FIP is a life-threatening disease that occurs in a small percentage of cats infected with a very common feline virus, feline coronavirus (FCoV). It’s a complicated disease to diagnose and to understand and it’s important to work closely with your vet and keep asking questions if you are not sure of anything.

©Ian Ramsey

Feline infectious peritonitis (FIP) is the severe inflammatory disease caused by FCoV. However, not all affected cats will have peritonitis (inflammation of the abdominal cavity lining). How the disease appears in an individual cat is poorly understood but likely depends upon individual viral strain and cat factors. These factors impact upon which organs in the body are affected the most (eg lymph node, kidney, brain, eye) and in what way (eg inflammation of blood vessels causing fluid leakage or inflammation within tissue forming masses). Often the initial clinical signs are vague, non-specific signs such as a fever, weight loss, appetite loss, lethargy or a dull hair coat. In kittens there can be a failure to gain weight. Cats rapidly or slowly progress to any of four recognized groups of signs of “forms”:

1. The most common form is called ‘wet’ or ‘effusive’ FIP; in this form, blood vessels leak protein-rich fluid into body cavities. Most commonly this fluid accumulates in the abdomen, causing it to swell leading to a ‘pregnant’ appearance. Fluid accumulation may also occur in the chest, causing life-threatening breathing problems.
2. Less commonly inflammatory nodules develop in the cat’s internal organs causing a wide variety of clinical signs dependent upon which organ is affected. This form of the disease is termed ‘dry’ or ‘non-effusive’ FIP as fluid does not significantly accumulate in body cavities.
3. Ocular FIP is where the inflammation occurs within one or both eyes and is often considered a sub-form of dry FIP. This can result in a change to the color of the iris (eg from blue to green) or result in the eye appearing to be painful, cloudy, or contain blood or pus.
4. Neurological FIP is where the inflammation occurs within the brain or spinal cord and is often considered a sub-form of dry FIP. This inflammation can result in signs such as seizures, abnormal walking, or behavioral change.

The disease, FIP, does not spread between cats and it is believed that the mutated form of the virus that triggers FIP is in fact less capable of being spread to other cats than the common, non-mutated form of FCoV. The common non-mutated form that usually only causes mild signs such as diarrhea is highly contagious and is shed in feces (with small amounts in saliva). Intestinal infection with FCoV is very common; however, only about one in a hundred cats with intestinal infection is likely to go on to develop FIP. The greater the viral burden within a household and the greater the viral turnover within an individual cat, the more likely that FIP with occur. Cats living in high density situations (eg rescue shelters, breeding catteries, and other large multi-cat households) are at increased risk owing to a combination of increased environmental contamination, increased access to cats shedding virus, alongside increased viral turnover within individual cats as a consequence of the stress of concurrent infections and other factors (eg limited access to resources, inter-cat conflict, and high frequency of interventions such as neutering, breeding, and vaccination). The most vulnerable cats are those with weaker defenses against infectious diseases, eg kittens, elderly cats, and those already suffering from another condition. In some studies, some pedigree breeds, eg Birman, Ragdoll, Bengals and Rexes, appeared to be at increased risk, as compared to the ordinary domestic cats. However, in other studies the majority of the affected cats were non-pedigree cats.

There is no evidence that FIP can cause any disease in humans or other animals such as dogs. Rarely it can affect non-domestic cats such as cheetahs.

In the early stages of disease, FIP has the potential to cause a variety of clinical signs such as weight loss, lethargy, a dull coat, diarrhea, poor appetite, and fever. These are also common to many other diseases. The first signs owners notice may reflect the form of FIP your cat has. For example, in the ‘wet’ form of FIP fluid may build up in your cat’s abdomen (causing swelling of the tummy) or chest (causing difficulty in breathing). In cats with the ocular form of FIP, your cat’s eyes may change color or appear cloudy or bloodshot. In cats with the neurological form of FIP your cat may become wobbly when walking, have tremors, or even have seizures. Frustratingly, there is no reliable blood test to confirm that your cat has FIP and interpreting test results can be challenging, although strong suspicions can be raised in some cases. Changes your vet will be looking for on blood results include high protein levels and low red blood cells. There may also be increased bilirubin (which can result in jaundice) and an increase in markers of inflammation. Testing for FCoV antibodies in blood will reveal if a cat has been exposed to FCoV, but this is not a test for FIP. Many healthy cats - who do not have FIP and will never go on to develop FIP - will test positive, while some cats with FIP will test negative. Testing for FCoV in feces (poo) will reveal if a cat is shedding virus; however, this is also not a test for FIP. Again, many healthy cats - who do not have FIP and will never go on to develop FIP - will test positive, while some cats with FIP will test negative. Imaging the chest and abdomen is recommended to look for fluid, enlarged lymph nodes, and changes to the appearance of other organs. Where fluid has built up in the chest or abdomen this can be sampled and analyzed. In many cases, analysis of this fluid can give a strong suggestion of FIP or even confirm infection. Diagnosis can be much more challenging in non-effusive (dry) cases and is often a case of doing a few tests and interpreting the results holistically – through a combination of increasing the suspicion of FIP and excluding alternative possibilities. In some cats, a confirmed diagnosis requires surgery to biopsy affected tissues from internal organs. Internal organs may be biopsied, sometimes using ultrasound guidance to take a small sample, and samples examined under a microscope. When the clinical signs and physical examination findings only identify changes in the eye, referral to an eye specialist for sampling may be recommended. When the clinical signs and physical examination findings only identify neurological changes, referral to a feline specialist or neurologist may be recommended for more advanced imaging (MRI or CT scans) and sampling of the fluid from around the spinal cord (cerebrospinal fluid; “lumbar tap”). Your vet may discuss the risks and benefits of trialing antiviral treatments prior to receipt of send-away test results or instead of more advanced diagnostics. The biggest risk is that time and money may be spent treating the wrong disease, and once antivirals are started it can be much more challenging to confirm a diagnosis of FIP (eg if initial tests are negative or if response to treatment is slow) – this is a particular issue when there is little physical and laboratory evidence on initial testing to support a diagnosis of FIP. The biggest benefit is that a positive response to treatment strongly supports a diagnosis of FIP and delaying start of treatment increases the risk of irreversible decline (particularly for those with effusive and neurological forms of FIP). Your vet may also discuss the anticipated minimum cost of the antiviral treatment course, in case that impacts on decisions regarding referral for specialist care, more advanced imaging, and more invasive sampling (eg requiring surgery).

The good news is that there are now antiviral medications that can effectively treat FIP in the majority of cats.

• GS-441524: this is given orally as a tablet or a liquid, usually once or twice daily, and is usually the preferred treatment.
• Remdesivir: this is given as an intravenous infusion or by injection under the skin. This treatment is usually reserved for hospitalized cats who are seriously ill or those that cannot be given oral medication. The injections under the skin can be painful.
• EIDD-1931 (and in some countries the related molnupiravir): this medication is reserved for cats that are not responding to GS-441524 or Remdesivir. There is increased risk of side effects, and it must be handled with gloves (ideally not at all if you could be pregnant).

Giving oral medications to cats can be a challenge with some cats. While some of the antivirals are meat- or fish-flavored, voluntary intake can be enhanced by giving them in a tasty treat (eg pill putty, Lick-e-Lix ‘yogurt’). Both tablets and liquid antivirals are best given on an empty stomach, to maximize absorption and to further encourage voluntary intake. They can be offered their regular meal 30-60 minutes after administering the medication. It is important to make lots of positive associations with medication, and this will depend on what each individual cat enjoys. Administration of remdesivir by injection can be a negative experience for cat and owner, as the acidity of this injection makes it painful for cats. Luckily, use of remdesivir in the home setting is no longer considered necessary in nearly all cases owing to the efficacy and acceptance of newer oral preparations.

The causative virus (FCoV) is spread in saliva, phlegm, and, primarily, in feces. Most cats that contract the virus are able to clear the infection. Some cats may continue to carry the virus in their intestines and are a source of infection for other cats through chronic, sometimes intermittent, shedding. If a cat with FIP is already living with other cats, then there is no need to isolate the affected cat as the other cats are already likely to have been exposed to the underlying FCoV. However, you should avoid bringing new cats into the household. In most circumstances, FCoV is fragile and can be easily inactivated with cat-safe disinfectants, including a dilute solution of household bleach (about five tablespoons of bleach in a gallon of water), used to clean feeding bowls, litter trays etc. and to wipe down room surfaces. Remember that bleach is caustic – so wear gloves, skin, and eye protection. Ensure that surfaces are rinsed clean before allowing the cat to contact them. Viral particles can survive for up to a couple of months in dried feces, so it is essential to clean surfaces prior to disinfection.

An FIP vaccine has been developed in the US and some parts of Europe. Although this vaccine can reduce the risk of FIP, it is impractical to use as kittens most at risk will have been exposed to FCoV prior to the window in which it can be given (from 16 weeks) which would negate any protection. It is not currently recommended by feline specialists.