H5N1 Bird Flu Spreads to Sea Otters, Sea Lions on San Mateo Coast

Wildlife experts warn of outbreak in marine mammals along California's central coast

Mar. 12, 2026 at 8:20pm

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Researchers have detected the H5N1 bird flu virus in sea otters and sea lions along the San Mateo County coast, after the virus was first discovered in elephant seals at Año Nuevo State Park. The strain contains a mutation that allows it to spread more easily between mammals, and is a different variation than the one found in dairy cows and commercial poultry. While the outbreak appears to be contained to the San Mateo area so far, wildlife officials are closely monitoring the situation.

Why it matters

The spread of the H5N1 virus to marine mammals is concerning, as it could have significant impacts on vulnerable populations like the southern sea otter. The virus has already decimated seal and sea lion populations in other parts of the world, and experts warn this outbreak could foreshadow wider spread along the Pacific Coast if not contained.

The details

After the H5N1 virus was first detected in elephant seals at Año Nuevo State Park in late February, researchers have now confirmed cases in a sea otter and several California sea lions. The virus contains a mutation that allows it to transmit more easily between mammals, and is a Eurasian strain that was first seen in 2022. So far, the outbreak appears to be limited to the San Mateo County coastline, but testing is ongoing along the entire California coast.

  • In late February, researchers found the H5N1 virus in 7 dead elephant seal pups at Año Nuevo State Park.
  • The number of confirmed elephant seal cases has now risen to 16, with researchers finding 2 new symptomatic and 2 new dead animals each day.
  • A 'handful' of California sea lions are also 'in the queue' for testing after showing signs of infection.

The players

Christine Johnson

The director of UC Davis' Center for Pandemic Insights, who said the A3 variation of the H5N1 virus found in the marine mammals is likely a new introduction to North America.

Patrick Robinson

The Año Nuevo reserve director and a marine biologist at UC Santa Cruz, who said 47 elephant seals on the mainland have died since the start of the outbreak.

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What they’re saying

“Outbreaks affect a wide range of birds and mammals, and these animals all share the near shore ecosystem. It's especially tragic when infections impact less common species in the southern sea otter.”

— Christine Johnson, Director, UC Davis Center for Pandemic Insights (latimes.com)

“The outbreak is not over, and we're not really sure what's going to happen in the future. I remain hopeful about this thing right now.”

— Patrick Robinson, Año Nuevo Reserve Director, Marine Biologist at UC Santa Cruz (latimes.com)

What’s next

Researchers will continue monitoring the situation and conducting widespread testing along the California coastline to determine the full extent of the H5N1 outbreak in marine mammals.

The takeaway

This outbreak highlights the potential for zoonotic diseases like H5N1 to spill over from birds to other wildlife, underscoring the need for vigilant disease surveillance and preparedness to protect vulnerable species and ecosystems.