We know the seals so well that abnormality is immediately visible.
Along the fog-shrouded shores of Año Nuevo State Park, a virus that has reshaped wildlife populations across the Southern Hemisphere has made its first confirmed appearance in California's elephant seals. Since February 19, thirty animals have died — most of them newly weaned pups — after H5N1 bird flu was detected in seven individuals, confirmed by federal laboratory testing. The outbreak arrives not as a surprise but as the fulfillment of a long-held fear, caught early only because scientists had spent years watching and waiting for precisely this moment. What remains unknown — how the virus arrived, whether it is spreading between mammals, and what mutations it carries — will determine whether this is a contained tragedy or the beginning of something larger.
- Thirty elephant seals are dead within days of the first visible symptoms, with pups convulsing and dying in locations that had been clear just twenty-four hours before.
- The catastrophe unfolding in California echoes a devastating precedent: South American colonies lost up to 97% of their pups to H5N1 between 2022 and 2024, and tens of thousands of sea lions perished across Peru and Chile.
- Scientists do not yet know whether the virus jumped from infected seabirds sharing the beach or whether mammal-to-mammal transmission — the more alarming possibility — has already taken hold.
- Genetic sequencing of the virus is pending, and its results will reveal whether mutations enabling easier mammal spread have arrived on the California coast.
- Researchers who monitored the colony 260 days a year caught the outbreak at its earliest detectable edge, buying critical time for investigation and response.
- Año Nuevo State Park is closed to the public, but thousands of apparently healthy seals remain on the beach — and major colonies at Piedras Blancas and Point Reyes are watching and waiting.
The dread had been building since 2023. When researchers from UC Davis and UC Santa Cruz confirmed H5N1 bird flu in seven weaned elephant seal pups at Año Nuevo State Park, just north of Santa Cruz, it was the arrival of something long anticipated. Federal laboratory testing in Iowa made it official. By then, thirty seals were already dead.
It began on February 19, when monitoring teams noticed animals that had been healthy the previous day now showing tremors, convulsions, and muscle weakness. Twenty-nine weaned pups and one adult male died before samples could be collected and results returned. The virus had moved quickly — but not faster than the scientists who had been watching.
The fear behind that vigilance was rooted in what H5N1 had already done elsewhere. Beginning in late 2022, the virus tore through elephant seal and sea lion populations along South America's Pacific coast and sub-Antarctic islands. Some Argentine colonies lost 97% of their pups. The breeding female population on South Georgia Island fell by nearly half. More than thirty thousand sea lions died across Peru and Chile. Scientists had long wondered why North Pacific populations remained untouched, theorizing that prior exposure to milder strains offered some protection. That protection, it now appears, was not sufficient.
What distinguished the California outbreak was the speed of detection. Christine Johnson of UC Davis called it exceptionally rapid for a free-ranging marine mammal population. The reason was deliberate: since 2023, teams from UC Santa Cruz, UC Davis, and California State Parks had monitored the Año Nuevo colony with extraordinary consistency — 260 days a year, and every single day from mid-December through March 1. Roxanne Beltran, who leads UC Santa Cruz's elephant seal research program, said her team knew these animals well enough that abnormality was immediately visible. On the morning of February 19, they saw behavior they had never witnessed before. They believed they had caught the very beginning.
How the seals were exposed remains under investigation. The virus has not yet been fully sequenced. Researchers suspect infected seabirds sharing the beach may be the source, but they are also examining whether mammal-to-mammal transmission occurred — a possibility made more urgent by the fact that the South American strain had acquired mutations enabling it to jump between mammals.
Áño Nuevo typically hosts around five thousand elephant seals during winter breeding season; roughly thirteen hundred were present when the outbreak began. Major colonies at Piedras Blancas and Point Reyes each hold around nine hundred weaned pups. Beltran urged perspective: the virus has so far affected only a small fraction of the population, and thousands of apparently healthy animals remain. The park is closed to visitors. Researchers are now awaiting genetic sequencing results that will reveal what mutations the virus carries — and whether this outbreak stays contained or moves further along the California coast.
The call came in on a Wednesday afternoon in late February, but the dread had been building since 2023. Researchers at UC Davis and UC Santa Cruz announced what they had feared for years: H5N1 bird flu had arrived in California's elephant seals. Seven weaned pups at Año Nuevo State Park, just north of Santa Cruz, tested positive for the virus. The confirmation came from the U.S. Department of Agriculture's National Veterinary Services Laboratory in Iowa.
The discovery was made almost by accident—or rather, by the kind of accident that only happens when you're paying very close attention. On February 19, teams monitoring the colony noticed something wrong. Animals that had been healthy the day before were now showing tremors, convulsions, and muscle weakness. Some were dying. By the time researchers had collected samples and sent them for testing, thirty seals were dead: twenty-nine weaned pups and one adult male. The virus had moved fast, but not faster than the scientists watching.
This was the outcome researchers had been bracing for since late 2022, when H5N1 tore through elephant seal populations in South America and the sub-Antarctic. In Argentina, some colonies lost ninety-seven percent of their pups. On South Georgia Island, the breeding female population dropped by nearly half between 2022 and 2024. Tens of thousands of animals died. In Peru and Chile, more than thirty thousand sea lions perished. The virus did not discriminate—it killed fur seals, sea lions, and elephant seals with equal efficiency. Scientists had wondered why the northern Pacific populations remained untouched for so long. The leading theory was that previous or milder strains had conferred some immunity. That protection, it turned out, was not enough.
What made the California detection remarkable was not the outbreak itself, but how quickly it was caught. Christine Johnson, director of the Institute for Pandemic Insights at UC Davis' Weill School of Veterinary Medicine, called it "exceptionally rapid detection of an outbreak in free-ranging marine mammals." The reason was simple: the researchers had been waiting for this. Since 2023, teams from UC Santa Cruz, UC Davis, and California State Parks had been monitoring the Año Nuevo colony with obsessive intensity—two hundred sixty days a year, and every single day from mid-December through March 1, when the seals come ashore to breed and give birth. Roxanne Beltran, who leads UC Santa Cruz's northern elephant seal research program, explained that the team knew these animals so well that abnormality was immediately visible. On the morning of February 19, her team saw it: behavior they had never witnessed before, mortality in locations where there had been none the day prior. They were confident they had caught the beginning.
The virus itself is no stranger to North America. It has ravaged dairy herds across the country, infecting millions of cows and dozens of workers. It has killed millions of commercial poultry. Since 2024, two Americans have died from H5N1, and seventy-one have been infected—most of them dairy or poultry workers. The virus first appeared in geese in China in 1996 and has since spread to every continent except Oceania. Two previous outbreaks in U.S. marine mammals occurred in Maine and Washington, affecting gray and harbor seals. But the scale of what happened in South America suggested that elephant seals might be uniquely vulnerable.
How the California seals were exposed remains unclear. The virus has not yet been fully sequenced. Johnson suspects transmission came from dead and dying seabirds living among the colony, but the team is investigating whether mammal-to-mammal spread occurred. Genetic analysis of the South American virus showed it had acquired mutations that allowed it to jump between mammals—a detail that adds urgency to understanding what is happening in California.
Áño Nuevo State Park typically hosts about five thousand elephant seals during winter breeding season. When the outbreak began, roughly thirteen hundred were on the beach. Other major California colonies at Piedras Blancas and Point Reyes hold about nine hundred weaned pups each. Beltran emphasized the need for perspective: so far, the virus has affected only a small fraction of the weaned pups, and thousands of apparently healthy animals remain in the population. Public access to the park has been closed, and guided tours have been canceled. The researchers are now waiting for genetic sequencing to reveal how the virus arrived and what mutations it carries—information that will determine whether this outbreak remains contained or spreads further along the California coast.
Notable Quotes
This is exceptionally rapid detection of an outbreak in free-ranging marine mammals. We have most likely identified the very first cases here because of coordinated teams that have been on high alert with active surveillance.— Christine Johnson, UC Davis Institute for Pandemic Insights
We know the seals so well that it's very obvious to us when something is abnormal. My team observed abnormal behaviors in seals and increased mortality that we had not seen the day before in those exact same locations.— Roxanne Beltran, UC Santa Cruz
The Hearth Conversation Another angle on the story
Why did it take until now? The virus was devastating South American seals for years.
The northern Pacific populations may have had some immunity from earlier, milder strains. But we don't fully understand why. That's part of what makes this so urgent—we need to know what changed.
How did you catch it so fast?
We were watching every day during breeding season. We knew these individual seals. When behavior shifted overnight and animals started dying in specific spots, we saw it immediately. That's the only reason we got samples out so quickly.
Is this going to spread to other colonies?
That's the question we're trying to answer. We don't know yet if it's spreading mammal-to-mammal or only coming from dead birds. The genetic sequencing will tell us a lot.
What happens to the seals that are still healthy?
Most of them appear fine right now. But we're monitoring closely. The virus moves fast when it takes hold, so we're watching for any new signs.
Should people be worried about getting this from seals?
The virus has infected people, but almost always through direct contact with infected animals in occupational settings—dairy workers, poultry handlers. The risk to the general public is very low. We closed public access to the park out of caution, but this is primarily a wildlife crisis.