Cat Diseases/microbiology; Central Nervous System Diseases/veterinary; Cerebrospinal Fluid/microbiology; Retroviridae Infections/veterinary; Retroviridae/physiology; Animals; Antibodies, Viral/biosynthesis; Antibodies, Viral/cerebrospinal fluid; Astrocytes/microbiology; Astrocytes/pathology; Brain/microbiology; Brain/pathology; Cat Diseases/cerebrospinal fluid; Cats; Central Nervous System Diseases/cerebrospinal fluid; Central Nervous System Diseases/microbiology; Cerebrospinal Fluid/cytology; Disease Models, Animal; Immunoblotting; Immunoglobulin G/cerebrospinal fluid; Leukocyte Count/veterinary; Macrophages/microbiology; Macrophages/pathology; Radioimmunoprecipitation Assay; Retroviridae/immunology; Retroviridae Infections/cerebrospinal fluid; Retroviridae Infections/microbiology; Retroviridae Proteins/immunology; Specific Pathogen-Free Organisms