Officials Confirm Small Bird Flu ‘Outbreak’ in Elephant Seals at Año Nuevo State Park

Scientists have confirmed that seven weaned northern elephant seal pups at the park tested positive for highly pathogenic avian influenza, known as H5N1.

The confirmation, provided by the U.S. Department of Agriculture’s National Veterinary Services Laboratory, marks California’s first confirmed detection of the virus in a marine mammal.

Researchers estimate that about 30 seals, mostly recently weaned pups, plus one adult male, have died so far. Additional samples are still being processed, and officials say the outbreak appears to have been caught early.

“We think we were able to witness the very start,” said Christine Johnson, a professor of epidemiology at UC Davis.

Field teams already monitoring the colony noticed a slight uptick in dead seals late last week and observed animals showing neurological symptoms, including tremors, weakness and seizure-like activity.

During a post-mortem exam on one known female weaned pup, veterinarians found signs that the disease moved quickly.

“The animal was in excellent nutritional condition,” said Megan Moriarty, a wildlife veterinarian at UC Santa Cruz. “That means she likely died quickly from a disease process that happened rapidly, as opposed to something more chronic.”

Moriarty said the seal showed significant damage to the brain and lungs — findings consistent with the neurological symptoms researchers had observed in the seals out in the field.

Highly pathogenic avian influenza was first identified in 1996 and has since spread globally, largely through poultry. The current North American outbreak began in late 2021 and has affected wild birds, poultry, dairy cows and multiple mammal species. Two prior U.S. marine mammal outbreaks — in Maine in 2022 and Washington state in 2023 — were linked to bird-to-seal transmission and were relatively short-lived.

“For people, the risk is low,” Johnson said. But she emphasized that the bird flu is a zoonotic virus, meaning it can spread from animals to humans through close contact, in rare instances. Officials are urging visitors to stay at least 150 yards away from marine mammals, keep pets leashed and avoid touching sick or dead wildlife.

At the park, public elephant seal tours have been canceled for the remainder of the season. California State Parks said the decision was made “out of an abundance of caution” to protect both wildlife and to avoid inadvertently spreading the virus through foot traffic in affected areas.

Año Nuevo State Park hosts one of the most intensively studied elephant seal colonies in the world, led by researchers at UC Santa Cruz. About 3,000 seals use the mainland site during the winter breeding season, and scientists have tracked more than 55,000 individuals over six decades through flipper tags and long-term monitoring.

“That long-term individual-based data set gives us a really unparalleled opportunity to understand how this virus affects uniquely identifiable animals,” said Roxanne Beltran, who leads the program at UC Santa Cruz.

So far, the outbreak appears concentrated among weaned pups — young seals that have recently been left behind after their mothers return to sea. Two weeks ago, researchers counted roughly 930 pups and weanlings on the beach. Beltran said about 95% of adult females had already departed on their foraging migrations when the outbreak began, a detail scientists hope may limit broader impact.

“Avian influenza has affected only a small proportion of the weaned pups at this time,” Beltran said. “There are still thousands, apparently healthy animals in this population.”

Still, researchers are bracing for uncertainty. In South America in 2023, H5N1 devastated southern elephant seals in Argentina, with major pup losses that altered the population’s trajectory.

“A change in the number of pups that survive in a given year has a really, really long-lasting consequence,” Beltran said. Northern elephant seals can live more than 20 years, and population shifts ripple across decades.

Scientists do not yet know how the virus is spreading at Año Nuevo — whether through direct contact with infected birds, environmental exposure such as feces, or seal-to-seal transmission. Genetic sequencing of the virus is underway and could take weeks to clarify whether it matches the dominant bird strain circulating now.

Meanwhile, monitoring has intensified. Teams are conducting systematic beach surveys, collecting samples from sick animals, flying drones to assess colony-wide health and coordinating across agencies, including NOAA Fisheries and The Marine Mammal Center.

At The Marine Mammal Center in Marin County, responders have temporarily paused hands-on responses for elephant and harbor seals while assessing risks.

“My biggest concern is that this perpetuates and continues to spread and paralyzes the operations,” said Dominic Travis, the center’s chief executive. “We’re going to be assessing that day by day.”