operations, breaking out in Europe in 2020.

Since then, it has leapt the species barrier, affecting at least 485 bird species and 50 mammal species, killing seals, sea otters, dolphins, foxes, California condors, albatrosses, bald eagles, cougars, polar bears and a zoo tiger. Animals as diverse as rats, elephant seals, bears, house cats, and humans — with an unprecedented outbreak in U.S.dairy cattle having affected 17 states and more than 1,000 herds. 

It is carried by migratory birds, and has been found on every continent except Australia, including Antarctica. In some bird and mammal species, the toll has been devastating. Some imperiled species could be pushed to extinction, with wildlife already fighting to survive against a changing climate, disappearing habitat and other stressors.

Since 2024, 71 human cases have been diagnosed in the U.S., with two deaths. Current public health risk is rated Low, with no person-to-person transmission reported.

“The more a virus like this is able to mutate and find its way into a wide range of species, especially farmed species that live in close contact with people like poultry and now cattle, the more the odds go up that a viral strain will more easily make that leap to people,” wildlife veterinarian Steve Osofsky, D.V.M., the Jay Hyman professor of wildlife health and health policy at Cornell University’s College of Veterinary Medicine, wrote in Stat in June 2025.

Rapid transmission

The virus’ ability to change rapidly, and its record of infecting other species makes it a cause of intense concern. 

Pinnipeds such as elephant seals, which congregate in large groups, are among the vulnerable species. The virus swept along South America’s Atlantic and Pacific coastlines, slaying more than 30,000 sea lions in 2022-23, when the H5N1 virus moved down from North and Central America to invade both the Atlantic and Pacific coasts of South America. In 2023, it killed an estimated 17,000 Southern elephant seal pups  (Mirounga leonina) on Argentina’s Península Valdés — the species’ largest die-off ever.

No more tours this season

At Año Nuevo, a California State Parks spokesperson said that 134 tours totaling 1,203 tickets were canceled through March 1, including 22 school tours. Thousands ofvisitors pay $11 per person to hike out a mile and a half to two miles with a guide to view the elephant seals during the mid-December through March pupping and mating season.

The park’s Mainland Camera remains active to watch the seals on the beach.

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The Marine Mammal Center has suspended its rescue response to elephant seals and harbor seals, until further information indicates whether this is an isolated cluster or the beginning of a wider outbreak.

“If it’s a cluster, we will figure out,” Dominic Travis, Chief Programs Officer, The Marine Mammal Center, said. “If it’s perpetuated, it will be really tricky. We will assess it day by day with NOAA.”

 

Most of the adult seals have left the beach on their next migration, escaping from what may be only a cluster of infection. As of February 25, only a small number of seals have been affected.