How to Diagnose Feline Infectious Peritonitis | Common Methods and Research

Feline infectious peritonitis (FIP) is a lethal disease, the pathogen is feline coronavirus (FCoV), this virus is relatively special, FCoV can survive in the intestinal tract of healthy cats, if the general onset of the disease, it will cause general gastrointestinal symptoms, such as diarrhea, at this time, we call the cat coronavirus for feline enterocoronavirus (FECV), if the mutation of the invasion of this virus will get In case of mutation, the invasive ability of this virus will be enhanced, and it can come from the intestines to the peritoneum, and further spread throughout the body, resulting in an incurable feline peritoneum, and then we call the feline coronavirus Feline Infectious Peritonitis Virus (FIPV). There are still no clear criteria for veterinary diagnosis of feline transmissible abdominal disease, especially dry transmissible abdominal disease, which is a headache for many veterinarians. This article mainly illustrates the characteristics of various diagnostics, with the aim of providing more ideas on diagnostic methods and choosing better diagnostics when encountering FIP that is difficult to determine.

Hematology

The most common hematologic change is anemia (decreased red blood cells and hemoglobin, with or without reticulocytopenia); other common hematologic findings are lymphocytopenia, microcytic hypochromic anemia, neutrophilia, and thrombocytopenia, observed in approximately 50% of FIP cats, and significantly more common in wet passerine cats.Hematology

Biochemical examination

Blood samples: the most common abnormality is hyperglobulinemia, which is found in 89% of FIP cats, with a decreased white globule ratio (typically A/G < 0.6, the probability of transmitting the abdomen increases substantially when A/G < 0.4)

Blood samples: the most common abnormality is hyperglobulinemia, which is found in 89% of FIP cats, with a decreased white globule ratio (typically A/G < 0.6, the probability of transmitting the abdomen increases substantially when A/G < 0.4)

AGP>1.5 mg / ml (when AGP>3 mg / ml when the probability of transmitting abdominal substantially increased), the normal range is AGP<0.48 mg / ml, positive can not confirm the diagnosis, negative can basically rule out FIP.Changes in blood biochemistry are not a specific manifestation of FIP and need to be combined with specific symptoms for diagnosis.

Fluid samples: The typical fluid in FIP cats is viscous, straw-colored, clear to moderately turbid, and may have clump formation.

FIP biochemical tests are high in protein, low A/G ratio, and low total cell count. Positive and negative Levantine tests are useful to rule out FIP, but cannot be used to identify effusions due to bacterial peritonitis/pleurisy or lymphoma, which can be achieved by cytology and bacterial culture.

RT-PCR

Since its first application in the detection of FCoV, RT-PCR has often been used to amplify FCoV RNA in different materials and to diagnose FIP. however, in the case of transmissible abdomens, PCR can only be used as a powerful aid to diagnosis, but not as confirmation of diagnosis. This is because FCoV RNA can also be amplified outside the gastrointestinal tract of cats without FIP. However, cats with FIP exhibit much higher viral loads than healthy FECV-infected cats. Therefore, positive PCR results with high viral loads are very relevant for the diagnosis of FIP.

Tissue samples: In cats with FIP, large amounts of FCoV RNA can only be found in tissues with inflammatory changes. tissues not involved in the disease process contain little or no viral RNA. the organs with the highest viral loads are the greater omentum, the mesenteric lymph nodes, and the spleen, and therefore these tissues are the most suitable for analysis by RT-PCR.

Blood samples: FCoV RNA can be detected in the blood of asymptomatic cats as well as in diseases other than FIP, so blood samples are not a good choice for RT-PCR.

Fluid samples: RT-PCR using fluid is more sensitive and specific than other rapid tests, but in other diseases that can cause ascites, the virus can leak from the blood into the fluid, e.g., bacterial peritonitis, and bacterial culture and cytology are recommended to identify this. Thermostatic amplification, while having good specificity, is less sensitive and prone to false negatives.

Sample Type	Sample Size	Detection method	Sensitivity	Specific
Ascites, pleural fluid or pericardial effusion	FIP (n=20), healthy cats (n=23)	RT-PCR Real-time RT-PCR	85%	100%
Ascites or pleural fluid	FIP (n=36), other disease cats (n=33)	RT-PCR Real-time RT-PCR	89%	100%
Ascites or pleural fluid	FIP (n=35), other disease cats (n=28)	isothermal amplification	35%-39%	95%-97%

Detection of FCoV mutation sites

FCoV replication, like that of all RNA viruses, is error-prone. Multiple individual mutations occur during each cycle of viral replication. One of these mutations, the S gene, is thought to be an alteration in viral cytophilicity The S gene contains two loci, the M1058L locus mutation, which results in the conversion of methionine to leucine, and the M1060A mutation locus, which results in the conversion of serine to alanine. Although this method has good sensitivity and specificity, the results of the study found that this new assay does not have enough advantages over the usual PCR methods.

In one study, tissue samples from FIP cats and non-FIP cats were used for comparative testing, which found that the majority of tissue samples from FIP cats had the S gene mutation (leucine) (39/43; 91%), and the majority of tissue samples from non-FIP control cats also had the S gene mutation (leucine) (8/9; 89%).

On the other hand, methionine was detected in fecal samples from cats with FIP for proteins expressed at the mutated site (10/13; 77%), and all fecal samples from cats without FIP (6/6; 100%) had methionine at this position. This suggests that the M1058L locus of the S gene does not represent the presence of FIP and is more favorable to systemic transmission of FCoV.

Antibody testing:

In general antibodies are not very helpful in diagnosing FIP and do not have a high specificity.

Blood antibodies: contrary to previous beliefs, it is now widely accepted that antibody testing cannot distinguish between antibodies against FECV and FIPV, and therefore even high antibody titers in the blood are not a specific indicator of FIP.There is also evidence of cross-reactivity of FCoV with other viruses such as Canine Coronavirus (CCV) and TGEV, and that a large portion of cat populations (up to 80% or more, especially in multi-cat households) have serum antibodies against FCoV, but the majority of these cats never develop FIP.In addition, antibodies can be detected in the sera of cats vaccinated against FCoV. Therefore, the presence of antibodies for diagnosis of FIP in individual cats is of very limited significance. Whereas a rise in high titers of antibodies in the blood can also be seen in FECV, a rise in low to intermediate titers is of no significance for diagnosis, and in end-stage FIP the antibodies bind to the virus in large quantities, resulting in a drop in titer.

Antibodies to fluid: Antibody testing of fluid is of similarly low diagnostic value and has the same limitations. Antibodies to effusions have similar results to those of blood and can also be false-negative.

Immunostaining to detect FCoV antigen in macrophages

Tissue samples: Immunohistochemical staining of FCoV antigen within characteristic histopathologic tissue lesions is still considered the gold standard for the diagnosis of FIP. Immunocytochemical or immunohistochemical staining using liver, kidney, or lymph node samples obtained by minimally invasive ultrasound-guided fine-needle aspirates (FNA) or cutting biopsies (TCB) has been shown to result in low sensitivity.

Atrial fluid samples: 60% of FIP cats that do not present with effusion will have ocular signs, with uveitis being the most common. Secondary to inflammation of ocular structures and disruption of the blood-ocular barrier, FCoV-carrying macrophages are present in the eye and FCoV antigens can be detected in macrophages in the aqueous humor. This technique is only moderately sensitive and specific for the diagnosis of FIP, and can be helpful in dry transmissions.

Ref: Felten S, Hartmann K. Diagnosis of Feline Infectious Peritonitis: a Review of the Current Literature. viruses. 2019;11(11):1068. published 2019 Nov 15. doi:10.3390/v11111068