Introduction

Feline infectious peritonitis (FIP) virus, a mutated feline coronavirus, is the most common infectious cause of uveitis in cats. Histopathologic characterization of FIP ophthalmitis is vague, and intralesional immunohistochemical identification of viral antigen is historically unsuccessful.

Objectives

The aims of this study were to further characterize the histopathologic features of FIP ophthalmitis; use immunohistochemistry (IHC) to identify viral antigen in affected eyes; and use RT-PCR to identify viral RNA in affected eyes.

Methods

One eye from 22 cats, and both eyes from 8 cats with FIP ophthalmitis, were included in this study. Formalin fixed and paraffin embedded tissues were examined via histology, IHC and RT-PCR.

Results

The dry form of FIP was over-represented and the most common extraocular lesions were nephritis, meningoencephalitis, and hepatitis. Common ocular lesions in order of occurrence were cyclitis, iritis, scleritis, proteinaceous exudate in ocular chambers, choroiditis, and retinitis. Plasmacytic anterior uveitis was often accompanied by marked pyogranulomatous exudate overlying the ciliary body. Vasculitis was noted in 9 cases. Positive IHC labelling was identified within macrophage cytoplasm in 10 cases. RT-PCR performed on deparaffinized, formalin-fixed tissue identified coronavirus RNA in 9 cases. Vasculitis and pyogranulomatous infiltrate were suggestive of positive IHC or PCR.

Conclusions

Histopathology remains the most reliable and important post mortem diagnostic method. The combination of histopathology, IHC and PCR is suggested. The presumptive immune-mediated pathogenesis of chronic FIP lesions is proposed as a potential reason for the number of negative IHC and RT-PCR.