This is the first of a two-part series about FIP.
Part one outlines what FIP is and several methods used for a possible diagnosis.

FIP is a viral disease of cats that was almost uniformly lethal until recently. A recently discovered effective treatment is now available to veterinarians in the United States, Canada, Australia, Japan, and most of Europe.

The virus responsible for FIP is Feline Infectious Peritonitis Virus (FIPV), which is a mutated form of Feline Enteric Coronavirus (FeCV). FIPV is an RNA virus, so it encodes its genetic information in RNA instead of in DNA. It is a common misconception that FeCV is the same virus responsible for causing COVID in humans. The SARS-CoV-2 virus, while also a coronavirus, differs from FeCV, which does not cause COVID.

The current consensus is that dogs and cats do not develop disease from infection with SARS-CoV-2 and do not transmit infection to other animals or people.

FeCV is commonly present in the digestive tract of cats. Infection is typically asymptomatic or causes several days of diarrhea or mild upper respiratory symptoms, like sneezing and congestion. The virus spreads through feces, saliva, and possibly sneezing. Cats that share litter boxes and groom each other are at risk of being infected, which is why infection is more common in multi-cat households, shelters, catteries, and pet stores. In these situations, the FeCV infection affects 74-100% of cats.

In 5-12% of FeCV cases, the virus mutates into a form that can leave the intestine and infect white blood cells. This type is known as the Feline Infectious Peritonitis Virus (FIPV). 3-10% of cats infected with FIPV develop FIP. It is still unclear why only some cats are affected.

In a newly diagnosed cat with FIP, there is often a history of a stressful event (e.g., re-homing, relocation, or new household member, surgery, or corticosteroid use weeks before illness onset. The disease affects 0.3-1.4% of cats worldwide.

FIP can also affect African lions, mountain lions, leopards, cheetahs, jaguars, lynx, servals, caracals, European wildcats, sand cats, Pallas’ cats, and almost all other large cats.

In cats affected by FIP, the virus causes an intense inflammatory reaction in blood vessels, where infected cells typically settle in the abdomen, kidney, or brain, but they may also settle anywhere.

The disease takes two forms: the “wet” (effusive) form is acute, and the “dry” (non-effusive) form is more slowly progressive, though the two types often overlap. Some experts question the usefulness of classifying the disease as wet or dry. The dry form may progress to the wet form.

In the wet form, reactive fluid leaks out of damaged blood vessels, causing effusions inside body cavities such as the abdomen (peritoneal effusion) with increased abdominal girth. Effusion in the chest cavity (pleural effusion) or around the heart (pericardial effusion) can cause difficulty breathing, and muffled breath or heart sounds.

In rare cases, effusion involves the scrotum. Inflammation in the abdomen may cause organs to adhere and cause a palpable mass. Common symptoms also include lethargy, loss of appetite, and weight loss (or failure to thrive in a kitten), and a fluctuating fever.

Cats may also present with jaundice, enlarged lymph nodes, and lameness because of tissue involvement surrounding the joints. A vet may observe fluffy-looking white plaques along blood vessels, which are white cell accumulations on a retinal exam, which is less common than in the dry form.

Multiple areas of white cell accumulation in various organs, including the kidney, liver, intestine, brain, and eyes, characterize dry or non-effusive FIP. Abdominal organs and lymph nodes may also become enlarged. Involvement of the intestine may cause palpable thickening, vomiting, diarrhea, or constipation. There may be intermittent fever. Eye involvement may be the only abnormality seen.

Each of the above signs is not unique to FIP. You must also consider other diseases, including septic peritonitis or pleuritis (a bacterial infection inside the peritoneal or pleural sac), cancer, such as lymphoma or histiocytic sarcoma, infection with Toxoplasma, Bartonella or mycobacteria, pancreatitis, primary myocarditis, congestive heart failure, primary Immune Mediated Hemolytic Anemia, and primary liver disease.

There is currently no single test for FIP. In a laboratory investigation, white blood cells may be high and red blood cells low. A high serum protein is frequent, but the albumin is low. The serum protein elevation is due to high gamma globulins. This results in an abnormal albumin to globulin (A:G) ratio. Normally, there is more albumin than gamma globulin in the blood. (A:G is greater than 1).

This ratio often inverts to FIP (both wet and dry), and it aids in diagnosis. An A:G ratio of less than 0.4 makes FIP more likely, while a ratio of over 0.8 makes it less likely. High bilirubin (the yellow pigment in jaundice) and jaundice itself are common, as are mild elevations in transaminases (“liver enzymes”). BUN (Blood Urea Nitrogen) might be high. The Rivalta test, a bedside test on the fluid obtained from an effusion, is positive. AGP (alpha-1-acid glycoprotein) is sensitive and specific for FIP; FIP is if AGP is over 1000 mg/dL.

A low titer of anti-coronavirus antibodies might not be useful in the diagnosis because the body could use the antibodies to fight the infection.

The “gold standard” of FIP diagnosis is immunocytochemistry on effusion fluid or immunohistochemistry on tissue obtained by biopsy. In this test, they stain cells or tissue with fluorescent dye attached to an anti-FIPV antibody and then wash away the excess antibody. Antibody uptake causes affected cells to fluoresce, which confirms the FIP diagnosis. Unfortunately, these are invasive tests that are not always possible to perform on a sick kitten.

Footnotes and further reading/listening

1. Conference Proceedings: Am College Vet Internal Med Forum; ACVIM 2023. Sally J. Coggins
2. Pedersen, N. et al., J, Feline Med Surg. 2019 Feb 13; 21(4):271-281. Efficacy and Safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis.
3. Coggins, S.J. et al., J Vet Intern Med. 2023 Sep-Oct; 37(5): 1772-1783. Epub 2023 Jul 13. Outcomes of treatment of cats with feline infectious peritonitis using parenterally administered remdesivir, with or without transition to orally administered GS-441524.
4. Taylor, S., Talk at the International Society for Companion Animal Infectious Disease meeting 2024. An Update on Treatment of FIP Using Antiviral Drugs in 2024: Growing Experience but Still More to Learn.
5. Coggins, S.J., Talk at the American College of Vet Int Med meeting 2023. Updates in Feline Infectious Peritonitis Treatment.
6. Felten, S. et al., J Feline Med Surg. 2017 Apr; 19(4): 321-335. Detection of feline coronavirus spike gene mutations as a tool to diagnose FIP.
7. Sorrell, S., Tasker, S., Taylor, S., Barker, E., Gunn-Moore, D.: Feline Infectious Peritonitis (FIP) Information for Cat owners. Stokes Pharmacy website: https://www.stokespharmacy.com/wp-content/uploads/2024/05/Pet-Owner-Brochure-Printable-Version.pdf
8. Thayer, V. et al., J Feline Med Surg. 2022 Sep; 24(9):905-933. 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines.
9. https://bova.vet/
10. https://www.stokespharmacy.com/stokes-bova-partner-for-feline-infectious-peritonitis-treatment/
11. https://www.stokespharmacy.com/fip/cat-owner-resources/
12. https://bova.vet/fip-resource-page/#FIP-Webinars
13. https://vcahospitals.com/know-your-pet/feline-infectious-peritonitis
14. https://veterinarypartner.vin.com/default.aspx?pid=19239&id=11618072
15. https://www.merckvetmanual.com/cat-owners/disorders-affecting-multiple-body-systems-of-cats/feline-infectious-peritonitis-fip
    16.Merck Veterinary Manual, 11th edition, 2016, pp. 782-790
    17.https://www.wormsandgermsblog.com/tags/feline-infectious-peritonitis/
16. https://ccah.vetmed.ucdavis.edu/sites/g/files/dgvnsk4586/files/inline-files/Inappropriate%20use%20of%20GS.pdf
17. https://www.biorxiv.org/content/10.1101/2023.11.08.566182v3.full.pdf . Attipa, C., Warr, A. et al. BioRxiv preprint 3 Sep 2024: Emergence and spread of feline infectious peritonitis due to a highly pathogenic canine/feline recombinant coronavirus.
18. https://thewebinarvet.com/videos/what-you-should-know-about-fcov-23-and-the-outbreak-of-fip-in-cyprus
19. Gao, Y. et al: Virus Research V 326, March 2023: An updated review of feline coronavirus: mind the two biotypes.