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Background: Toxoplasmosis is caused by Toxoplasma gondii, an obligate intracellular protozoan that belongs to the Aplicomplexa phylum, coccidian subclass, and affects all warm-blooded animals. The role of opossums in the epidemiology of toxoplasmosis in Brazil is not fully understood, and there are very few descriptions of toxoplasmosis lesions in these animals. This report describes the anatomopathological, molecular and immunohistochemical findings of a case of encephalic toxoplasmosis in free-living white-eared possum (Didelphis albiventris).Case: A young male opossum (D. albiventris), was treated at the Veterinary Hospital of Wild Animals of the University of Brasília, Federal District. The animal was apathetic, uncoordinated, reluctant to move, and had an exposed proximal fracture in the left radius and ulna with laceration of muscles and adjacent tendinous structures. Amputation on the left thoracic limb was performed followed by analgesia and antibiotic therapy. The environment is frequented by other wild animals, and stray cats have access to the patio of the building. Twenty-five days after arriving at the hospital, the animal was found dead in its cage. After death, a necropsy was performed. Organ fragments from the abdominal cavity, thoracic and central nervous system were collected, processed routinely for histology and stained with hematoxylin and eosin. Macroscopic lesions in the central nervous system were not observed. On microscopy, the brain showed moderate random glial nodules throughout the neuropil associated with the presence of spherical to elongated parasitic cysts of about 20 µm, with a thin wall and with its interior full of bradyzoites, consistent with Toxoplasma gondii. There was also moderate fibrinoid necrosis and moderate multifocal lymphoplasmacytic infiltrate surrounding the blood vessels (perivascular cuffs) To investigate the etiology of the brain injury, brain sections were subjected to immunohistochemistry (IHC) and real-time polymerase chain reaction (qPCR) technique for detection of T. gondii and Neospora caninum. Immunostaining for T. gondii in the cyst wall and in bradyzoites and negative immunostaining for N. caninum. qPCR was positive for T. gondii and negative for N. caninum.Discussion: Diagnosis of encephalic toxoplasmosis in a Didelphis albiventris was possible based on histopathological, immunohistochemical and molecular findings. The morphological classification of the brain lesion was important for the diagnosis. Brain toxoplasmosis in opossums usually results in focal areas of malacia on macroscopy and focally extensive necrosis on microscopy, neutrophil infiltrate, calcified necrotic material, and perivascular cuffs of lymphocytes and plasma cells. In the present case, similar histopathological lesions were noted, but no significant macroscopic changes were observed. The etiology here was defined by immunohistochemistry and qPCR, techniques proven to be useful and with good specificity for diagnosing toxoplasmosis in mammals. It is believed that the positive immunohistochemical and molecular result for Toxoplasma gondii together with the negative result for Neospora caninum were conclusive for the diagnosis. Thus, we demonstrate here a post mortem diagnosis of toxoplasmosis in a free-living synanthropic opossum and the use of anatomopathology, immunohistochemistry and real-time polymerase chain reaction as a diagnostic option for this disease in opossums. Keywords: Toxoplasma gondii, marsupial, immunohistochemistry, Real time PCR, protozoal encephalitis.Título: Toxoplasmose encefálica em um gambá-de-orelha-branca (Didelphis albiventris)Descritores: Toxoplasma gondii, marsupial, imuno-histoquímica, Real time PCR, encefalite protozoal.

Encephalic Toxoplasm in a White-Eared Possum (Didelphis albiventris)