H5N1 bird flu hits California elephant seal pups, UC Davis confirms first cases in species, seal-viewing areas closed as wild-mammal reservoir risk grows

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H5N1 avian influenza has been confirmed in northern elephant seal pups at Año Nuevo State Park on California’s central coast, after samples from seven sick and dead animals tested positive, according to UC Davis and the U.S. Department of Agriculture’s National Veterinary Services Laboratory. The cases involve recently weaned pups in San Mateo County, where researchers began finding dead weanlings on 19–20 February.

California State Parks has closed public access to seal-viewing areas at Año Nuevo and cancelled guided tours for the rest of the season. Officials said the risk to the general public is low, but urged people to avoid contact with live or dead seals and to keep pets away.

The immediate story is a local wildlife outbreak; the larger issue is what happens when a virus stops being a “spillover” headline and becomes a recurring feature of mammal populations. In previous U.S. incidents—in Maine in 2022 and Washington state in 2023—investigators concluded seals were likely infected by wild birds, a transmission route that is hard to block once it becomes routine. UC Davis called the Año Nuevo detection “exceptionally rapid” because teams were already running active surveillance.

Año Nuevo hosts one of the largest mainland elephant seal colonies in the United States, with around 5,000 animals during the winter breeding season and roughly 1,350 seals on the beach when this outbreak began, UC Davis said. That density turns a coastal rookery into a natural amplification point: many animals in close contact, with frequent interactions with seabirds and shared shoreline space.

The wider risk profile shifts when H5N1 is repeatedly found in wild mammals. A virus that circulates mainly in birds can be fought with poultry biosecurity and culling; a virus that also establishes itself in free-ranging mammals forces a different kind of response—monitoring, closures, and guidance that can be maintained for years rather than weeks. The “eradication” language that fits farm outbreaks becomes less meaningful when the reservoir is the coastline.

The precedent is already visible in the southern hemisphere. H5N1 has caused major marine mammal die-offs in Argentina and Chile, including severe impacts on southern elephant seals, UC Davis noted. Each additional species and geography adds another set of hosts that cannot be fenced, vaccinated, or easily counted.

At Año Nuevo, the practical policy response is simple: restrict access and watch the beach. The longer-term reality is that the surveillance apparatus grows even when the outbreak ends, because the next one is now expected.

Año Nuevo’s seal-viewing areas are closed for the remainder of the season, and the confirmed cases so far involve seven recently weaned pups.