California Marine Mammals with H5N1
#19,099
Just over a month ago the Año Nuevo Natural Reserve in San Mateo County, CA announced the first U.S. detection of H5N1 in Elephant Seals after dozens of seals were observed either sick or dying and the virus was confirmed by the USDA’s NVSL lab.Since then, according to local news outlets (see CIDRAP report), the number of HPAI positive seals has increased, along with an H5 positive otter and a sea lion. The actual count is likely much higher, as only a limited number of mammals have been tested.
In early March we saw an update and health advice from California's DPH and just over a week ago a paper on the impact of HPAI H5 on pinnipeds (see UC Davis: High pathogenicity Avian Influenza in Pinniped conservation).
Late yesterday, SLO (San Luis Obispo) county announced the discovery of a dead sea lion (H5N1 positive) on Morro Strand State Beach - some 150 miles south of the first outbreak - in the following press release.Deceased Sea Lion Found in San Luis Obispo County Tests Positive for H5N1 (Bird Flu)
Author: Public Health
Date: 3/27/2026 2:07 PM
While the risk of H5N1 to the public remains low, officials ask community members and visitors to avoid approaching marine mammals and seabirds and report them instead through the appropriate channels. En español
San Luis Obispo, Calif— A deceased sea lion found along Morro Strand State Beach has tested positive for H5N1 (avian influenza or bird flu), marking the first confirmed case in a marine mammal in the county.
While the risk of H5N1 to the public remains low, officials from San Luis Obispo County Public Health Department, the California Department of Fish & Wildlife, and the Central California Marine Animal Response Team urge community members and visitors to avoid approaching marine mammals and seabirds. This is especially important for animals that appear sick, injured, or deceased.
“Staying 150 yards away from all marine mammals and seabirds; keeping children and pets away from sick, injured or dead wildlife; and not approaching, touching or attempting to assist marine mammals or seabirds,” states guidance from the California Department of Public Health.
Officials ask the public not to touch or help wildlife and instead to call:
- NOAA West Coast Marine Mammal Stranding Hotline at (866) 767-6114 for assistance with marine mammals
- California Department of Fish and Wildlife at 916-358-2790 for assistance with birds
“While the risk to the public remains low, it’s understandable that people may have questions about their own health if they find themselves unexpectedly exposed to a sick or dead animal,” said County Health Officer, Dr. Penny Borenstein. “If community members have questions about H5N1 as it relates to human health, our Public Health Department can help answer those questions.”
Community members can reach the County of SLO Public Health Department by calling 805-781-5500. To learn more about H5N1 in California by visiting the California Department of Public Health’s bird flu webpage or by reading updates from UC Davis regarding recent H5N1 activity in marine mammals.
History of bird flu in SLO County
Positive cases of bird flu have been found in SLO County since 2022 from various wild birds within the county. In 2024, many states, including California, experienced outbreaks of H5N1 in dairy cattle. There were 38 confirmed human cases in California linked to exposures from infected cattle between September 2024 and January 2025 with none in SLO County.
UC Davis maintains an H5 Influenza A Outbreak in Marine Mammals webpage which now lists 28 Elephant Seals, 2 Sea Lions, and 1 otter as having tested positive in California over the past 6 weeks.
Compared to what we've seen in South America, this remains a small outbreak, but it is likely we are only seeing the tip of the iceberg. Many animals may have died unnoticed at sea, or have washed up on remote beaches.
Although H5N1 has yet to crack the `transmission code' in humans, evidence suggests marine mammals may be closer to achieving sustained transmission (see Preprint: Massive outbreak of Influenza A H5N1 in elephant seals at Peninsula Valdes, Argentina: increased evidence for mammal-to-mammal transmission).
While that would be a terrible thing for marine mammals, it might not work out too well for us, either.
