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 marks the first detection of HPAI H5N1 in marine mammals in California and specifically in northern elephant seals.
The outbreak was identified after researchers observed abnormal respiratory and neurological symptoms such as weakness and tremors among seals at the park on February 19 and 20. Samples collected from sick and deceased animals were sent to UC Davis for testing, where initial results indicated avian influenza. The USDA lab later confirmed these findings as HPAI H5N1.
“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 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.”
Scientists from UC Santa Cruz and UC Davis had increased monitoring efforts over recent years due to concerns about avian influenza spreading across North and South America. Roxanne Beltran, a professor at UC Santa Cruz leading research on northern elephant seals, noted: “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.”
Año Nuevo State Park is home to around 5,000 elephant seals during winter breeding season. At the start of this outbreak, approximately 1,350 were present on site. In response to health concerns, California State Parks has temporarily closed public access to seal viewing areas and canceled guided tours for the rest of the season.
For decades, researchers at UC Santa Cruz have tracked individual seals within this colony, which allows them to study both immediate and potential long-term effects on population health. Since 2024, scientists from UC Davis have also been testing samples from marine birds and mammals along various coastal regions.
Efforts are ongoing between university researchers and agencies such as NOAA Fisheries, California Department of Fish and Wildlife, and the West Coast Marine Mammal Stranding Network to monitor affected wildlife 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 said.
Officials advise that while risk to humans remains low, people should avoid contact with live or dead seals or allow pets near them. Anyone encountering a sick or dead marine mammal along California, Oregon or Washington coasts is urged not to touch or move it but instead call NOAA Fisheries’ stranding hotline at (866) 767-6114.
H5N1 was first detected in Southeast China in 1996 before spreading through poultry industries worldwide; it eventually reached wild birds and humans across several continents including North America by early winter 2021. Nearly all U.S. states now report cases among commercial poultry flocks as well as various wild bird species and some mammals.
Previous outbreaks involving U.S. marine mammals occurred in Maine (2022) and Washington (2023), typically resulting from direct transmission between infected birds and seals.
The NSF Center for Pandemic Insights at UC Davis collaborates with other institutions—including The Marine Mammal Center—to improve global surveillance methods aimed at preventing future outbreaks.
