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Improvement in Diagnostic Testing for Non-Effusion FIP

Dunbar D, Kwok W, Graham E, et al. Diagnosis of non-effusive feline infectious peritonitis by reverse transcriptase quantitative PCR from mesenteric lymph node fine-needle aspirates.  J Feline Med Surg. 2018 Nov 8.

A high index of suspicion of effusive (wet) feline infectious peritonitis (FIP) is usually straight forward and can be made based on history, signalment, and measurement of hematological, biochemical and serological parameters.  Recent studies have shown that the detection of FCoV by reverse transcriptase quantitative polymer chain reaction (RT-qPCR), using template RNA, can provide further support for a diagnosis of effusive FIP, with sensitivity of ~ 65-89% and specificity of ~96-100% when testing effusions.  On the other hand, a minimally invasive sampling technique has not been described for non-effusive (dry) FIP, and confirmation is often achieved only at the time of post-mortem examination.  Therefore, a sensitive, specific and minimally invasive method for supporting or refuting a diagnosis of non-effusive FIP is very much desired.

The use of a RT-PCR protocol, similarly as done for effusive FIP, may be capable of detecting the virus in a key anatomical site (ie,, mesenteric lymph node (MLN) tissue), and prove to be diagnostically useful for suspect cases of non-effusive FIP since many non-effusive FIP present with abnormally enlarged MLN.  Since surgically acquired biopsy material is stressful and risk-associated intervention, sample collection by ultrasound-guided FNA is a far less invasive procedure, and therefore could provide samples amendable to RT-PCR FIP.

Therefore, researchers from School of Veterinary Medicine, University of Glasgow, UK and their collaborators used RT-PCR protocol to assess cats with and without confirmed non-effusive FIP comparing detection of viral RNA in MLN FNA preparations. Disease status of the animals was determined using a combination of gross pathology, histopathology and/or ‘FIP profile’, consisting of serology, clinical pathology and clinical signs. Diagnosis of non-effusive FIP, MLN FNA RT-qPCR had an overall sensitivity of 90.0% and specificity of 96.1%.

Mesenteric lymph nodes are the local draining lymph nodes for the intestinal tract, and it is expected that at some point during corona viral intestinal infection there will be a transient viral presence in those nodes; therefore,  there is the risk of a false-positive result if the animal is tested in this early period of infection. Thus, it should be understood that this diagnostic test should only be applied when there is a strong index of suspicion of FIP, based on clinical presentation and other laboratory test parameters. In other words, this test is not designed to be a screening assay for healthy cats and its application is not meant to be a standalone method to diagnosing FIP, but, instead, should be used to complement the standard suite of hematological, biochemical and serological tests currently in use. [GO]