Año Nuevo State Park Hosts Rare Case of H5N1 Infection in Marine Mammals

Avian influenza has been confirmed in northern elephant seals at California’s Año Nuevo State Park, marking the first documented case of highly pathogenic avian influenza (HPAI) H5N1 in marine mammals in the state. The outbreak, detected in seven weaned elephant seal pups, highlights the virus’s potential to impact marine ecosystems. Scientists warn that the H5N1 virus, first identified in 1996, has spread globally, infecting poultry, wild birds, and mammals, including humans. Its 2023 outbreak in southern elephant seals in Argentina caused significant mortality, underscoring the threat it poses to marine mammal populations.

The rapid identification of the outbreak is attributed to coordinated surveillance efforts by researchers at the University of California, Davis (UC Davis), UC Santa Cruz, and the Marine Mammal Center. These teams intensified monitoring of the Año Nuevo State Park colony, home to approximately 5,000 seals during the winter breeding season, following concerns about the virus’s spread across North and South America. UC Davis scientists had been testing samples from marine birds and mammals along the coast since 2024, while UC Santa Cruz researchers increased disease surveillance due to the virus’s documented spillover from infected birds to seals.

The outbreak was first noticed on February 19 and 20, 2026, when researchers observed abnormal respiratory and neurological symptoms, including weakness and tremors, in seals at the Año Nuevo Reserve. Samples collected from sick and dead seals were tested at the California Animal Health and Food Safety Laboratory System, with confirmation of HPAI H5N1 provided by the U.S. Department of Agriculture’s National Veterinary Services Laboratory. Christine Johnson, director of the Institute for Pandemic Insights at UC Davis, noted that the rapid detection likely stems from sustained surveillance and collaboration among institutions.

As of February 26, 2026, most of the adult female seals had already departed the beach for their routine migrations, and the majority of the colony appears healthy. However, public access to the seal viewing areas has been temporarily closed, and guided tours have been canceled for the remainder of the season to prevent human contact with potentially infected animals. The California Department of Fish and Wildlife, NOAA Fisheries, and the West Coast Marine Mammal Stranding Network are collaborating to monitor the situation and track the virus’s potential spread.

Authorities have emphasized that the risk of HPAI H5N1 to the general public remains low. However, they urge people to maintain a distance of at least 150 yards from seals, avoid touching live or dead wildlife, and keep dogs leashed on beaches to minimize transmission risks. If individuals encounter sick, injured, or dead marine mammals, they are advised to report the incident to the NOAA Fisheries West Coast Region Stranding Hotline at (866) 767-6114.

In the United States, previous HPAI outbreaks in marine mammals occurred in Maine (2022) and Washington (2023), both linked to direct spillover from infected birds to seals. Scientists warn that the virus’s ability to adapt and transmit between species poses a growing threat to marine ecosystems. Researchers at the NSF Center for Pandemic Insights at UC Davis are developing innovative technologies, such as chemical sensors and drones, to enhance surveillance in hard-to-reach areas.

The Marine Mammal Center and its partners are conducting weekly updates and testing samples from affected animals to assess the outbreak’s trajectory. The Institute for Pandemic Insights at UC Davis will continue to monitor the situation, with findings shared publicly through its website. Researchers emphasize that sustained surveillance and international collaboration are critical to mitigating the virus’s impact on wildlife and human health.

This incident underscores the interconnectedness of terrestrial and marine ecosystems in the face of emerging infectious diseases. As scientists work to understand the virus’s behavior in marine mammals, the broader implications for global health and conservation remain a focal point of ongoing research.