Highly pathogenic avian influenza H5N1 has been confirmed in northern elephant seals at Ano Nuevo State Park in San Mateo County, California, forcing officials to shut down public access and cancel guided tours for the rest of the season. The cases represent the first time the virus has been detected in a marine mammal in California. With thirty seals, primarily weaned pups, already dead, the outbreak raises urgent questions about whether the virus can spread rapidly through densely packed breeding colonies along the Pacific coast.
How the Virus Was Detected and Confirmed
Researchers with the Ano Nuevo long-term monitoring program first noticed abnormal behavior in elephant seals on February 19 and 20, 2026. Animals were displaying respiratory distress and neurological signs, prompting field teams to collect samples from sick and dead seals and transport them to the UC Davis laboratory, which identified influenza A/H5. The USDA’s National Veterinary Services Laboratory then confirmed the pathogen as HPAI H5N1, completing a diagnostic chain that mirrors the protocol used in earlier West Coast seal cases and establishing that a highly virulent strain of avian influenza had reached this major rookery.
That confirmation triggered an immediate management response. State park officials announced a full closure of viewing areas and the cancellation of all remaining guided tours at Ano Nuevo, a popular winter destination where thousands of visitors typically watch elephant seals haul out to breed and nurse pups. The park’s decision reflects a dual concern: protecting the animals from human disturbance during an active disease event and reducing any risk of people or pets coming into contact with infected wildlife, even though current evidence still points to birds and other animals, rather than humans, as the primary hosts in this outbreak.
Thirty Dead Seals and a Vulnerable Population
The toll has mounted quickly. Thirty seals have died since late last week, with weaned pups accounting for most of the losses. That concentration among recently weaned animals is not surprising from an epidemiological standpoint. Pups that have just separated from their mothers are immunologically naive and tend to cluster together on beaches, creating conditions ripe for rapid transmission. The timing of the outbreak, landing squarely in the weaning window, may be amplifying the virus’s spread in ways that would not occur during other phases of the annual cycle, when animals are more dispersed at sea.
What remains unclear is whether the California strain carries the same mammalian-adaptation mutations found in related outbreaks abroad. No full-genome data from the Ano Nuevo cases has been publicly released yet, and without that sequencing information, scientists cannot say definitively whether the virus jumped from birds to seals multiple times or whether mammal-to-mammal transmission is already occurring in this colony. That distinction matters enormously for forecasting how far and how fast the outbreak could spread along the coast, and for determining whether interventions should focus primarily on limiting contact with infected birds or on monitoring direct spread within and between seal rookeries.
South America’s Warning: Mass Die-Offs and Population Crashes
The California cases did not emerge in a vacuum. H5N1 clade 2.3.4.4b has already torn through marine mammal populations in the Southern Hemisphere. A peer-reviewed study published in Nature Communications documented a 2023 outbreak among southern elephant seals at Peninsula Valdes, Argentina, where full-genome characterization and epidemiological analysis pointed to mammal-to-mammal transmission rather than repeated spillover from birds. The virus has also killed thousands of sea lions in Chile and Peru and thousands of elephant seals in Argentina, according to reporting on marine die-offs, offering a stark preview of what an uncontrolled outbreak can do to a pinniped population.
The population-level consequences are already measurable. Research published in Communications Biology used UAV aerial imagery to quantify a major decline in adult female southern elephant seals at South Georgia between 2022 and 2024, associating the drop with HPAIV. That finding is significant because adult females are the reproductive engine of any elephant seal colony, and their loss reverberates through future generations. Losing breeding-age females does not just reduce one season’s pup count; it compounds over years, shrinking the population’s ability to recover and increasing the risk that localized outbreaks could tip already stressed colonies into long-term decline if similar patterns emerge in northern elephant seals along the Pacific coast.
A Pattern Building Across U.S. Coastlines
California is not the first U.S. state to confront H5N1 in pinnipeds. A 2022 unusual mortality event in Maine, documented by NOAA Fisheries, linked seal deaths to HPAI H5N1 spillover from wild birds, with affected animals showing the same respiratory and neurological clinical signs now being observed at Ano Nuevo. On the West Coast, NOAA has also confirmed H5N1 in seals on Marrowstone Island in Puget Sound, where initial testing by a state diagnostic lab was followed by confirmation from the USDA’s National Veterinary Services Laboratory, underscoring that the diagnostic and response playbook now being used in California has already been tested in other regions.
The geographic spread of these cases, from the Atlantic to Puget Sound to central California, suggests that the virus is finding marine mammals wherever wild bird migration routes intersect with haul-out sites. Most coverage of H5N1 in the United States has focused on dairy cattle and poultry, but the marine mammal angle deserves sharper attention because seals and sea lions congregate in dense aggregations that can function as amplifiers once the virus gains a foothold. With multiple coastal states now grappling with pinniped infections, agencies are under pressure to coordinate surveillance, standardize testing, and share genomic data quickly enough to detect any shift toward more efficient transmission between mammals.
Managing Risk for Wildlife, People, and Coastal Access
For now, the primary public health message remains one of caution rather than alarm. State and federal agencies emphasize that people should avoid close contact with sick or dead wildlife, keep pets away from beaches where marine mammals haul out, and respect temporary closures such as the restrictions now in place at Ano Nuevo. The state government has used its online platforms to direct residents to updates from wildlife, agriculture, and public health departments, reflecting a broader “One Health” approach that treats animal, human, and environmental health as interconnected in managing this outbreak.
At the same time, land managers are balancing disease control with the public’s strong interest in coastal recreation. California’s park system, which spans traditional state parks as well as specialized units like off-highway vehicle areas, is accustomed to closing or rerouting access in response to fire, erosion, or sensitive wildlife nesting seasons. The H5N1 event at Ano Nuevo adds a new kind of emergency to that list, one in which limiting human presence is intended not only to shield vulnerable animals from stress but also to reduce the chance that people or pets might inadvertently carry contaminated material between sites. How long the closures last will depend on what ongoing testing reveals about the virus’s behavior in northern elephant seals, and whether swift action now can prevent California’s outbreak from echoing the catastrophic marine mammal losses already seen in South America.
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*This article was researched with the help of AI, with human editors creating the final content.
