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

In total, 699 tissue, fluid and faeces samples were analysed from 102 cats, 57 with FIP and 45 without FIP. This is a marked increase in the number of samples analysed as compared to most previous studies [20, 21, 29, 30] , and contains similar numbers of effusion samples to two previous studies [19, 31] . Some studies have examined the use of FCoV RT-PCR alone in the diagnosis of FIP using fluid samples [29, 31] . Other studies [19, 21] compared FCoV RT-PCR to FCoV RT-PCR in combination with characterisation of S gene mutations in the diagnosis of FIP using body cavity fluids, these derived similar sensitivity (72 to 85% for FCoV RT-PCR alone and 60 to 64% for FCoV RT-PCR and S gene mutations characterisation) and specificity (100% for both FCoV RT-PCR alone and FCoV RT-PCR and S gene mutations characterisation) as obtained in this study (sensitivity 78.4% for FCoV RT-PCR alone and 60% FCoV RT-PCR and S gene mutations characterisation; specificity 97.9 and 97.9% respectively). As expected, in cats with histological changes consistent with FIP, individual tissues without FIP lesions were more likely to be FCoV RT-qPCR-negative or have lower copy numbers than those tissues in which FIP lesions were present. Overall, however, FCoV RT-qPCR was more sensitive (89.7% vs. 62.1%; calculated from Table 4 ) than IHC at detecting FCoV in tissues from cats with FIP. Therefore, not surprisingly, to maximise the sensitivity of both IHC and RT-qPCR at detecting FCoV in cats with FIP, biopsies should be collected from tissues with imaging changes or gross visual changes consistent with granulomata, and from lesion sites, whenever possible. Distribution of granulomatous lesions within a tissue, and virus laden macrophages within a lesion, is not uniform, which could account for the failure to detect viral antigen in some FIP lesions, and the negative FCoV RT-qPCR result in some cats in which the combined histological and IHC examination confirmed FIP. However, application of the calculated sensitivity to suspect FIP cases has to be viewed with caution, as not all samples included in the study were selected on the basis of clinical signs and gross pathological changes. In contrast, 10% of tissue samples from cats without FIP had a positive FCoV RT-qPCR result even though there were neither histological changes consistent with FIP nor was there evidence of viral antigen expression in tissue macrophages by IHC, resulting in a specificity of 90% for FCoV RT-qPCR, as compared to 100% for the reference standard of IHC. Persistence of FCoV in both intestinal and extraintestinal tissue macrophages in the absence of disease has been reported in healthy cats experimentally infected with FCoV, albeit with authors reporting IHC to be a relatively insensitive method of FCoV detection compared with RT-qPCR [7] .