Strong, healthy and newly weaned from her mom on February 18, the plump elephant seal pup on the beach at Año Nuevo State Park represented one more success story in the long and remarkable conservation effort to restore a wild species once on the brink of extinction.
But within two days, she was dead—killed by a highly pathogenic strain of avian flu virus called H5N1. If other deaths follow, the park’s entire population is at risk, say scientists.
The deadliest and most infectious bird flu ever to strike North America, the strain has ravaged domestic poultry flocks, and sickened and killed more species of wild birds across a greater geographic area than any previous outbreak. But birds are not the only creatures the virus has felled. With this new outbreak, northern elephant seals are the latest victims in the growing list of vulnerable species.
So far, six other pups are confirmed to have died from the virus, the first-ever cases in northern elephant seals. A total of 30 deaths are suspected. Their ends were gruesome, with weakness, tremors, convulsions and respiratory distress. “We are completely devastated that highly pathogenic avian influenza has reached this population,” says Roxanne Beltran, a professor in ecology and evolutionary biology at UC Santa Cruz. Her lab leads UC Santa Cruz’s northern elephant seal research program at Año Nuevo. “A change in the number of pups that survive in a given year has a really, really, really long-lasting consequence on the population of seals,” she says.
How bad is it? We’re not sure yet.
The good news is that more than 1,300 other weanlings at Año Nuevo are still healthy. And their parents, who recently left the crowded beaches in search of food, are at lower risk. No illnesses or deaths have been reported in colonies at Point Reyes National Seashore, the Piedras Blancas near San Simeon, or other California beaches.
Wildlife biologists hope that the outbreak will stay contained, then vanish. That’s what happened when the virus infected harbor seals in Maine in 2022 and in Washington in 2023. Perhaps Ano Nuevo’s tragic cluster will be a mere footnote in some future scientific report.
But they’re braced for the worst. A 2023 epidemic in Argentina’s Southern elephant seals was the greatest calamity to affect marine mammals in South America in recent history. There, the three beaches where the species breeds were littered with over 17,000 dead pups, representing 70 percent of all animals born in 2023, according to research co-led by UC Davis. It could take more than a century for that population to return to pre-outbreak numbers.
The deadly infections spread between the Argentine seals rather than directly from birds—marking a significant and dangerous shift in the virus’s behavior.
Get Bay Nature’s Free Weekly Newsletter
A similar outbreak at Año Nuevo would be a huge setback for a large breeding colony that has taken decades to build.
Why the outbreak was detected so early
Elephant seals were once presumed extinct. But in 1892, a small colony of fewer than 100 animals was discovered off the coast of Baja California.
Now protected, populations are expanding north, turning long-empty beaches into a ruckus of burps and belches, moans and snorts. Every one of today’s pups descends from these early survivors.
But the Año Nuevo colony, established in 1955, is unique. With a coordinated team of scientists from UC Davis, UC Santa Cruz, and California State Parks, its animals are among the best-studied on Earth. Monitoring has intensified in recent years since avian flu established a firm foothold in wild mammals. So an uptick in the number of dead seals at Año Nuevo last Thursday (Feb. 19) and Friday (Feb. 20) was instantly detected.
“Of immediate concern was that the seals were showing signs of respiratory and neurologic disease,” says Christine Johnson, director of the Institute for Pandemic Insights at the UC Davis Weill School of Veterinary Medicine. The neurologic symptoms—weakness, tremors and seizures—suggested a viral infection.
Inside the hunt to understand a death
The team collected mucus and respiratory specimens from the animals, then rushed them to UC Davis for testing. The U.S. Department of Agriculture’s National Veterinary Services Laboratory confirmed the test results on Tuesday evening. “This is exceptionally rapid detection of an outbreak in free-ranging marine mammals,” says Johnson.
The body of one of the dead pups, born in early February to a six-year-old tagged female, was carefully transported to the UCSC campus for a necropsy led by UCSC wildlife veterinarian Megan Moriarty.
Removing her skin in big sheets, the team found that she was in excellent nutritional condition, which told them she had died quickly. Her chest cavity was filled with watery fluid and her lungs were very swollen and red-purple. Her lymph nodes, dark red, were abnormally large. Her spleen was swollen. On the surface of her brain, many blood vessels were dilated. A detailed microscopic examination of preserved tissues is under way. Necropsies, like human autopsies, help veterinarians understand an animal’s precise cause of death.
These findings hit hard, says Beltran.
“We know these individuals. We know where they spend their time, we know their family lineages across generations,” says Beltran, exhausted by nonstop 20-hour work days. “This makes it especially difficult for our team.”
The origin of an outbreak
How was she infected? No one knows for sure. But it’s likely that the virus originated from contact with the feces of sick birds, says scientists. Brandt’s cormorants, Western Gulls and brown pelicans, the most abundant birds at Año Nuevo Island, are very susceptible to H5N1.
Avian flu, first identified in 1996, started out as just a bird problem. In 2020, a new, highly pathogenic form emerged in Europe and spread quickly around the world. Just like our more familiar flu viruses, H5N1 constantly changes as it multiplies. Mutations have helped it jump from birds to domestic animals like cats and dogs, zoo animals like tigers and leopards, and wild animals like bears, foxes, seals, and skunks. The severity varies among species from mild to severe. (There is no evidence of human-to-human transmission.)
How to help your local seals: Stay away
About 75 miles north at Point Reyes National Seashore, biologists are actively monitoring the population for signs of illness. Staff are conducting daily visual assessments and, when necessary, entering colonies in personal protective equipment to evaluate any animals that show signs of illness, says spokesman Scott Carr of the Pacific West and Alaska Regions of the National Park Service.
Año Nuevo has canceled tours, to protect the public and prevent viral spread. The public should not attempt to rescue sick animals, warned Dominic Travis, wildlife veterinary epidemiologist at the Marine Mammal Center.
The Sausalito-based nonprofit center, which traditionally treats ailing elephant seals, has paused its rescue efforts “until we understand this situation better,” he says.
No easy cure—but research may help
There is no approved marine mammal flu vaccine. While research is underway on a vaccine to protect endangered monk seals, it is not designed for use in California elephant seals.
But Año Nuevo’s decades of research will make it possible to closely watch the outbreak’s trajectory. Many of its animals are tagged, identified and tracked on their travels. This epidemiologic work will reveal if there is seal-to-seal transmission, like what happened in Argentina Genetic sequencing could offer insights about the virus, its evolutionary history and its virulence. Does the Año Nuevo strain match the virus that killed so many southern elephant seals? If so, that is ominous.
Immunologic study could show whether some elephant seals are able to fend off the pathogen. More benign flu viruses routinely infect the animals, so it is possible that the seals carry protective antibodies.
For now, “we know not to make any predictions, because we’re unsure how the situation will proceed,” says Johnson. “We’ve caught this outbreak at the very, very beginning stages.”
