Limitations of using feline coronavirus spike protein gene mutations to diagnose feline infectious peritonitis

Barker, Emily N.; Stranieri, Angelica; Helps, Chris R.; Porter, Emily L.; Davidson, Andrew D.; Day, Michael J.; Knowles, Toby; Kipar, Anja; Tasker, Séverine

Immunohistochemistry data were available for 224 of the 260 tissue samples, histopathological data alone were available for one further tissue sample and no histopathological data were available for 35 tissue samples from cats with FIP. Of the 57 cats with FIP, 56 (98.2%) had at least one tissue sample that was FCoV RT-qPCR-positive. One cat (#74) with severe pyogranulomatous lymphadenitis and pleuritis in association with FCoV antigen in intralesional macrophages, consistent with FIP, tested negative for FCoV by RT-qPCRs on all tissues. However, the use of alternative PCR primers detected FCoV in three of the four tissue samples available for analysis, whilst sequencing revealed mutations at the RT-qPCR primers (P009; P010) and probe (P1) binding sites. Four other tissue samples from three cats (#61, #94, and #96) that had histopathological changes consistent with FIP, and were positive for FCoV antigen by IHC, but negative for FCoV by RT-qPCR, had limited, focal changes as confirmed by the histological examination. Tissues with histopathological changes (incl. IHC) consistent with FIP were more likely (χ 2 = 10.9, p ≤ 0.001) to be FCoV RT-qPCRpositive (132/139) than those without (69/85), and with a greater relative FCoV copy number (U = 2472.5, p ≤ 0.001; Table 4 ).