First Cases of Highly Pathogenic Avian Influenza in Northern Elephant Seals Confirmed in California

Wildlife health officials have confirmed the first known cases of highly pathogenic avian influenza (HPAI) H5N1 in northern elephant seals in California following detections in pups at Año Nuevo State Park.

The cases were confirmed by the United States Department of Agriculture National Veterinary Services Laboratories, marking the first detection of the virus in marine mammals in California and the first known infection in northern elephant seals. The disease has previously caused severe mortality events in related species, including southern elephant seals in South America in 2023.

Scientists from University of California, Davis said rapid detection was made possible through ongoing surveillance programs. Researchers involved in the response noted that early warning systems and coordinated monitoring efforts helped identify abnormal health signs quickly within the seal colony.

Seven weaned elephant seal pups tested positive after researchers observed abnormal respiratory and neurological symptoms, including weakness and tremors. Samples from sick and deceased animals were analyzed at the California Animal Health and Food Safety Laboratory System, where initial testing suggested avian influenza infection before federal confirmation was received.

Researchers have increased monitoring efforts across coastal marine wildlife populations due to global spread of HPAI. Scientists from UC Davis and University of California, Santa Cruz have worked together to track disease activity among marine mammals and seabirds along the West Coast.

The research team, including marine mammal biologists studying the colony at Año Nuevo, reported that the colony typically hosts roughly 5,000 seals during peak breeding season. When the outbreak was detected, about 1,350 seals were present on the beach. State park officials temporarily closed public viewing areas and canceled guided tours to reduce human disturbance and limit potential disease transmission.

Marine mammal researchers have monitored this colony for more than 60 years, creating a rare long-term dataset that allows scientists to track individual animals through flipper tagging and behavioral observation. This long-term research effort is helping scientists evaluate both short- and long-term population health impacts.

Additional monitoring efforts are being coordinated with several federal and state agencies, including National Oceanic and Atmospheric Administration Fisheries, the California Department of Fish and Wildlife, and the West Coast Marine Mammal Stranding Network.

Scientists emphasized that public health risk remains low, but advised people to avoid contact with live or dead seals and to keep pets away from wildlife. Officials said reporting sick or dead marine mammals helps support surveillance efforts and protect both wildlife and public health.

The H5N1 virus was first identified in poultry farms in Southeast China in 1996 and later spread globally through bird populations before spilling over into mammals, including humans. The current North American outbreak was first detected in 2021 and has since been reported across most U.S. states and Canadian provinces in both wild and domestic animals.

Researchers are also exploring new surveillance technologies, including drone-based monitoring and environmental biosensors, to improve early detection of wildlife disease outbreaks. The work is supported through collaborations with multiple academic and conservation institutions focused on pandemic preparedness in wildlife populations.

Scientists will continue weekly monitoring of the colony and testing of wildlife samples while tracking the potential long-term ecological impacts of the outbreak.