H5N1 confirmed in northern elephant seals in California
Avian influenza H5N1 has been confirmed in northern elephant seal pups in California, the first known cases in marine mammals in the state and in this species.
On Thursday, UC Davis said the U.S. Department of Agriculture’s National Veterinary Services Laboratory confirmed highly pathogenic avian influenza H5N1 after samples from seven sick and dead seals tested positive. The cases involve recently weaned pups at Año Nuevo State Park in San Mateo County.
Researchers first observed multiple weanling pups dead on February 19 and 20. Other young seals showed tremors, seizures, weakness and respiratory problems, according to the Institute for Pandemic Insights at UC Davis.
Año Nuevo hosts one of the largest mainland elephant seal colonies in the United States, with about 5,000 animals during the winter breeding season. Roughly 1,350 seals were on the beach when the outbreak began.
“This is exceptionally rapid detection of an outbreak in free-ranging marine mammals,” said Christine Johnson, who directs the Institute for Pandemic Insights at UC Davis’ veterinary school. “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.”
Roxanne Beltran, who leads UC Santa Cruz’s northern elephant seal research program at Año Nuevo, said her team had increased monitoring in recent years as H5N1 spread across North and South America.
“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.
H5N1 has caused significant marine mammal die-offs elsewhere. The virus decimated southern elephant seal populations in Argentina and Chile. In the United States, outbreaks were reported in seals in Maine in 2022 and Washington state in 2023. Russian authorities have also reported infections in marine mammals.
Investigators in previous U.S. outbreaks determined the virus likely spread from infected wild birds to seals.
California State Parks has closed public access to seal viewing areas at Año Nuevo and canceled guided tours for the remainder of the season. Officials said the risk to the general public is low but advised people not to touch live or dead seals or allow pets near them.
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