Clinicopathologic Features and Magnetic Resonance Imaging Findings in 24 Cats With Histopathologically Confirmed Neurologic Feline Infectious Peritonitis

Crawford, A.H.; Stoll, A.L.; Sanchez-Masian, D.; Shea, A.; Michaels, J.; Fraser, A.R.; Beltran, E.

Limitations of our study include its multi-institutional and retrospective nature, leading to variability in the clinical and pathologic information available as well as the precise MRI sequences obtained. Magnetic resonance imaging and pathologic lesion severity were graded to facilitate analysis of the cases and to group them according to the extent of pathology. However, the grading schemes used have not been validated and further large-scale, prospective studies are needed to fully evaluate the extent and severity of MRI findings and pathologic lesions. A difference in grade of MRI and histopathologic findings was identified in 1 cat, in which euthanasia was performed 6 days after MRI. Although lesion progression could have occurred during this time period, the difference in grading may have arisen from lesion variation among CNS locations and emphasizes the need to standardize histopathologic examination across cases. Confirmation of FIP was primarily based on histopathologic identification of consistent lesions. Feline infectious peritonitis is distinguished pathologically from other infectious causes of meningitis and encephalitis, such as feline immunodeficiency virus, feline leukemia virus, rabies, pseudorabies, Toxoplasma gondii, and Cryptococcus neoformans, by the presence of vascular and superficially orientated pyogranulomatous inflammation in the absence of visible fungal and bacterial pathogens. 5, 8 The gross and histologic lesions of FIP are typical, and a thorough postmortem examination with adequate histopathologic examination of diseased tissues can be an accurate way to confirm a diagnosis. 1, 8 Should there be an element of uncertainty in the pathologist's interpretation of the lesions, immunohistochemistry to detect viral antigen can aid in obtaining a definitive diagnosis. 1, 24 Although a definitive diagnosis of FIP was made in each case in our series based on characteristic histopathologic findings, immunohistochemistry was performed in 10 cats, which confirmed the presence of viral antigen within the CNS of all 10 cats.