Quick Take
The H5N1 bird flu outbreak in California elephant seals has spread to other marine mammals, including a sea otter and sea lion. But wildlife officials are hopeful the outbreak will remain contained.
Last month’s outbreak of deadly bird flu among northern elephant seals in Año Nuevo State Park has spread to other species of marine mammals, including a sea otter and a sea lion, wildlife specialists said at a media briefing on Thursday.
Confirmed laboratory testing brought the total number of elephant seals infected with highly pathogenic H5N1 avian influenza to 16. Although the count reflects only the animals that have been sampled and tested, “there are likely more animals with signs of infection,” said Christine Johnson, director of the National Science Foundation Center for Pandemic Insights at UC Davis.
According to Johnson, it’s not unusual for an avian influenza outbreak to affect other mammal species. “H5N1 outbreaks infect a wide range of birds and mammals that share the north shore ecosystem,” she said.
The new cases confirmed Tuesday are connected to the original outbreak cluster in San Mateo County, though scientists have also found some animals suffering from H5N1 symptoms on beaches slightly to the north and south of Año Nuevo State Park, just north of the San Mateo-Santa Cruz county line.
A veterinary pathologist at the California Department of Fish and Wildlife evaluated the elephant seal and sea otter cases and found symptoms of brain infection and pneumonia consistent with what scientists have observed among sick animals in San Mateo County.
Testing conducted by the U.S. Department of Agriculture has determined the A3 genotype of the H5N1 virus is the cause of these new cases. This genetic marker is known to adapt and spread among mammalian hosts.
This is the first time the A3 genotype has reached California. In the summer of 2023, the same genotype caused a mass mortality event in northern seals on an island in eastern Russia.
As research teams ramp up testing, they are finding an average of two dead animals and two newly symptomatic animals each day, said Patrick Robinson, director of the Año Nuevo Reserve. However, his team is observing only a small number of potentially affected animals daily, compared to the total population of elephant seals that breed on the beach or marine mammals that pass through or live there year-round.
Wildlife specialists said an estimated 47 elephant seals have died since the start of the outbreak. Weaned pups represent about 5% of the dead animals, while large males represent about 6%.
“This outbreak is not as bad as what you see in some areas,” Robinson said, while also noting this year’s mortality rate is about four times higher than last year’s.
When the outbreak began on Feb. 19, about 80% of adult female seals had already departed on their foraging migration. Right now, nearly all of the seals have left the beach, and no adult female seals have been observed to be symptomatic or dead. Robinson said this is “great news” for preventing further spread of the virus.
Año Nuevo State Park remains closed to tours, while local, state and federal agencies plus academic researchers monitor the virus and conduct testing in the field.
The California Department of Public Health urges beachgoers to avoid dead animal carcasses along the coast. While the risk to the public remains low, contact with sick or dead wildlife can increase chances of the virus transmitting to humans.
Although scientists don’t know the trajectory for this virus outbreak, researchers are hoping it burns out, and will continue to share updates on the UC Davis Institute for Pandemic Insights webpage.
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