Feline calicivirus localization in central nervous system cells of cats with neurological lesions.

Feline calicivirus (FCV) is primarily associated with upper respiratory disease in cats; however, it can cause severe multisystemic illness, occasionally accompanied by neurological signs. Despite sporadic reports of neurological involvement, the neurotropic potential of FCV remains poorly defined. This retrospective study investigated central nervous system (CNS) involvement in domestic cats presenting with clinical signs suggestive of FCV infection, including neurological, respiratory, and/or systemic manifestations. Brain tissues from 67 cats were examined, of which 27 exhibited neurological signs, while the remaining cases presented primarily with respiratory or systemic involvement. Samples were screened for FCV RNA using reverse-transcription polymerase chain reaction (RT-PCR) targeting a partial fragment of the viral polymerase gene, followed by partial genome sequencing. Positive cases were further evaluated using histopathology, immunohistochemistry (IHC), and in situ hybridization (ISH). Transmission electron microscopy (TEM) was used to confirm CNS localization in a selected FCV-positive case. FCV was detected in 4 of 67 cats (5.97%). All cats exhibited neurological lesions characterized by perivascular cuffing, gliosis, and perineuronal satellitosis. IHC demonstrated FCV capsid protein within neurons and glial cells across multiple brain regions, with additional identification in neuron ganglions in tongue and intestine, while ISH confirmed the presence of viral genomic material in corresponding areas. TEM identified intact viral particles within the glial cells in one case. Although our study identified FCV in CNS tissue, a direct association between FCV infection and neurological signs in cats requires further investigation; however, these findings indicate that FCV can localize within the feline CNS.