Parasite genotype is a risk factor for Sarcocystis neurona-associated mortality in southern sea otters (Enhydra lutris nereis).

The protozoal parasites Sarcocystis neurona and Toxoplasma gondii are important causes of mortality for threatened southern sea otters (Enhydra lutris nereis) in California. Sarcocystis neurona causes more sea otter deaths than T. gondii and has caused two mortality events, yet S. neurona is comparatively understudied. Little is known about the role of parasite genotype on the outcome of infection (fatal versus non-fatal) for S. neurona-infected sea otters. The objective of this study was to evaluate the effect of parasite genotype in addition to host and environmental factors (animal age, sex, location, and season of stranding) on disease outcome in S. neurona-infected sea otters. A multilocus sequence typing (MLST) approach was used to characterize the S. neurona genotype in fatal (n = 92) and non-fatal (n = 33) sea otter infections. In the northern study region, the IIg/j genotype was more likely to result in fatal infections. In the southern study region, the Ia and Ib/c/d/gg genotypes were the dominant genotypes implicated in fatal infections and were responsible for mortality events that occurred in 2004 (Ib/c/d/gg) and 2021 (Ia). Subadult sea otters were more likely to die from S. neurona than adults. Stranding during the California wet season during peak rainfall events that facilitate land-to-sea flow of infective sporocysts may also play a role in fatal infection outcomes. These findings suggest that parasite genotype, as well as certain host and environmental factors, all contribute to disease outcome following S. neurona infection in southern sea otters.