Ante mortem diagnosis of FIP remains a challenge. Clinical diagnosis of FIP should be based on a range of parameters including age, breed, number of cats in the household, clinical findings, haematology, biochemistry, analysis of effusions and specific tests for coronavirus or FIP.
A range of tests are available to evaluate cats with a clinical history and / or haematological and serum biochemical changes or effusion cytology supportive of FIP.
Histology
Histology is the gold standard diagnostic method. Typical pyogranulomas with vasculitis are diagnostic. Histology can successfully diagnose both effusive and dry forms of FIP with suitable sample selection. True cut biopsies have a lower diagnostic sensitivity than larger tissue samples.
Immunohistochemistry
Immunohistochemistry for Coronavirus can be used to confirm histological diagnoses, or to confirm the presence of FIP when non typical histological changes are identified.
PCR
Abdominal fluid:
PCR detection of Feline Coronavirus in effusion fluids has a reported sensitivity of 72-100%. Several studies have reported a 100% specificity, but some studies have detected Feline Coronavirus in effusion fluids from cats with non FIP disease.This has been reported variably in 3-12% of control cats in these studies,usually with a low level of virus present.
The presence of Feline Coronavirus RNA, particularly in high levels, in an effusion that also has cytological and biochemical features suggestive of FIP, is highly supportive of a diagnosis of FIP.
Abdominal lymph node aspirates:
PCR for feline Coronavirus in mesenteric lymph nodes should only be performed when there is a strong index of suspicion for FIP based on clinical presentation and laboratory diagnostic tests. This should not be used as a screening assay in healthy cats.
A controlled study of Feline Coronavirus RNA detection in FNAs collected from the mesenteric lymphnodes from 20 cats with FIP without effusions reported a sensitivity of 90% and specificity of 96%.
Collectionguideline:
Add 0.2 - 0.5 ml of saline into an EDTA tube.
Under ultrasound guidance, aspirate the mesenteric lymph node(s) using suction and add to the EDTA tube. Redraw the fluid from the EDTA tube though the needle and gently return into the EDTA tube. Repeat this process if necessary – the sample in the EDTA tube should be cloudy.
CSF:
Non-effusive FIP is associated with neurological signs in 25-33% of cases and ocular signs are common. Altered mental status, nystagmus, ataxia and seizures are reported. In cats with neurological FIP, CSF is often viscous and pleocytosis is identified in 67% of cases.
In samples from unwell cats (cats both with and without confirmed FIP), PCR of CSF had a sensitivity of 42% and a specificity of 100%. In cats with neurological or ocular signs, sensitivity was 86%.
Faeces:
Most Feline Coronavirus infections are subclinical. Self-limiting diarrhoea can occur on first infection although sometimes this can be severe. This is a small intestinal diarrhoea which usually resolves in 1-2 weeks but may become chronic. Chronic large bowel diarrhoea may occur in older cats who are Feline Coronavirus carriers. Post infection faecal shedding commonly occurs for 2-3 months but may be chronic, and some cats are lifelong shedders. Faecal shedding of Coronavirus is NOT diagnostic for FIP. Less than 10% of cats that have faecal shedding of Coronavirus will develop FIP.
Immunofluorescenceof effusion fluids
This assay detects direct immunofluorescent staining of FIP-related antigens in macrophages in effusion fluids. This test has high specificity (near 100%) but a moderate sensitivity of 75%.
Effusion fluids need to be collected into an EDTA tube, and must be submitted as fresh as possible,as these samples deteriorate rapidly.
Serological Tests:
Feline Coronavirus Qualitative Antibody (Immunocomb), Feline Coronavirus Quanitiative Antibody (IFA): These assays assess for coronavirus antibody in the blood. They are NOT specific for FIP.
They are relatively sensitive tests, however, can give false negatives with either very high viral loads or early in infection (within the first 3 weeks). Thus, a negative result does not exclude coronavirus infection or FIP. Positive results are consistent with exposure to Coronavirus but do not confirm FIP. Higher titres do not correlate with a higher risk of developing FIP, however cats with higher IFA titres are more likely to be shedding virus.
