Seven weaned northern elephant seal pups at Año Nuevo State Park in California have tested positive for highly pathogenic avian influenza (HPAI) H5N1, officials confirmed Tuesday evening. The outbreak marks the first detection of the virus in marine mammals in California, and the first case identified in northern elephant seals, according to the U.S. Department of Agriculture’s National Veterinary Services Laboratory.
Researchers estimate that approximately 30 seals, primarily recently weaned pups, along with one adult male, have died as of February 27, 2026. Additional samples are currently being processed to determine the full extent of the outbreak, officials stated.
“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.”
Scientists at UC Santa Cruz and UC Davis had increased disease surveillance of the northern elephant seal population in recent years, driven by concerns over the spread of avian influenza throughout North and South America. “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, who leads the UC Santa Cruz’s northern elephant seal research program at Año Nuevo.
Veterinarians conducting post-mortem examinations have observed that the disease progresses rapidly. One female weaned pup examined was found to be in excellent nutritional condition, suggesting the virus is capable of quickly overcoming healthy animals, according to wildlife veterinarian Megan Moriarty at UC Santa Cruz.
Field teams monitoring the Año Nuevo colony noticed a slight increase in dead seals late last week and observed animals exhibiting neurological symptoms, including tremors, weakness, and seizure-like activity. The outbreak prompted officials to cancel public tours of the park.
The H5N1 virus decimated populations of southern elephant seals in Argentina in 2023, raising concerns about the potential impact on northern elephant seals. The current situation is being closely monitored by researchers, who are working to understand the transmission dynamics and potential for further spread.
