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

The University of Bristol FIP Biobank cats included in this study represent a convenience population, including cats for which FIP was not necessarily a differential diagnosis, and cats for which the samples available do not necessarily reflect clinical signs reported and, as such, clinically relevant samples of tissues essential for a definitive diagnosis may not be included. Differences between the cats with FIP and cats without FIP in this convenience population are clearly demonstrated when age at diagnosis is compared, which reflects the diseases for which these cats were euthanased, with neoplasia being the most common reason for euthanasia in the significantly older cats without FIP. This significant difference in the age could complicate comparison of faecal FCoV shedding between each clinical group (20% for non-FIP vs. 64.6% for FIP group), as younger cats are more likely to have been recently exposed to factors that could increase likelihood of enteric FCoV infection such as multicat households (i.e. breeding cattery or rehoming shelter). One study found FCoV shedding was most common in young cats (90% in cats aged 8-56 weeks cf. 39% for cats > 56 weeks) [1] ; however, another study of faecal FCoV shedding found no difference between different age categories (34.6% < 1 year of age, 31.6% 1-5 years, and 35.3% > 5 years) [40] . Ideally the cats without FIP would have been age and clinical sign matched to the cats with FIP; however, this was not possible and is generally very difficult to achieve in an unbiased clinical patient cohort. Previous case-controlled studies of FIP have either not reported age of either FIP or control cat populations [29, 31, 41] , or recruited healthy control cats [42] . One advantage of the increased age in the cats without FIP is that it does imply that these cats would be unlikely to have gone on to develop FIP had their concurrent disease not resulted in euthanasia. The absence of clinically relevant samples of tissue for some cats resulted in their exclusion from analysis, as a definitive diagnosis could not be achieved.