Between 2020 and 2022, four southern sea otters (Enhydra lutris nereis) stranded and died in California, USA. The most striking gross lesion in all four otters was marked discoloration and an unusual, granular texture of the subcutaneous and internal fat tissues. Histopathologic evaluation revealed that systemic toxoplasmosis was the primary cause of death for all four otters which was confirmed via immunohistochemistry and PCR. High parasite burdens were seen microscopically in many tissues, including fat tissues which were also markedly inflamed (steatitis). Protozoal-associated steatitis is highly unusual and has never before been documented in sea otters with systemic protozoal infections. Multilocus sequence typing across 13 loci revealed that all four otters were infected with the COUG (TgCgCa1) strain of Toxoplasma gondii, an atypical and rare North American strain that has never been reported in southern sea otters nor any other aquatic intermediate host. The COUG strain was first isolated from two mountain lions in British Columbia, Canada during an investigation into a waterborne outbreak of toxoplasmosis in humans in 1995. Similar to other T. gondii strains, COUG strain oocysts presumably travel from terrestrial felid feces via surface water runoff to contaminate the coastal nearshore environment where sea otters become infected, but the felid population shedding COUG oocysts in California remains unknown. This genotype appears to have been recently introduced to this environment given that it has not previously been detected in more than two decades of T. gondii surveillance in the southern sea otter population. The discovery of this highly virulent strain is concerning for threatened southern sea otters, whose population is already under pressure from other stressors including infectious agents, predation, anthropogenic activities, and resource limitations. Although no reports of the COUG strain infecting humans exist to-date, the detection of this virulent strain in sea otters also raises public health concerns for humans that share the same marine environment and food source (e.g. shellfish) as sea otters. This presentation will discuss our findings from the initial case series of four sea otters, as well as new insight from additional sea otters infected with the COUG strain that have been identified since 2022.