Mongabay
By Christine Heinrichs
Ever since a deadly strain of avian influenza, H5N1, killed some 17,000 southern elephant seal pups on South American coastlines in 2023 and 2024, researchers and public officials have kept an extra-close eye on California’s northern elephant seals. Fears of infection have now become reality: Lab tests just proved the virus has breached this colony.
In mid-February, six young, newly weaned seals on Año Nuevo State Park beaches fell ill. They had obvious respiratory problems and also suffered from neurological symptoms, including weakness, tremors and seizures — all of which pointed to H5N1.
The research team collected samples from sick and dead elephant seals, which were analyzed at the California Animal Health and Food Safety Laboratory System. Initial screening revealed that the samples were positive for avian influenza; it was then confirmed to be the highly pathogenic H5N1 strain.
As of Feb. 24, seven pups had tested positive for the virus, according to the USDA’s National Veterinary Services Laboratory. At time of publication, 30 seals had died, 29 of them weaned pups, but the cause has not yet been confirmed for all the victims.
The outbreak marks the first cases of H5N1 in marine mammals in California and the first time it’s been found in northern elephant seals (Mirounga angustirostris). This highly contagious virus has been circulating the planet as a panzootic — an animal pandemic — since 2020, infecting and killing some 700 species of birds and mammals.
Because of the constant monitoring of these seals, the virus was detected “very early in the outbreak,” Roxanne Beltran, an assistant professor in ecology and evolutionary biology at UC Santa Cruz, said during a press conference. Beltran’s lab leads the university’s northern elephant seal research program at Año Nuevo.
Her colleague, Christine Johnson, elaborated. “This is exceptionally rapid detection of an outbreak in free-ranging marine mammals,” she said. Johnson directs the Institute for Pandemic Insights at the University of California, Davis. “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.”
On Monday, Feb. 23, California State Parks barred the public from the elephant seal viewing area of the Año Nuevo Coast Natural Preserve. Then, with confirmation that H5N1 was responsible, tours have been canceled for the rest of the season.
A deadly virus
Avian flu — which, in another, milder strain is much like the common cold in wild birds — morphed and became pathogenic when chickens and other poultry at industrial-scale producers were exposed to the virus through contact with migrating flocks of wild birds. Since it first appeared in Europe in 2020, this “Highly Pathogenic Avian Influenza” strain has devastated wildlife worldwide, the largest avian flu outbreak ever. And this panzootic is obviously not over.
H5N1 has raged on, leaping the species barrier to infect animals on six continents, pole to pole. Animals that gather in large groups, like pinnipeds and birds, are particularly vulnerable. Proximity is a big factor in a virus’s ability to spread, as the world learned too well during the COVID-19 pandemic.
Animals can be infected by contact with an infected bird or animal or their droppings. Both scavengers and carnivores may catch H5N1 by eating an infected carcass. But in 2024, researchers made a startling discovery about how this quickly mutating virus had changed: They discovered that elephant seals were passing the virus between themselves. This method of transmission makes a virus infinitely more dangerous. Since then, animal-to-animal transmission has been confirmed in the wild, in zoos and on farms.
Some of the wildlife victims are endangered species, and this virus’s ability to spread to new hosts is astounding. As of December 2025, H5N1 had infected some 598 types of bird and 102 mammal species, according to the United Nations. The numbers have jumped substantially over the past 18 months: As of August 2024, the U.N. tally was 485 bird and 48 mammal species.
H5N1 has stricken or killed animals as diverse as sea otters, house cats, terns, dolphins, foxes, California condors, rats, albatrosses, cougars, polar bears, zoo tigers — and many, many others, including humans. An outbreak in imperiled species could push them to extinction: Wildlife is already fighting to survive against a changing climate, disappearing habitat and other stressors.
By Christine Heinrichs
Ever since a deadly strain of avian influenza, H5N1, killed some 17,000 southern elephant seal pups on South American coastlines in 2023 and 2024, researchers and public officials have kept an extra-close eye on California’s northern elephant seals. Fears of infection have now become reality: Lab tests just proved the virus has breached this colony.
In mid-February, six young, newly weaned seals on Año Nuevo State Park beaches fell ill. They had obvious respiratory problems and also suffered from neurological symptoms, including weakness, tremors and seizures — all of which pointed to H5N1.
The research team collected samples from sick and dead elephant seals, which were analyzed at the California Animal Health and Food Safety Laboratory System. Initial screening revealed that the samples were positive for avian influenza; it was then confirmed to be the highly pathogenic H5N1 strain.
As of Feb. 24, seven pups had tested positive for the virus, according to the USDA’s National Veterinary Services Laboratory. At time of publication, 30 seals had died, 29 of them weaned pups, but the cause has not yet been confirmed for all the victims.
The outbreak marks the first cases of H5N1 in marine mammals in California and the first time it’s been found in northern elephant seals (Mirounga angustirostris). This highly contagious virus has been circulating the planet as a panzootic — an animal pandemic — since 2020, infecting and killing some 700 species of birds and mammals.
Because of the constant monitoring of these seals, the virus was detected “very early in the outbreak,” Roxanne Beltran, an assistant professor in ecology and evolutionary biology at UC Santa Cruz, said during a press conference. Beltran’s lab leads the university’s northern elephant seal research program at Año Nuevo.
Her colleague, Christine Johnson, elaborated. “This is exceptionally rapid detection of an outbreak in free-ranging marine mammals,” she said. Johnson directs the Institute for Pandemic Insights at the University of California, Davis. “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.”
On Monday, Feb. 23, California State Parks barred the public from the elephant seal viewing area of the Año Nuevo Coast Natural Preserve. Then, with confirmation that H5N1 was responsible, tours have been canceled for the rest of the season.
A deadly virus
Avian flu — which, in another, milder strain is much like the common cold in wild birds — morphed and became pathogenic when chickens and other poultry at industrial-scale producers were exposed to the virus through contact with migrating flocks of wild birds. Since it first appeared in Europe in 2020, this “Highly Pathogenic Avian Influenza” strain has devastated wildlife worldwide, the largest avian flu outbreak ever. And this panzootic is obviously not over.
H5N1 has raged on, leaping the species barrier to infect animals on six continents, pole to pole. Animals that gather in large groups, like pinnipeds and birds, are particularly vulnerable. Proximity is a big factor in a virus’s ability to spread, as the world learned too well during the COVID-19 pandemic.
Animals can be infected by contact with an infected bird or animal or their droppings. Both scavengers and carnivores may catch H5N1 by eating an infected carcass. But in 2024, researchers made a startling discovery about how this quickly mutating virus had changed: They discovered that elephant seals were passing the virus between themselves. This method of transmission makes a virus infinitely more dangerous. Since then, animal-to-animal transmission has been confirmed in the wild, in zoos and on farms.
Some of the wildlife victims are endangered species, and this virus’s ability to spread to new hosts is astounding. As of December 2025, H5N1 had infected some 598 types of bird and 102 mammal species, according to the United Nations. The numbers have jumped substantially over the past 18 months: As of August 2024, the U.N. tally was 485 bird and 48 mammal species.
H5N1 has stricken or killed animals as diverse as sea otters, house cats, terns, dolphins, foxes, California condors, rats, albatrosses, cougars, polar bears, zoo tigers — and many, many others, including humans. An outbreak in imperiled species could push them to extinction: Wildlife is already fighting to survive against a changing climate, disappearing habitat and other stressors.
On the lookout
Scientists from UC Davis have been testing samples from marine birds and mammals along the coast since 2024. With colleagues from UC Santa Cruz, they’d increased surveillance at elephant seal beaches over the past two months in anticipation of a possible disease outbreak: From mid-December through March, the area becomes a nursery, as mothers arrive and give birth to their pups. The beaches are literally littered with seals, often in very close proximity.
“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,” Beltran said.
That wasn’t only because of the massive seal die-off in South America. “We had two prior outbreaks in U.S. marine mammals; not elephant seals, but other types of seals, one in Maine in 2022 and [another] in Washington state in 2023,” Johnson said. “Because of these trends and global trends in H5N1 outbreaks around the world, our teams, both at UC Davis and UC Santa Cruz, increased disease surveillance at Año Nuevo and other locations in anticipation of a possible spillover into seals.”
The team is now working closely with NOAA Fisheries, the California Department of Fish and Wildlife and the West Coast Marine Mammal Stranding Network to closely monitor marine mammals along the coast.
The rich eastern Pacific coast is a marine mammal hotspot, with about 350,000 northern elephant seals that haul out on at least 14 rookery beaches along the U.S. West Coast, offshore islands and Mexico.
Elephant seals congregate at various locations along the West Coast. The size of the circle shows the relative number of seals at that site. The seals’ flippers are tagged with different colors according to their birthplace. Image courtesy of Richard Condit, Population Biology of Northern Elephant Seals.
They share that coast with five other pinnipeds: 250,000-300,000 California sea lions (Zalophus californianus), about 66,000 northern or Steller sea lions (Eumetopias jubatus), some 14,000 northern fur seals, (Callorhinus ursinus), 35,000-44,000 Guadalupe fur seals (Arctocephalus townsendi) and perhaps 31,000 harbor seals (Phoca vitulina richardsi).
With some six decades of study, researchers have amassed astounding data on this elephant seal population. It includes some 380,000 observations of 55,000 individuals.
They’ve tracked individuals, built family trees, and they knew the history of one of the victims, a dead “weaner.” It was the offspring of a mother in the study who was herself born on that beach. The pup entered the researcher’s database when she was 15 days old. She was weaned when her mother left the beach; two mornings later, she was convulsing on the beach. By afternoon, she was dead.
“It’s tough to watch animals we have followed and watched for years get sick,” Beltran said. “We know their family lineages.”
This large body of research will greatly inform assessments of the long-term effects on the population: how many pups survive, whether females are affected and future births.
Rapid transmission
The virus’s ability to mutate rapidly and its record of infecting other species make it a cause of intense concern, and seal populations have suffered catastrophic losses. In 2022-23, H5N1 swept along South America’s Atlantic and Pacific coastlines, slaying more than 30,000 sea lions in addition to the devastation of the southern elephant seal (M. leonina) population on Argentina’s Península Valdés, which was the species’ largest die-off ever.
It’s also infected people. Since 2024, 71 human cases have been diagnosed in the U.S., with two deaths. Most cases involved hands-on contact with infected cows or poultry. Current public health risk is considered low, experts say, with no person-to-person transmission reported.
“The more a virus like this is able to mutate and find its way into a wide range of species, especially farmed species that live in close contact with people like poultry and now cattle, the more the odds go up that a viral strain will more easily make that leap to people,” wildlife veterinarian Steve Osofsky, a professor and wildlife health expert at Cornell University’s College of Veterinary Medicine, wrote in Statin June 2025.
Cautionary measures
To protect the public and limit virus transmission, the public has been barred from the area for the rest of the season. A California State Parks spokesperson said 4,363 tickets for Año Nuevo tours were canceled. Visitors pay $11 each to hike out 1.5-3 kilometers (1-2 miles) with a guide to view the elephant seals during the mid-December through March mating and pupping season.
Since this pathogen is zoonotic and can spread between wildlife, livestock and humans, surveillance extends beyond animals. With each leap to a new mammal host, it raises concern that the virus could more easily infect people. Since 2021, there have been 131 human infections globally, according to the Centers for Disease Control and Prevention. But thus far, it hasn’t spread between humans.
Over the past 50 years or so, zoonotic diseases have emerged and spread at ever-faster rates, facilitated by human conversion of wild habitats and global travel and trade. This allows humans and animals to swap germs that are quickly transported across the globe and shared with species that have no immunity to them. These emerging diseases rarely have a cure and are often fatal. Examples include HIV and Ebola.
Christian Walzer, executive director of health at the nonprofit Wildlife Conservation Society, called H5N1 “an existential threat to the world’s biodiversity.”
For now, the hope is that this is a small outbreak. “If it’s a cluster, we will figure it out,” said Dominic Travis, the chief programs officer at The Marine Mammal Center. “If it’s perpetuated, it will be really tricky. We will assess it day by day with NOAA.”
The timing of the outbreak may lean in the seals’ favor. “We are cautiously optimistic, as most of the adult females had already departed the beach for their routine migrations before the outbreak began, and most seals on the colony seem healthy,” Beltran said.
This article includes reporting by Sharon Guynup.
Source: This article was published by Mongabay
Citation: Uhart, M., Vanstreels, R. E., Nelson, M. I., Olivera, V., Campagna, J., Zavattieri, V., … Rimondi, A. (2024). Massive outbreak of influenza A H5N1 in elephant seals at peninsula Valdes, Argentina: Increased evidence for mammal-to-mammal transmission. doi:10.1101/2024.05.31.596774
Mongabay
Mongabay is a U.S.-based non-profit conservation and environmental science news platform. Rhett A. Butler founded Mongabay.com in 1999 out of his passion for tropical forests. He called the site Mongabay after an island in Madagascar.
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