Seven northern elephant seal pups at Año Nuevo State Park in California have tested positive for highly pathogenic avian influenza (HPAI) H5N1, according to confirmation from the U.S. Department of Agriculture’s National Veterinary Services Laboratory. This is the first time HPAI H5N1 has been detected in marine mammals in California and specifically in northern elephant seals.
The disease had previously caused significant losses among southern elephant seals in Argentina in 2023. In response to these concerns, researchers at UC Santa Cruz and UC Davis increased their monitoring efforts for signs of avian influenza within the local seal population.
“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 UC 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.”
Professor Roxanne Beltran, who leads the northern elephant seal research program at UC Santa Cruz, noted that her team had heightened their vigilance given what happened to related species elsewhere. “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,” she said.
On February 19 and 20, Beltran’s team observed several seals displaying unusual respiratory and neurological symptoms such as weakness and tremors. Samples collected from sick and dead animals were sent to UC Davis for testing; initial results indicated avian influenza, which was subsequently confirmed as HPAI H5N1 by federal authorities.
Año Nuevo State Park typically hosts around 5,000 seals during its winter breeding season. At the time of the outbreak, approximately 1,350 seals were present on site. In light of these developments, California State Parks has temporarily closed public access to seal viewing areas and canceled guided tours for the rest of this season while investigations continue.
Researchers at UC Santa Cruz have monitored this colony for over sixty years. Their long-term work allows them to track health changes among individually identified seals. Since 2024, scientists from UC Davis have also tested samples from both marine birds and mammals along various coastal regions.
Efforts are ongoing as researchers collaborate with NOAA Fisheries, the California Department of Fish and Wildlife, and other organizations to monitor marine mammal populations along the coast.
“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 added.
Authorities emphasize that while infection risk to humans is low, people should avoid contact with live or dead seals and keep pets away as well. Those who encounter sick or dead marine mammals along West Coast beaches are urged not to approach or move them but instead call regional wildlife hotlines.
H5N1 was first discovered on a domestic goose farm in Southeast China in 1996 before spreading globally through poultry industries and wild birds. The current North American outbreak began in Canada’s Newfoundland and Labrador region during winter 2021; since then it has been confirmed across nearly all U.S. states and Canadian provinces among poultry flocks, wild birds, mammals—including humans—and now northern elephant seals.
Past outbreaks involving U.S. marine mammals occurred in Maine (2022) and Washington (2023), resulting from direct transmission between infected birds and seals.
Sustained wildlife surveillance remains a challenge worldwide but is seen as essential for early detection and prevention efforts. The NSF Center for Pandemic Insights at UC Davis works with collaborators including The Marine Mammal Center and UC Santa Cruz to develop new methods aimed at improving monitoring capabilities across multiple institutions.
