Sarcocystis sp. infection associated to encephalomyelitis and myositis in a Brazilian dog

Sarcocystis sp.-induced disease is known especially in horses, in which S. neurona is the primary causative agent of Equine Protozoal Myeloencephalitis in the Americas. While horses are aberrant hosts, the range of susceptible species includes marine mammals, raccoons, and cats. Reports of clinical disease in dogs are rare, remaining unreported in South America. This study describes the clinical, pathological, and immunohistochemical findings of a naturally occurring, fatal case of meningoencephalomyelitis and myositis caused by Sarcocystis sp. in a dog in Brazil. A 5-year-old mixed-breed bitch presented with acute non-ambulatory tetraparesis, generalized muscle atrophy, and blindness. Despite supportive therapy, the patient deteriorated and was euthanized. Gross findings included enlarged lymph nodes, moderate splenomegaly and hepatomegaly, with multifocal mild fibrin deposition on the liver capsule, accompanied by multifocal mild hemorrhages in the pancreas. Histopathology demonstrated severe, multifocal necrotizing meningoencephalomyelitis, myositis, splenitis, lymphadenitis and interstitial pneumonia, associated with intralesional protozoal schizonts. Immunohistochemistry supported immunolabelling consistent with Sarcocystis sp. while excluding Toxoplasma gondii and Neospora caninum . To the authors’ knowledge, this represents the first report of clinical canine sarcocystosis with this severity in South America supported by histopathological and immunohistochemical findings. These findings support that Sarcocystis sp. should be considered a differential diagnosis for canine progressive neurological disease, highlighting the potential role of wildlife reservoirs in the disease’s epidemiology.