On Feb. 25, 2026, seven weaned elephant seal pups 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.

The outbreak marks the first cases of HPAI H5N1 in marine mammals in California and the first detection in northern elephant seals. The disease decimated populations of a related species, southern elephant seals, in Argentina in 2023. 

“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.” 

Scientists at UC Santa Cruz and UC Davis increased disease surveillance of this population in recent years due to concerns about avian influenza’s spread throughout North and South America. On Feb. 19 and 20, a team lead by Roxanne Beltran, a UC Santa Cruz professor in Ecology and Evolutionary Biology noticed seals at Año Nuevo Reserve with abnormal respiratory and neurological signs, including weakness and tremors. 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.

Año Nuevo State Park, just north of Santa Cruz, is home to an elephant seal colony with approximately 5,000 seals during the winter breeding season. About 1,350 seals were present on the beach when the outbreak began. California State Parks has temporarily closed public access to seal viewing areas and cancelled its popular guided elephant seal tours for the remainder of the season while officials investigate the wildlife health threat.

For six decades, researchers at UC Santa Cruz have been closely monitoring the colony, providing a unique opportunity to investigate the outbreak in individually identifiable seals and understand the potential near and long-term impacts to population health. 

Sustained wildlife surveillance and monitoring is a critical but ongoing challenge for preventing and detecting outbreaks globally. The NSF Center for Pandemic Insights at UC Davis, with collaborators at 12 other institutions, including The Marine Mammal Center and UC Santa Cruz, has been working to develop innovative technologies and methodologies — from chemical sensors to drones — to be able to scale up HPAI surveillance, especially in populations that are less monitored and often harder to access. The team will generate weekly updates and continue to closely monitor and test samples collected from sick animals, conduct drone surveys and ensure systematic observations.