Gross Findings: 

On external examination, all teeth are present and in excellent condition with no wear, and there is no grizzling of the fur (subadult animal).  There is soft swelling of the neck and subcutaneous tissue, so that there are loose, soft folds of skin over the back of the head and dorsal aspect of the neck when the animal is in dorsal recumbency. On internal examination, there is scant to adequate subcutaneous, visceral, and epicardial fat. A large area of subcutaneous and intramuscular hemorrhage extends from the caudal aspect of the skull over the entire cervical region to the cranial regions of the shoulder muscles. The left hyoid bone is fractured near its attachment to the thyroid cartilage. Severe muscle rupture and fragmentation is present over the ventral aspect of cervical vertebrae 2 and 3, and generally in a zone extending from C1 to C4.  Trauma, likely due to predation, is diagnosed as the cause of death. 

Histopathological Findings: 

Multifocally, there is mild to moderate gliosis and lymphocytic perivascular cuffing in the cerebellum and cerebrum in the brain. Protozoal schizonts in varying stages of development are found within or adjacent to some areas of inflammation (Fig 1A). A focus of mostly mononuclear inflammation is present in the retina of the left eye, and in the affected area the inner and outer nuclear layers are disorganized and irregular, and numerous protozoal schizonts are present (Fig 1B). Immunohistochemistry for Sarcocystis neurona in both the brain and the eye is positive. In the right atrium of the heart, there is a thick-walled, mature Sarcocystis sp. tissue cyst within a cardiomyofiber near the atrioventricular junction (Fig 1C). A moderate number of Sarcocystis-like tissue cysts are also present within myofibers in the skeletal muscle; metrocytes are present toward the perimeter and small zoites are in the center (Fig 1D).

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Morphologic Diagnoses:

Brain – Acute to subacute, moderate, multifocal lymphocytic meningoencephalitis with intralesional Sarcocystis neurona protozoa.

Eye – Acute to subacute, moderate, focal retinochoroiditis with intralesional Sarcocystis neurona protozoa.

Disease: Sarcocystis neurona infection

Etiology:  Sarcocystis neurona. S. neurona is a coccidian parasite belonging to the order Apicomplexa, family Sarcocystidae.  

Distribution: The definitive hosts of S. neurona are believed to be members of the opossum family (Didelphis virginiana in North America and other members of the Didelphidae family in Central and South America) and therefore disease caused by this parasite is confined to these geographic areas. In sea otters, disease has been found only in the Pacific Coast of North America.

Seasonality: Cases in sea otters occur most commonly in the spring and early summer and are associated temporally with large rainwater runoff events. 

Host range: S. neurona is best known as the main cause of equine protozoal myeloencephalitis. However, along with sea otters, it has been reported to cause disease in a variety of other species, though generally less commonly. Affected species currently reported include Pacific harbor seals (Phoca vitulina richardsi), California sea lion (Zalophus californianus), and (rarely) domestic cats, dogs, raccoons, ferrets, fisher, mink, lynx, and skunks. 

Transmission: Infection occurs most often through the direct ingestion of sporocysts shed in opossum feces, but may also occur through the consumption of muscle containing tissue cysts. Sea otters are believed to be infected either by direct consumption of sporocysts or by consumption of sporocysts in filter-feeding invertebrates such as clams.

Clinical signs: S. neurona infection in sea otters is often diagnosed in stranded animals that are found dead. In live animals, a variety of neurologic signs referable to the central nervous system may occur, such as ataxia, seizures, and cranial nerve deficits.

Pathology: Infection with S. neurona in sea otters causes multifocal to coalescing, nonsuppurative meningoencephalitis which may occur anywhere within the brain but with a predilection for the cerebellum and midbrain. Immature and mature schizonts or free merozoites are variably present in areas of inflammation. In addition, immature sarcocysts may be found within the skeletal muscle or the heart, where they may be associated with mild myofiber degeneration or necrosis.

Diagnosis: Definitive diagnosis requires the use of immunohistochemistry to distinguish S. neurona from Toxoplasma gondii.

Public health concerns: Humans may be infected by a number of Sarcocystis species, however no cases of infection with S. neurona have as yet been reported.

Wildlife population impacts: Sarcocystosis is a significant direct cause of mortality in sea otters. It may also play a role in deaths due to predation or other traumatic events due to the neurologic deficits it produces, however these cases have not been quantified. 

Management: No management strategies have been described at this time.

References: 

• Burgess TL, Tinker MT, Miller, MA et al. 2020. Spatial epidemiological patterns suggest mechanisms of land-sea transmission for Sarcocystis neurona in a coastal marine mammal. Sci Rep 10:3683. https://doi.org/10.1038/s41598-020-60254-5
• Dubey JP, Saville WJ, Lindsay DS, Stich RW, Stanek JF, Speert CA, Rosenthal BM, Njoku CJ, Kwok OC, Shen SK, Reed SM. 2000. Completion of the life cycle of Sarcocystis neurona. J Parasitol. 86:1276-80. https://doi.org/10.1645/0022-3395(2000)086[1276:COTLCO]2.0.CO;2
• Dubey JP, Thomas NJ. 2011. Sarcocystis neurona retinochoroiditis in a sea otter (Enhydra lutris kenyoni). Vet Parasitol. 183(2011):156-159. https://doi.org/10.1016/j.vetpar.2011.06.022
• Dubey JP, Howe DK, Furr M, Saville WJ, Marsh AE, Reed SM, Grigg ME. 2015. An update on Sarcocystis neurona infections in animals and equine protozoal myeloencephalitis (EPM). Vet Parasitol.  209(2015):1-42. https://doi.org/10.1016/j.vetpar.2015.01.026
• Lindsay DS, Thomas NJ, Dubey JP. 2000. Biological characterization of Sarcocystis neurona isolated from a Southern sea otter (Enhydra lutris nereis). Int J Parasitol. 30(2000):617-624. https://doi.org/10.1016/S0020-7519(00)00034-5
• Miller MA, Crosbie PR, Sverlow K, Hanni K, Barr BC, Kock N, Murray MJ, Lowenstine LJ, Conrad PA. 2001. Isolation and characterization of Sarcocystis from brain tissue of a free-living southern sea otter (Enhydra lutris nereis) with fatal meningoencephalitis. Parasitol Res. 87:252-7. https://doi.org/10.1007/s004360000340
• Miller MA, Conrad PA, Harris M, Hatfield B, Langlois G, Jessup DA, Magargal SL, Packham AE, Toy-Choutka S, Melli AC, Murray MA, Gulland FM, Grigg ME. 2010. A protozoal-associated epizootic impacting marine wildlife: mass-mortality of southern sea otters (Enhydra lutris nereis) due to Sarcocystis neurona infection. Vet Parasitol. 172(2010):183-194. https://doi.org/10.1016/j.vetpar.2010.05.019
• Rosenthal BM. 2021. Zoonotic sarcocystis. Res Vet Sci. 136:151-157. https://doi.org/10.1016/j.rvsc.2021.02.008
• Thomas NJ, Dubey JP, Lindsay DS, Cole RA, Meteyer CU. 2007. Protozoal Meningoencephalitis in Sea Otters (Enhydra lutris): a Histopathological and Immunohistochemical Study of Naturally Occurring Cases. J Comp Path. 137:102-121. https://doi.org/10.1016/j.jcpa.2007.05.001