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

Feline coronavirus (FCoV) infection is ubiquitous in domestic cats, with up to 90% of cats within multicat households being infected [1, 2] . The majority of FCoV infections are asymptomatic or are associated with mild intestinal disease. However, an estimated 1 to 5% of infected cats develop feline infectious peritonitis (FIP) [3, 4] , characterised by the development of a variable combination of pyogranulomatous polyserositis, vasculitis and granulomatous lesions in organs, and an extremely high mortality rate [5, 6] . Avirulent or self-limiting FCoV infection was believed to be confined to the intestines, but we now know that healthy FCoV-infected cats can implications, alternative diagnostic techniques have been investigated, such as detection of FCoV and characterisation of FCoV genomic sequences, using pyrosequencing, Sanger sequencing or sequence specific hydrolysis probes. Recent studies have identified mutations in the gene encoding spike (S) protein of serotype 1 FCoVs that are implicated in monocyte/macrophage tropism [18] . Two amino acid substitutions, M1058L and/or S1060A corresponding to nucleotide mutations 23531A>T/C and 23537T>G respectively in the S gene, together distinguished FCoVs found in the tissues of FIP cats from those found in the faeces of healthy cats without FIP in > 95% of cases. A subsequent study, which compared detection of FCoV by RT-qPCR alone to detection of FCoV by RT-qPCR combined with sequence analysis to confirm the presence of nucleotide mutations 23531A>T/C and 23537T>G, concluded that the addition of an assay for S gene mutation analysis did not alter the specificity of the FIP diagnosis, which was already 100%, but did decrease the sensitivity from 9.4 to 6.3% for serum/plasma samples and from 72 to 64% for effusion samples [19] . However, we have shown that the nucleotide mutations 23531A>T/C and 23537T>G are likely to be markers of systemic FCoV infection rather than FIP per se, being present in 91% of the FCoV-positive tissue samples from cats with FIP and 89% of the FCoV-positive tissue samples from cats without FIP [20] .