Situation Update
- 05/12/2026: The HPAI H5N1 outbreak in marine mammals in California, which occurred between February 12 and March 24, 2026, is now officially over, with no new positive cases found in tested marine mammals for over 30 days. The outbreak resulted in 64 positive marine mammal cases out of 233 tested animals (27% overall positivity). Positive cases included 57 elephant seals, 6 California sea lions, and 1 southern sea otter. The majority of cases in marine mammals (57) were concentrated in San Mateo County, with additional positive cases in Santa Cruz, Monterey, and San Luis Obispo counties. Preliminary analyses have shown that the sequences from the outbreak cluster tightly as the HPAI H5N1 A3 genotype.
Scientists are returning to normal research efforts, including routine surveys to look for sick elephant seals at Año Nuevo Reserve. Post-outbreak investigations of stranded marine mammals throughout California include screening of animals that could be suspect for HPAI H5N1 infection, based on all that we have learned during this outbreak. We plan to increase surveillance activities in marine mammals during the next bird migration in the fall of 2026.
We are very grateful to the partners listed below that supported the outbreak investigations, the many federal, state, and local agencies that oversaw aspects of this response, and the frontline responders that constitute the NOAA Fisheries West Coast Marine Mammal Stranding Network. - 03/26/2026: Scientists have confirmed a case of HPAI H5N1 in a California sea lion in San Luis Obispo County, California. The young (1-2 year old) dead sea lion was sampled by a researcher from Central California Marine Animal Response Team (CCMART) on March 16, and samples were sent to UC Davis and the USDA’s NVSL lab where presence of the virus was confirmed on March 25. This is the first report of HPAI in a marine mammal from San Luis Obispo County which is south of the main outbreak response area in San Mateo County. Juvenile sea lions are known to forage along the central California coast traveling as far south as the primary sea lion breeding colonies on the Channel Islands. Researchers are continuing to monitor marine mammal populations all along the coast.
- 03/12/2026: A recording of the Press Briefing held today can be viewed here.
- 03/10/2026: Scientists have confirmed a case of HPAI H5N1 in a California sea lion in San Mateo County, California. The adult male sea lion was found dead on February 27 near Año Nuevo State Reserve by the California Academy of Sciences. Samples were sent to UC Davis and the USDA’s NVSL laboratory, where presence of the virus was confirmed on March 9. To date, all positive HPAI H5N1 cases have been detected in San Mateo County. A network of responders continue to investigate stranded or dead marine mammals in neighboring areas to assess influenza activity in seals, sea lions, and sea otters. People and their pets should avoid all contact with sick or dead wildlife and can report dead or distressed marine mammals on the west coast to the NOAA Fisheries West Coast Region Stranding Hotline: (866) 767-6114.
- 03/06/2026: Scientists have confirmed a case of HPAI H5N1 in a southern sea otter in San Mateo County, California. The southern sea otter was found dead by UC Santa Cruz researchers on February 26. A postmortem examination was conducted by trained wildlife disease experts at the California Department of Fish and Wildlife’s Marine Wildlife Veterinary Care & Research Center in Santa Cruz, and samples were sent to UC Davis and the USDA’s NVSL lab, where presence of the virus was confirmed on March 5. Researchers continue to monitor marine mammal populations to understand the full extent and impact of the disease outbreak.
- 03/05/2026: We are continuing to monitor progression of the outbreak and investigate the impact to animals. To date, approximately 60 marine mammals are currently being tested for suspected influenza virus infection at Año Nuevo State Park. In addition, we are evaluating stranded or dead marine mammals in neighboring areas to determine if influenza has spread to other species or other regions. If new cases are confirmed, updates will be posted to this site.
- 02/25/2026: On February 19th and 20th, multiple northern elephant seal weanling pups (recently weaned from nursing) were observed dead at Año Nuevo State Park in San Mateo County, California. Live elephant seal weanlings were also observed with tremors and seizure activity. In collaboration with University of California, Davis, researchers from University of California, Santa Cruz responded to collect samples from 7 sick and dead animals for testing at the California Animal Health & Food Safety Laboratory System (CAHFS) laboratory at UC Davis. Initial screening revealed that the samples were positive for influenza A, subtype H5. The USDA National Veterinary Services Laboratory (NVSL) later confirmed infection with influenza A subtype H5N1 (Highly Pathogenic Avian Influenza, HPAI).
