H5N1 bird flu spreads to sea otters and sea lions along San Mateo coast, wildlife experts say

"The strain the animals have contains a mutation allowing it to more easily transmit between mammals. It is also a different variation than the ones found in dairy cows and commercial poultry. This one is Eurasian in origin, first seen in 2022. It has been detected in birds that fly along the Pacific Flyway, and is responsible for a mass mortality event in 2023 in northern fur seals on an island in eastern Russia."

"Johnson said researchers think this is the first detection of the A3 variation of the virus on the Pacific Coast and therefore likely a new introduction into North America. In late February, a team of researchers from UC Davis, UC Santa Cruz, the California Department of Fish and Wildlife and the Marine Mammal Center announced they had found the virus in seven dead elephant seal pups collected from the beach at Año Nuevo State Park. That number is now 16."

"This count reflects only the animals that have gone through sampling and confirmatory testing in multiple labs. We know there are more animals with signs of infection that we have sampled that are being tested across the different laboratory systems."

H5N1 bird flu outbreak in California elephant seals has expanded to other marine mammals including a sea otter and sea lion. The virus detected is a Eurasian strain originating from 2022, distinct from dairy cow and poultry variants, with a mutation enabling easier mammal-to-mammal transmission. This represents the first detection of the A3 virus variation on the Pacific Coast and likely a new introduction to North America. Initially 16 dead elephant seal pups were confirmed at Año Nuevo State Park in late February, with researchers expecting more confirmations. Wildlife officials remain cautiously optimistic about containment, though testing continues along the coastline beyond the current San Mateo County detection area.

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