Felis ISSN 2398-2950

FIP / FCoV tests

Synonym(s): Feline coronavirus (FCoV) test, Feline infectious peritonitis (FIP) test, Feline enteric coronavirus (FECV)

Contributor(s): David Godfrey, Diane Addie

Overview

  • Feline infectious peritonitis (FIP) is the name of the disease which is caused by feline coronavirus (FCoV). Feline enteric coronavirus (FECV) is sometimes used interchangeably with FCoV, but the proper scientific name, as determined by the International Committee on Taxonomy of Viruses, is FCoV. However, in databases such as Genbank, early strains of FCoV may be named “FIPV-etc” or “FECV-etc” as indicators of whether they originated from a cat with or without FIP.

RT-PCR 

  • Feline infectious peritonitis (FIP) Feline infectious peritonitis is the disease which occurs in a minority of FCoV-infected cats: there is no such thing as a “FIP test,” tests claiming to be FIP tests are actually tests for feline coronavirus RNA or antibodies. 
  • The advent of sensitive real time (quantitative) reverse-transcription polymerase chain reaction (RT-qPCR) test PCR (Polymerase chain reaction) for FCoV Feline corona virus: FIP have made in vivo feline infectious peritonitis (FIP) diagnosis much simpler and less invasive than previously. 
  • Positive RT-qPCR tests on effusions are 96% specific for effusive (wet) FIP. 
  • Positive RT-qPCR tests on mesenteric lymph node (MLN) fine needle aspirates (FNA) (Veterinary Diagnostic Services, University of Glasgow Veterinary School) are 96% specific for non-effusive (dry) FIP.  
  • RT-qPCR tests on MLN FNA have reduced the need for biopsy diagnosis in non-effusive (dry) FIP cases, but at post mortem, organ sections for histopathology should still be taken, especially in cases where a pedigree cat purchaser intends to sue the breeder. 
  • The usefulness of negative RT-qPCR tests depends on the sensitivity of the laboratory test used and that the correct sample has been submitted (eg not blood or feces for FIP diagnosis). 
  • RT-PCR testing to detect viral messenger RNA (mRNA) should, in theory, demonstrate replicating virus, however the primers occasionally detect human DNA, resulting in false positive results. 
  • Mutation M1058L and S1060A tests (Idexx Laboratories) have poor sensitivity but positive results on effusions are 95.8% specific for FIP.  (Specificity on FNAs has not been determined.) 
  • Quantitative RT-PCR yields a threshold cycle (CT) result which is an indirect indicator of viral RNA quantity: the lower the CT, the more viral RNA was present in the sample. Some laboratories translate CT results into RNA quantities, others report CT, and others simply report positive or negative: the latter is inadequate - ideally CT or viral quantity should be given on the report. 
  • Blood should not be sent for FCoV RT-qPCR testing since most cats with FIP are not viremic and will test negative; in addition around 5% of cats without FIP test positive. 
  • Fecal samples are useful for monitoring FCoV shedding, but positive RT-PCR results are not predictive of a cat having FIP, negative tests do not rule out FIP, but mostly rule out FCoV as a cause of diarrhea. 

FCoV antibody tests 

  • Infection with coronavirus results in the production of circulating antibodies detectable by serological tests Serology
  • FCoV antibody tests have a poor positive predictive value for FIP and FCoV-associated diarrhea, but negative antibody tests are excellent for ruling out a diagnosis of FIP or FCoV-associated diarrhea, provided the test is sufficiently sensitive. 
The diagnosis of FIP must never be made on antibody titers alone, even if they are rising. 

FIP profile 

  • Various laboratories offer an FIP profile which usually consists of FCoV antibody titer; hematology (checking for non-regenerative anemia, lymphopenia); biochemistry (checking for hypergammaglobulinemia leading to decreased albumin to globulin ratio, raised bilirubin, raised alpha-1 acid glycoprotein acute phase protein) Acute phase proteins, cytology (if effusion); plus possibly FCoV RT-PCR test. 

Immune staining 

  • Immune staining of post-mortem and biopsy samples is often regarded as the gold standard of FIP diagnosis Indirect immunofluorescence. However, due to the property of feline tissue, especially macrophages, to non-specifically bind antibodies, false positive results are a problem, unless the laboratory uses a control antibody (ie one not against FCoV) on every tissue sample to control for non-specific signaling. 
  • Immune staining of FNA or effusion cells also have problems of both sensitivity and specificity and are no longer widely used, having been supplanted by RT-PCR.

Sampling

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Tests

Methodologies

  • FCoV RT-PCR: quantitative technique is preferable to traditional nested RT-PCR because there is less risk of a false positive result, it is often more sensitive, and should give an indication of viral load. 
  • FCoV antibody test: indirect immunofluorescent antibody (IFA) Indirect immunofluorescence, enzyme-linked immunosorbant assay (ELISA) Enzyme linked immunosorbent assay (ELISA) and rapid immunomigration (lateral flow) antibody tests are all available. Virus neutralization tests are not commercially available. The sensitivity of the antibody test is extremely important: see below.

Validity

Sensitivity and specificity of FCoV RT-qPCR tests  

  • No independent comparison of various laboratory RT-qPCR tests has been published. 
  • Mutations M1058L and S1060A tests (Idexx Laboratories) were reported to correlate with FIP (Chang et al, 2012). However, the methionine to leucine substitution at position 1058 in the FCoV spike protein was found in 89% of tissue samples from 14 cats without FIP suggesting that the mutation is indicative of systemic spread of FCoV from the intestine, rather than that the mutation is specific for FIP (Porter et al, 2014). Sensitivity and specificity of the mutation tests on effusions were reported to be 68.6% and 95.8% respectively (Felten et al, 2017), but on FNAs is unknown. 
  • The veterinary surgeon who is screening a fecal sample by RT-PCR test should find out if his or her laboratory spikes the samples to control for fecal PCR inhibitors. 

Sensitivity and specificity of FCoV antibody tests 

  • Laboratories which begin sample dilutions at over one in 25 should be avoided: the laboratory lower cut-off FCoV antibody titer should be 1:25 or less. 
  • Use of TGEV rather than FCoV in immunofluorescent antibody tests can decrease specificity.
  • A comparison of available FCoV antibody tests showed the following sensitivity and specificity (some laboratories and test manufacturers requested their data not to be shown (others, eg Idexx, Antech, declined to take part in the study): 
 
IFA FCoV 
IFA TGEV
ELISA  

Rapid Immunomigration

 

Biobest  

VDS  

EVNA  

Zurich  

FCoV 
Immunocomb  

Speed
F-Corona  

FASTest 
FIP  

Anigen
Rapid 
FCoV  

Sensitivity %

96.1  

100  

96.2  

100  

100  

92.4  

84.6  

64.1  

Specificity %

100  

100  

97.5  

83.3  

100  

100  

100  

100  

(Addie et al, 2015)
Biobest Laboratories Ltd,, Penicuik, Scotland.
Veterinary Diagnostic Services (VDS), University of Glasgow Veterinary School, Scotland.
UMR 1161-Virologie-INRA-ENVA-ANSES, Maisons-Alfort, France.
Clinical Laboratory, Vetsuisse Faculty, University of Zurich, Switzerland. 
FCoV Immunocombe, Biogal, Israel.
Speed F-Corona, Virbac, Nice, France.
FASTest FIP, MegaCor Diagnostik, Hoerbranz, Austria.
Anigen Rapid FCoV Ab Test Kit, Bionote Inc, Seoul, Korea.

Predictive value

  • FCoV antibody tests should only be used in combination with the presentation, history, clinical signs, and other diagnostic tests. 
  • Around one third of FCoV seropositive cats sheds virus in their feces, ie in 66% of cats FCoV antibodies indicate previous exposure to virus, not active infection. 

Sensitivity and specificity of immunostaining  

  • No independent comparison of various laboratory immunohistochemistry tests Immunohistochemistry (IHC) has been published: the veterinary surgeon should ask the laboratory what negative controls are in place: whether they are performed on each individual sample or not. 

Result Data

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Addie D D, Curran S, Bellini F, Crowe B, Sheehan E, Ukrainchuk L, Decaro N (2020) Oral Mutian® X stopped faecal feline coronavirus shedding by naturally infected cats. Res Vet Sci 130, 222-229 PubMed Full Article.
  • Addie D, Houe L, Maitland K, Passantino G, Decaro N (2020) Effect of cat litters on feline coronavirus infection of cell culture and cats. J Feline Med Surg 22(4), 350-357 PubMed Full Article.
  • Dunbar D, Kwok W, Graham E, Armitage A, Irvine R, Johnston P, McDonald M, Montgomery D, Nicolson L, Robertson E, Weir W, Addie D D (2019) Diagnosis of non-effusive feline infectious peritonitis by reverse transcriptase quantitative polymerase chain reaction from mesenteric lymph node fine needle aspirates. J Feline Med Surg 21(10), 910-921 PubMed.
  • Fish E J, Diniz P P V, Juan Y C, Bossong F, Collisson E W, Drechsler Y, Kaltenboeck B (2018) Cross-sectional quantitative RT-PCR study of feline coronavirus viremia and replication in peripheral blood of healthy shelter cats in Southern California. J Feline Med Surg 20(4), 295-301 PubMed.
  • Tasker S (2018) Diagnosis of feline infectious peritonitis: Update on evidence supporting available tests. J Feline Med Surg 20(3), 228-222 PubMed Full Article.
  • Felten, S, Leutenegger C M, Balzer, H J, Pantchev N, Matiasek K, Wess G, Egberink H, Hartmann K (2017) Sensitivity and specificity of a real-time reverse transcriptase polymerase chain reaction detecting feline coronavirus mutations in effusion and serum/plasma of cats to diagnose feline infectious peritonitis. BMC Vet Res 13, 228 PubMed.  
  • Felten S, Matiasek K, Gruendl S, Sangl L, Wess G, Hartmann K (2017) Investigation into the utility of an immunocytochemical assay in body cavity effusions for diagnosis of feline infectious peritonitis. J Feline Med Surg 19(4), 410-418 PubMed.  
  • Longstaff L, Porter E, Crossley V J, Hayhow S E, Helps C R, Tasker S J (2017) Feline coronavirus quantitative reverse transcriptase polymerase chain reaction on effusion samples in cats with and without feline infectious peritonitis. J Feline Med Surg 19(2), 240-245 PubMed.   
  • Riemer F, Kuehner K A, Ritz S, Sauter-Louis C, Hartmann K (2016) Clinical and laboratory features of cats with feline infectious peritonitis - a retrospective study of 231 confirmed cases (2000-2010). J Feline Med Surg 18(4),348-356 PubMed.  
  • Addie D D, le Poder S, Burr P, Decaro N, Graham E, Hofmann-Lehmann R, Jarrett O, McDonald M, Meli M L (2015) Utility of feline coronavirus antibody tests. J Feline Med Surg 17(2), 152-162 PubMed
  • Porter E, Tasker S, Day M J, Harley R, Kipar A, Siddell S G, Helps C R (2014) Amino acid changes in the spike protein of feline coronavirus correlate with systemic spread of virus from the intestine and not with feline infectious peritonitis. Vet Res 45(1), 49 PubMed.  
  • Meli M L, Burr P, Decaro N, Graham E, Jarrett O, Lutz H, McDonald M, Addie D D (2013) Samples with high virus loads cause a trend toward lower signal in feline coronavirus antibody tests. J Feline Med Surg 15(4), 295-299 PubMed
  • Addie D D, McDonald M, Audhuy S, Burr P, Hollins J, Kovacic R, Lutz H, Luxton Z, Mazar S, Meli M (2012) Quarantine protects Falkland Islands (Malvinas) Cats from Feline Coronavirus Infection. J Feline Med Surg 14 (2), 171-176 PubMed.
  • Chang H W, Egberink H F, Halpin R, Spiro D J, Rottier P J (2012) Spike protein fusion peptide and feline coronavirus virulence. Emerg Infect Dis 18(7), 1089-1095 PubMed
  • Giori L, Giordano A, Giudice C, Grieco V, Paltrinieri S (2011) Performances of different diagnostic tests for feline infectious peritonitis in challenging clinical cases. J Small Anim Pract 52(3), 152-157 PubMed.  
  • Kipar A, Baptiste K, Barth A, Reinacher M (2006) Natural FCoV infection: cats with FIP exhibit significantly higher viral loads than healthy infected cats. J Feline Med Surg 8, 69-72 PubMed.   
  • Simons F A, Vennema H, Rofina J E, Pol J M, Horzinek M C, Rottier P J, Egberink H F (2005) A mRNA PCR for the diagnosis of feline infectious peritonitis. J Virol Methods 124(1-2), 111-116 PubMed.   
  • Addie D D et al (2004) Evaluation of an in-practice test for feline coronavirus antibodiesJ Feline Med Surg 6(2), 63-67 PubMed.
  • Addie D D, Schaap I A T, Nicolson L, Jarrett O (2003) Persistence and transmission of natural type I feline coronavirus infection. J Gen Virol 84(10), 2735-2744 PubMed.
  • Addie D D & Jarrett J O (2001) Use of a reverse-transcriptase polymerase chain reaction for monitoring feline coronavirus shedding by healthy cats. Vet Rec 148, 649-653 PubMed.  
  • Herrewegh A A, Smeenk I, Horzinek M C, Rottier P J, de Groot R J (1998 ) Feline coronavirus type II strains 79-1683 and 79-1146 originate from a double recombination between feline coronavirus type I and canine coronavirus. J Virol 72(5), 4508-4514 PubMed.  
  • Duthie S, Eckersall P D, Addie D D, Lawrence C E, Jarrett O (1997) Value of α1-acid glycoprotein in the diagnosis of feline infectious peritonitis. Vet Rec 141(12), 299-303.
  • Herrewegh  A A, Mahler M, Hedrich H J, Haagmans B L, Egberink H F, Horzinek M C, Rottier P J, de Groot R J (1997) Persistence and evolution of feline coronavirus in a closed cat-breeding colony. Virology 234, 349-363 PubMed.  
  • Herrewegh A A, de Groot R J, Cepica A, Egberink H F, Horzinek M C, Rottier P J   (1995) Detection of feline coronavirus RNA in feces, tissue, and body fluids of naturally infected cats by reverse transcriptase PCR. J Clin Microbiol 33, 684-689 PubMed.  
  • Sparkes A H, Gruffyd-Jones T J, Harbour D A (1994) An appraisal of the value of laboratory tests in the diagnosis of FIP.JAAHA30, 345-350.  
  • Stoddart M E, Gaskell R M, Harbour D A, Gaskell C J (1988) Virus shedding and immune responses in cats inoculated with cell culture-adapted feline infectious peritonitis virus. Vet Microbiol 16, 145-158. 
  • Stoddart M E, Whicher J T, Harbour D A (1988) Cats inoculated with feline infectious peritonitis virus exhibit a biphasic acute phase plasma protein response. Vet Rec 123, 621-624. 

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