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FIP – A Straightforward Compendium

Posted in Cat

com·pen·di·um [ kuhm-pen-dee-uhm ], noun: a brief treatment or account of a subject, especially an extensive subject; concise treatise

Like my previous post about Athena’s road to diagnosis, this post will be more technical. I’ve done my best to link to highly technical information — rather than include it in the text — in an attempt to make this more “readable.” Where I’m unable to easily link to the information, or when I have to use medical terms that don’t have a plain language equivalent, I’ve provided explanations through [def] (definition) links.

Because there is so much information about feline infectious peritonitis (FIP) to share, I’m providing TLDR — too long, didn’t read — paragraphs at the beginning of each section, offset with narrow separator lines. The TLDRs provide you a general overview to get an initial idea of the topic before diving into more detail.

It’s not contagious

Before I get started, probably the most important thing to know is that FIP, the disease, is not contagious. I’ve been asked this question countless times since Jim and I announced Athena’s diagnosis. You do not need to isolate or quarantine a cat that has been diagnosed.

What is FIP?

This is likely to be the first question a pet parent has after their vet suggests it as a potential diagnosis. Unfortunately, It’s difficult to answer adequately. Despite being identified as a specific disease in 1963, FIP remains one of the least understood of all cat diseases. So, what do we know?

FIP is a viral cat disease caused by a type of coronavirus. While it infects less than 10% of cats, FIP has historically been fatal. It is a multi-step disease — think of the way HIV eventually becomes AIDS. FIP begins with a strain of Feline Coronavirus (FCoV) known as feline enteric coronavirus (FeCV or FECV). IF an FeCV infection mutates into FIP Virus (FIPV) AND the infected cat has an improper immune response to the FIPV infection, the result is FIP.

FIP’s cause is a bit complex. So let’s start at the beginning.

FCoV is a cat-specific version of the coronavirus. Most coronaviruses tend to cause upper respiratory or gastrointestinal symptoms; FCoV is the latter sort.

While I dove deep down the rabbit hole of FCoV, FeCV, and FIPV, I’m still not positive I’ve grasped the relationship between the three. That said, I’m certain that:

Depending on the source, between 80 and 100 percent of cats are exposed to FeCV at some point in their lives, with 20-40% of cats in single-cat households, and 80-100% of cats in multi-cat households or colonies, becoming infected. The most common form of transmission is the ingestion of infected feces, because infected cats shed particles of the virus in their feces. However, transmission through saliva — by mutual grooming, sharing food bowls, sneezing, or close contact — and inhalation are also possible.

FeCV typically causes a short-term, mild intestinal infection. While most cats infected with FeCV completely recover and eliminate the disease, some develop a persistent infection. These cats can be a continual source of infection for other cats in their environment because they are generally asymptomatic and often shed large amounts of virus in their feces.

RNA-based viruses, like FCoV, make many mistakes when replicating, allowing them to quickly mutate. One of the FeCV mutations is FIPV which alters FeCV’s biological behavior, resulting in white blood cells becoming infected with the virus and spreading it throughout the cat’s body. Research has not yet discovered what causes this particular mutation.

While FIP results from the FIPV infection, it is ultimately an immune-mediated disease [def]. Again, research has not determined the precise cause, but some cats with FIPV have an abnormal immune response. This causes widespread and severe inflammation in the body that can cause organ failure, fevers unresponsive to antibiotics, and in many cases, an accumulation of thick yellow fluid in the chest or abdomen.

Symptoms of FIP

Allow me a short disclaimer to say that the list of symptoms and potential symptoms I found seems never-ending. So, rather than try to include them all, I’m listing the symptoms most often included in FIP overview articles.

Initial FIP symptoms may include lethargy, decreased or absent appetite, weight loss, and fluctuating fever. As the disease progresses, a cat will develop either the “wet” or “dry” form, with the former being more common, somewhat easier to diagnose, and typically causing a faster decline. The most obvious sign of the wet form is a buildup of fluid in the abdominal or thoracic cavities. The dry form can present in the form of neurological disease; eye symptoms; lesions in the liver, kidneys, or other internal organs; and several other symptoms. Many cats with dry FIP will have eye problems as their only clinical symptom.

Initial Symptoms

The first signs of FIP may be vague, with listlessness, lethargy, decreased or absent appetite, weight loss, and fluctuating fever commonly reported. After several days to a few weeks other symptoms typically begin to occur.

More advanced clinical symptoms vary depending on the affected organs, such as the membranes lining the abdominal and thoracic cavities, liver, kidneys, central nervous system, and eyes. FIP usually causes vasculitis — an inflammatory disease of blood vessels.

Those more advanced symptoms are the basis for the general division of FIP into “wet” (effusive) and “dry” (non-effusive) forms, with the former more common and more quickly fatal. Either form can apparently evolve into the other, but I couldn’t find any information about what causes the transition, so that’s likely something research hasn’t determined yet.


Most cats with FIP will develop the wet form in which the vasculitis results in fluid leaking from the blood into body cavities. Generally, the fluid buildup is within the abdominal cavity — resulting in a pot-bellied appearance — and/or the thoracic cavity — resulting in breathing difficulties. This fluid is typically thick, bright yellow, and contains a lot of protein.

The fluid accumulation in the abdomen led to the naming of FIP; ‘peritonitis’ refers to the inflammation that occurs in the lining of the abdominal cavity.

Cats with the wet form of FIP are gravely ill and, when untreated, survival is measured in days to occasionally weeks.


With the dry form of FIP, little to no fluid accumulates. Instead, it often involves severe inflammation in one or more organs including the eyes, brain, liver, intestines, kidneys, lungs, and skin. While the dry form of FIP develops more slowly than the wet form, the range of symptoms seems to be wider.

Possible symptoms of dry FIP include neurological disease — such as a wobbly and unsteady gait or seizures — bleeding in the eyes, and lesions in the liver, kidneys, or other internal organs. Making it even more difficult to diagnose, many cats with dry FIP will have ocular (eye) symptoms as their only clinical symptom.

Regardless of the form a cat’s FIP takes, without treatment, most cats deteriorate rapidly after symptoms develop, although some cats remain normal for several weeks. Unfortunately, FIP will eventually result in death in almost every untreated case.


FIP Vet Guide has an excellent page dedicated to diagnosing FIP. Because FIP is difficult to diagnose, it often becomes a case of ruling out other options. When considered together, a cat’s history, clinical symptoms, blood work, and imaging results are generally enough to confirm a diagnosis. Additional testing specific to FCoV may provide the final information necessary to commit to an FIP diagnosis. Contact the FIP Warriors Facebook group as soon as possible after diagnosis, or when an FIP diagnosis appears to be the most likely answer.

FIP is difficult to diagnose because there are no clinical symptoms that are specific to it, and no conclusive tests to confirm a diagnosis. There is often little trouble in placing FIP high, or at the top, of a diagnostic list, but veterinarians can have great difficulty in confirming their diagnosis.

FIP can be a diagnosis of exclusion, meaning other possibilities have been ruled out, leaving FIP as the most likely, or only, answer. An FIP diagnosis can also be reached by putting together pieces of the FIP diagnostic puzzle as examinations are performed and test results interpreted.

The first set of facts to consider is the cat’s history:

  • Age: 50% of cases occur in kittens less than 7 months of age, and 70% of cases occur in cats under 1.5 years of age
  • Breed: Pure-bred cats have a three times greater incidence of FIP than random-bred cats — often just referred to as domestic short/medium/long hair — due to genetic factors passed in bloodlines
  • Origin: Cats from crowded environments — shelters, catteries, etc. — are more likely to develop FIP.
  • Recent medical history: cats who have had a recent stressful event (surgery, re-homing, vaccination, other illness), significant weight loss, or suffer from a general failure to thrive are at a higher risk of developing FIP

These facts should be evaluated alongside any clinical symptoms that are present. Common symptoms that raise suspicion for FIP include:

  • A cyclical fever unresponsive to antibiotics
  • Jaundice
  • Abdominal distention (with suspicion of fluid buildup)
  • Shortness of breath (with suspicion of thoracic fluid buildup)
  • Uveitis or retinitis (in one or both eyes)
  • Neurological symptoms (a loss of coordination and balance, seizures, rear leg weakness, changes in gait, tremors, etc)

The next step is basic diagnostic tests, specifically CBC and CHEM panels for both diagnostic purposes and to serve as a baseline for treatment progress. Not all cats with FIP will have abnormal blood work, especially those in the earliest stages. Therefore, if FIP is suspected but blood work is normal, it should be repeated to determine if there are any changes.

While there are no FIP-specific blood work anomalies, some findings of diagnostic significance include:

  • Anemia
  • High white blood cell count, specifically:
    • Increased numbers of neutrophils [def]
    • Reduced numbers of lymphocytes [def]
  • Abnormal concentration of protein in the bloodstream, including
    • Increased concentration of globulins [def]
    • Reduced concentration of albumin [def]
    • An albumin:globulin ratio below 0.6
  • Increased concentration of bilirubin (and associated presence of bilirubin in the urine)
  • Abnormal liver enzyme values — like decreased alanine transaminase (ALT) [def]

Depending on the results of blood work, diagnostic imaging — x-ray, MRI, or ultrasound — of the abdominal and/or thoracic cavities may be useful to detect the presence of fluid buildup, and/or organ or lymph node involvement. Results that may be indicative of FIP include:

  • Abdominal or thoracic fluid buildup
  • Swelling of abdominal lymph nodes
  • Enlargement of organs
    • Many different organs, including the heart may be affected​
  • Evidence of granulomas [def]

Depending on the imaging findings, a fine needle aspiration to take a non-invasive biopsy for testing may be useful. Invasive biopsies and exploratory surgeries should generally be avoided unless absolutely necessary. If a fluid buildup is discovered, draining will likely be suggested both to relieve pressure and to test a sample, as few other diseases produce the same type of fluid that FIP creates.

There are some tests — immunoperoxidase staining [def] and immunofluorescence staining [def] — that are meant to detect the presence of viral proteins in affected tissue. A newer technology, polymerase chain reaction (PCR), can detect viral RNA in tissue and fluid samples. It is not enough to just find the virus, as cats without FIP may have FIPV. However, identifying FIPV outside of the normal areas FeCV infects, especially when the cat has other characteristic clinical symptoms, is considered confirmation of an FIP diagnosis.

An important aspect to consider with these tests is the turnaround time to get a result. Depending on the severity of a cat’s illness, it may be prudent to start treatment in parallel with diagnostic tests, as some require as much as a week before getting results.

Further tests that can also sometimes be helpful include:

  • Further analysis of proteins in the blood (e.g. measuring the protein acid-1-alpha glycoprotein)
  • MRI scans of the brain
  • Evaluation of a cerebrospinal fluid sample (the fluid that surround the brain and spinal cord) in cases where there are neurological signs
  • Evaluation of fluid from the eye in ocular cases

I’m going to make the assumption that you are likely in a country where the most recent treatments — at the time of writing — are not legally available for your vet to prescribe. If your cat is being evaluated for an FIP diagnosis, contact the FIP Warriors Facebook group when an FIP diagnosis appears to be the most likely answer, or as soon as possible after the diagnosis has been confirmed. They will help get you started on your journey to a healthy cat.

Treatment Options

Historically, FIP was considered to be a non-treatable disease, fatal in almost all cases. However, there have been significant recent developments, particularly in the form of research by Professor Niels Pedersen of University of California, Davis.

A new treatment, currently only approved in UK and Australia, is administered as injections and/or oral tablets over 12 weeks. During treatment, a monthly standard complete blood panel (CBC) and chemistry panel (CHEM) should be run, and continued through the 12-week observation period post treatment. Vitamin B12 and probiotics are also recommended during treatment.

Studies in both laboratory and household settings have shown that newer anti-viral medications, currently referred to as GC-376 and GS-44152 are effective treatment options. Remdesivir, a prodrug — a drug that changes in the body before it starts working — of GS-44152, has been legalized in the UK and Australia for the treatment of FIP.

UPDATE, 7/1/2024: The U.S. has taken a step forward in FIP treatment. The U.S. Food & Drug Administration (FDA) announced that they have effectively decriminalized vets providing treatment to FIP patients. Stokes Pharmacy started selling a compounded treatment — in this case a pill form — on June 1.

This treatment is administered as injections and/or oral tablets over the course of 12 weeks (84 days). Dosage is dependent on the weight of the infected cat, so it will change over the course of treatment. When appropriately administered, the effective rate is between 80% and 90% depending on the source.

UPDATE, 7/1/2024: In the UK, treatment is transitioning to oral tablets, with injections reserved for cats with “severe neurological signs and inability to swallow or tolerate oral medication; extremely dehydrated/unwell cats; cats that cannot be orally medicated for other reasons.”

During treatment, and for a 12-week observation period after treatment, your cat should have regular blood work — CBC and CHEM panels — the first being four weeks after starting treatment, and continuing on every four weeks. This blood work is to monitor:

  • Anemia, which should resolve
  • Neutrophils, which should be within the normal range
  • Bilirubin, which should return to normal
  • Globulins, which should return to normal limits
  • A/G ratio, which should resolve to 0.7 or higher. Some cats won’t achieve this during treatment.  An A/G level lower than 0.7 is less worrisome if it is the result of normal globulins and low (or low-normal) albumin rather than the reverse.

Vitamin B12 shots are recommended during treatment to prevent anemia, maintain nervous system integrity, and keep the intestines healthy for the proper digestion and absorption of food. These should be administered once a week for 6 weeks, then monthly for as long as your vet recommends, but at least through treatment.

For cats that have significant diarrhea, probiotic supplements, food mix-ins, and/or foods are recommended to promote a healthy digestive system and alleviate the diarrhea.

Unfortunately, there is no data to point to any type of FIP treatment being legalized or commercially available in the USA anytime soon. And I can’t find any information about treatment being legalized elsewhere. However, I did find reference to veterinarians in India, New Zealand, South Africa, and some parts of Europe accessing GS-44152/Remdesivir from suppliers for human medication.

“Off-Label” Prescriptions

While researching FIP, I came across information about “off-label” prescribing.

Unapproved use of an approved drug is often called “off-label” use. This term can mean that the drug is:

  • Used for a disease or medical condition that it is not approved to treat, such as when a chemotherapy is approved to treat one type of cancer, but healthcare providers use it to treat a different type of cancer.
  • Given in a different way, such as when a drug is approved as a capsule, but it is given instead in an oral solution.
  • Given in a different dose, such as when a drug is approved at a dose of one tablet every day, but a patient is told by their healthcare provider to take two tablets every day.
Understanding Unapproved Use of Approved Drugs “Off Label”

Apparently many medicines used for veterinary applications are done so “off-label.” This, of course, led me to research why GS-44152 can’t be used “off-label.” While I can’t find anything that specifically spells it out, the best I can determine is approval status.

When a medication has been approved for one use, it can be prescribed for another. But, when it hasn’t been approved for a use, it can’t be prescribed at all. While it’s not relevant to this post, I have to say that this stance seems hypocritical. “Off-label” prescribing is unapproved prescribing, and taken with an understanding of risk. Why, then, can we not use unapproved medications with an understanding of risk? But I digress.

Cat life after FIP

What does cat life after FIP look like? Unfortunately, we don’t really know yet. Consider how recently the treatment was discovered, mixed with how few countries have approved it. I couldn’t find any data on relapse or long-term effects that would be accepted as medical fact. There are simply too many variables to draw definite conclusions.

What is clear is that treatment is highly effective against FIP. Whether it’s a permanent cure or only sends FIP into remission is yet to be seen. Current anecdotal evidence — the data comes from households, not official studies — puts relapse at around 3% of cases, which isn’t too bad.

For now, I can only hope the cure is permanent. However, given the lack of understanding about the precise causes of FIP, Jim and I intend to take some precautions for Athena.

We’re having all of our boys tested for FCoV/FeCV. We’d rather not expose Athena to it again. It’s my belief, given that FIP is immune-mediated, that if she were re-infected with FCoV and it mutated to FIPV, she would get FIP again. The only guaranteed way to prevent that is to ensure she is not re-exposed to FCoV.

Unfortunately, there isn’t a cure for for FCoV/FeCV, either. So, if one of our boys turns out to have it, we’ll have to evaluate our options to keep Athena as safe as possible. The internal medicine specialist doesn’t think we’d be putting Athena at increased risk to keep her in the house with a FCoV/FeCV positive cat, but, for my own peace of mind, I’d want to research extra precautions.

Also for our peace of mind, Jim and I plan on quarterly exams for a year after Athena’s observation period, then twice yearly for the rest of her life. It would only be an additional 3 years, as our local vet suggests starting “senior” biannual exams around at 8 years old.

Thanks for reading to the end, I hope you found this post useful. If you’re here because you have a cat with FIP, or suspect your cat has FIP, I wish you the best. May your furbaby recover and spend many happy years with you.

If you’re just here because you’re curious, please share this post as much as you can to help get information to those who need it.

Make sure to subscribe to my blog or follow me on Instagram so you don’t miss any posts. I generally post at least once a week.

Useful Resources

Be aware that the treatment sections of several pages below do not include the “new” treatment. If a page states that FIP is incurable, know that it is out-of-date

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