Encephalitis associated with giant cells in a cat with naturally occurring feline immunodeficiency virus infection demonstrated by in situ hybridization.

This report describes the clinical, pathological, immunocytochemical, and in situ hybridization characteristics of encephalitis associated with feline immunodeficiency virus (FIV) infection in a 4-year-old domestic cat. Lesions were identified throughout the brain, affecting the cerebrum, medulla, and cervical spinal cord. Perivascular lymphocytic cuffing, gliosis, and white matter vacuolation were most severe in the cerebrum, affecting the white matter and the deep laminae of the grey matter. Gemistocytes were prominent, and many bizarre cells with large, sometimes multinucleate, hyperchromatic nuclei were evident. Immunostaining with antibody specific for FIV p24 nucleocapsid protein produced staining in the gemistocytes and glial cells of the white matter. In situ hybridization with a 327-base pair fragment of the FIV gag gene produced staining that was most intense in the white matter and gemistocytes of the deep laminae of the grey matter. These findings indicated localization of FIV infection within the cerebrum, and the detection of FIV RNA by in situ hybridization confirms the infection as active.