Seronegative disseminated neosporosis with fatal outcome in an adult dog following prednisone therapy.

A 4-y-old, male English Mastiff with a history of inflammatory neuromuscular disease developed progressive tetraparesis, ataxia, and severe temporal muscle atrophy, culminating in non-ambulatory status and euthanasia. The autopsy revealed diffuse muscle atrophy and pale pulmonary nodules. Histologically, polyphasic myositis was evident, with apicomplexan organisms within skeletal muscle myofibers. Encephalomyelitis, myocarditis, and hepatitis with protozoal cysts and tachyzoites also were observed. Immunohistochemistry of brain tissue was strongly positive for Neospora caninum and Toxoplasma gondii, but PCR testing confirmed N. caninum and excluded T. gondii, establishing a diagnosis of disseminated neosporosis. Despite extensive tissue involvement, N. caninum antibody titers were below the diagnostic cutoff. Our case highlights that non-positive serologic results do not exclude neosporosis in clinically compatible cases, particularly in dogs undergoing corticoid therapy, and underscores the diagnostic value of histopathology with confirmatory molecular testing for definitive diagnosis.