Common Methods and Research on Diagnosis of FIP
Feline infectious peritonitis (FIP) is a fatal disease. Its pathogen is feline coronavirus (FCoV). This virus is relatively special. FCoV can survive in the intestinal tract of healthy cats. If it is a general disease, it will cause general gastrointestinal symptoms, such as diarrhea. At this time, we call it “feline enteric coronavirus” (FECV). If it mutates, the invasion ability of this virus will be improved and it can be transmitted from the intestines to the peritoneum and further spread throughout the body, causing FIP that is difficult to cure.At this time we call it cat coronavirus feline infectious peritonitis virus (FIPV). There is still no clear standard for veterinary diagnosis of FIP, especially dry FIP, which makes many veterinarians troubled. This paper mainly describes the characteristics of various diagnosis methods, the purpose is to provide more diagnostic methods and choose better diagnostic methods when encountering FIP that is difficult to determine.
Hematology tests:
The most common hemogram change is anaemia (decreased red blood cells and hemoglobin with or without reticulocytosis). Other common hemograms are lymphopenia, microcytic hypochromic anaemia, neutropenia and thrombocytopenia. Lymphopenia observed in approximately 50% of FIP cats is significantly more common in wet transmission cats.
Biochemical tests:
Blood samples: The most common abnormality is hyperglobulinemia, which can be found in 89% of FIP cats, and the ratio of albumin to globulin decreases (A/G<0.6 in general, and the probability of abdominal transmission increases greatly when A/G<0.4).
AGP>1.5 mg / ml (when AGP>3 mg / ml, the possibility of abdominal transmission increases greatly), the normal range is AGP<0.48 mg/ ml, positive can not be diagnosed, negative can basically exclude FIP. Changes in blood biochemistry are not specific manifestations of FIP and need to be diagnosed in conjunction with specific symptoms.
Fluid Sample:
Typical fluid for FIP cats is viscous, straw-colored, clear to moderately cloudy, with clumps forming.
FIP biochemical test showed high protein content, low A/G ratio and low total cell count. Rivalta test is significant for excluding FIP, but it cannot be used to identify effusions caused by bacterial peritonitis/pleurisy or lymphoma, which can be identified by cytology and bacterial culture.
RT-PCR:
Since its first application in the detection of FCoV, RT-PCR has been frequently used to amplify FCoV RNA in different materials and diagnose FIP. However, for abdominal transmission, PCR can only be used as a strong basis to help diagnose, but it cannot be used as evidence for a definite diagnosis. Because of 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 load are very significant for the diagnosis of FIP.
Tissue samples:
In cats with FIP, large amounts of FCoV RNA were found only in tissues with inflammatory changes. Tissues that are not involved in disease processes contain little or no viral RNA. The organs with the highest viral load were the greater omentum, mesenteric lymph nodes and spleen, thus these tissues were most suitable for analysis by RT-PCR.
Blood samples:
FCoV RNA can be detected in the blood of asymptomatic cats and diseases other than FIP, so blood samples are not a good choice for RT-PCR.
Fluid samples:
RT-PCR is more sensitive and specific than other rapid detection methods, but in other diseases that can cause ascites, viruses leak into the fluid with the blood, such as bacterial peritonitis, bacterial culture and cytological examination are recommended for identification. Although isothermal amplification has good specificity, but the sensitivity is low, easy to cause false negative.
| Sample type | Sample size | Detection method | Sensitivity | Specificity |
|---|---|---|---|---|
| Ascites, pleural effusion, or pericardial effusion | FIP(n=20), healthy cats (n=23) | Real-time RT-PCR | 85% | 100% |
| Ascites or pleural effusion | FIP(n=36), cats with other diseases (n=33) | Real-time RT-PCR | 89% | 100% |
| Ascites or pleural effusion | FIP(n=36), cats with other diseases (n=33) | isothermal amplification | 35%-39% | 95%-97% |
Detection of FCoV mutation sites: FCoV replication, like all RNA virus replication, is error-prone. Multiple individual mutations occur in each cycle of viral replication. Mutations in the S gene are thought to be alterations in virus cell tropism. The S gene contains two sites, the M1058L mutation, which converts methionine to leucine, and the M1060A mutation, which converts serine to alanine. Although this method has good sensitivity and specificity, the results show that this new detection method does not have enough advantages over the general PCR method: A study comparing tissue samples from FIP cats and non-FIP cats found the S gene mutation (leucine) in most tissue samples from FIP cats (39/43;91%) and in most tissue samples from non-FIP control cats (89;89%).
On the other hand, methionine was detected in faeces samples from FIP cats (10/13;77%) and in faeces samples from FIP-free cats (6/6;100%). This suggests that the M1058L locus of S gene does not represent the occurrence of FIP, but rather the systematic transmission of FCoV.
Antibody detection:
In general, antibodies are not very helpful in diagnosing FIP, and there is not much specificity.
Blood antibodies:
Contrary to previous belief, antibody testing is now generally believed to be unable to distinguish between antibodies to FECV and FIPV, and therefore even high antibody titers in the blood are not specific indicators of FIP. There is evidence that FCoV cross-reacts with other viruses, such as canine coronavirus (CCV) and TGEV. A large proportion of cat populations (up to 80% or more, especially in multi-cat households) have serum antibodies to FCoV, but most of these cats never develop FIP. In addition, antibodies can also be detected in the serum of cats vaccinated with FCoV. Therefore,the presence of antibodies for diagnosing FIP in individual cats has very little significance. High titers of antibodies in the blood can also be seen in FECV, moderate to low titers are useless for diagnosis, and in terminal FIP, antibodies bind to the virus in large quantities, resulting in a decrease in titers.
Effusion antibodies:
The antibody test for effusions is also of low diagnostic value and has the same limitations. Fluid antibodies are similar to blood tests, and false negatives can occur.
Immunostaining for the detection of FCoV antigen in macrophages:
Tissue samples:
Immunohistochemical staining of FCoV antigen within characteristic histopathological histopathological tissue lesions remains considered the gold standard for diagnosing FIP. Immunocytochemistry or immunohistochemical staining using liver, kidney, or lymph node samples obtained by minimally invasive ultrasound-guided fine-needle aspirate (FNA) or incisional biopsy (TCB) has been shown to result in low sensitivity.
Aqueous humor samples:
60% of FIP cats without effusion will have ocular symptoms, with uveitis being the most common. Secondary to inflammation of ocular structures and disruption of the blood-eye barrier, the presence of FCoV-carrying macrophages in the eye, and the ability to detect FCoV antigens in macrophages in aqueous humor, this technique has only moderate sensitivity and specificity for the diagnosis of FIP and is helpful for dry FIP.
References：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