Feline Infectious Peritonitis (FIP) is a common and highly fatal coronavirus (FCov) disease in cats.
FIP is caused by the feline enteric coronavirus (FECV). FECV is similar to coronavirus causing diarrhoea or bronchitis in humans, as well as other animals such as piglets, foals, ferrets, calves and poultry.
In about 10% of cats, mainly kittens, the enteric coronavirus experiences mutations. The corona virus usually attacks the gut lining causing an ooze, however FIP lies dormant and later attacks the lining of the blood vessels (with the wet form manifesting in the abdomen or chest and the dry form becoming evident in the brain and/or eye).
It escapes the cells that line the lower intestine and infects the most basic cell of the immune system, the macrophage. This mutated virus is known as feline infectious peritonitis (FIPV) and initially affects local lymphatic tissues associated with the lower intestine. FIPV infected macrophages then spread the infection to other sites within the abdomen and chest cavities, and sometimes to the eyes and brain.
More detailed information on FECV infection and clinical manifestations of FIPV disease can be found at ccah.vetmed.ucdavis.edu
Around 90% of affected cats experience diarrhoea which is usually undetected, and then recover. A positive Corona test does not necessarily mean a cat will go on to develop FIP. FIP has high mortality but low morbidity i.e., has a high death rate if not treated, approaching 99%, but is largely not contagious.
Studies of cats with FIP have identified a number of risk factors for its development. The highest prevalence is in young cats (9 weeks to 3 years of age) with the majority of cases in multi-cat environments (Pedersen, 1995: Rohrbach, et al., 2001). Males and sexually intact cats are at increased risk for development of FIP (Rohrbach, et al., 2001).
Two key factors are at play when looking at susceptibility to FIP:
Stress is also reported to be associated with increased disease prevalence, as are seasonal factors (more cases typically diagnosed in winter) (Pederson, 1995; Rohrbach, et al 2001)
The development of FIP depends on the health of the immune system. Stress impairs the immune system, and cats with a weakened immune system are those that are more susceptible to FIP. Kittens under stress are more susceptible to FIP as their immune systems are not fully developed. The immune system of a kitten usually develops fully between 10-14 weeks of age dependent on the breed.
FIP has been found to strike after a stressful event, for example re-homing, if the owner is away for a prolonged period of time, after veterinary treatment for an illness, even after sterilization.
Some research posits there may be a genetic predisposition to FIP in some cats, pointing to the importance of genes in designing the feline immune system and thereby affecting vulnerability to FIP. However, there is no evidence of this at this time of writing. More than one case of a kitten developing FIP from the same breeding pair is required to show a genetic link. The reasons for a possible genetic link are unknown, and no trials have been undertaken to explore the role of genetics in FIP.
FIP is thought to be more prevalent in some breeds.
FIP is more common in pure breed cats (Rohrbach, et al, 2001). Certain breeds have been found to be more likely to develop FIP, particularly Birman, Ragdoll, Bengal, Rex, Abyssinian, and Himalayans. Anecdotally FIP also tends to be prevalent in Sphynx. Other breeds including Burmese, Exotic Shorthair, Manx, Persian, Russian Blue and Siamese do not appear to be at an increased risk as compared to mixed breed cats.
The Radzai and Pressler 2006 study found some breeds, such as Maine Coons may possess unrecognised protective factors that influence susceptibility due to the low numbers presenting with FIP. However, in Australia the incidence in Maine Coons is higher accounting for around 7-10% reported to the Australia and New Zealand FIP Warriors page.Rescue cats are prone to develop FIP due to the stress the cat/kitten undergoes in its life prior to re-homing. The incidence of FIP in cats housed by rescue groups can be up to 5%, also reflecting high density cat populations and lack of secure food sources all contribute to this propensity to contract FIP.
FIP is on the rise due to greater use of rescue operations and re-homing.
We also have to be aware and note that increased awareness amongst veterinary practitioners is also contributing to a rise in diagnosed cases.
Wise cattery management can result in reducing the incidence of FIP.
Refer to section 9 in relation to potential preventative factors suggested for adoption in catteries by breeders.
Types of FIP
This initial macrophage infection is limited in all but 0.3-1.4% of cats, which are unable to develop the required protective immunity. These are the cats that go on to develop FIP. The disease in this small group of cats can manifest within days, several weeks, sometimes months, and rarely a year or more.
There are two forms of FIP:
- Wet (effusive)
- Dry (non-effusive)
Occasionally a cat may present with the dry form but develop wet FIP, or visa versa.
Wet form of FIP
The wet form occurs in around 75% of cats with FIP. The onset of wet is easier to detect. Early symptoms include:
- Antibiotic resistant fever
- Reduced appetite
- Weight loss
- Accumulation of fluid in the abdomen, less commonly in the chest.
Where fluid accumulates in the abdomen swelling is evident. Fluid on the chest results in breathing difficulties (dyspnoea). The fluid has a high specific gravity (exudate), i.e. is quite thick and tacky/sticky, and when shaken forms bubbles that pop quickly. Cats may appear to be jaundiced due to high levels of bilirubin and/or have strongly yellow-tinged serum and urine due to the proteins in the fluid. Almost all cats with wet FIP will either be euthanized or die within 2/24 weeks. It is not recommended to remove of large amounts of abdominal fluid due to the systemic impact caused by the sudden loss of high levels of protein; removal of chest fluid is always indicated when breathing is impaired but abdominal fluid is best left alone and if it must be removed, there is a recommended percentage to eliminate
Cats suffering from the dry form of FIP tend to be more chronically ill, less likely to have elevated serum bilirubin, and present with more vague clinical signs of disease.
The illness presents as:
- Failure to thrive or grow normally
- Poor hair, coat and skin condition
- Bouts of secondary respiratory or intestinal infections
- Bouts of diarrhoea
- Weight loss
The dry form is not characterized by inflammation and swelling but affects:
- Organs such as kidneys, cecum, colon, lungs, liver, lymph nodes via formation of tumour like lesions (granulomas)
- Eyes and brain
- Unlike the wet form, cats with dry FIP usually survive longer but die within a year.
Neurological and Ocular FIP
Brain and/or eyes are involved in around 9% of cases, neurological and or ocular disease is the main presenting clinical sign in over 70% of cats presenting with dry FIP.
Neurological and ocular FIP are associated with dry FIP and are seen in less than 1 in 10 cats with wet FIP. Many cats with dry FIP have been ill for weeks or months prior to diagnosis. They tend to be treated symptomatically before they are euthanized or die.
The main symptoms of neurological FIP which causes a breakdown of cells in the brain are:
- In appetence
- Weight loss
- Incoordination (especially in the posterior)
Less common symptoms are seizures and dementia.
Ocular disease may accompany neurological FIP because of the close connection of the eyes and brain. Ocular disease is characterised by:
- inflammation of anterior uveal tract (iris and ciliary body)
- discolouration of iris
- precipitates on back of cornea
- inflammation in the retina and optic nerve
Less than 5% of cats, usually those with milder forms of dry FIP, survive longer than one year with the best symptomatic care.
Veterinarians find FIP to be either easy or difficult to diagnose, depending mainly on its form (wet or dry), and how often it occurs in their cat practice population and on their knowledge of the disease forms and laboratory findings.
Because of its high mortality, they are often reluctant to diagnose FIP without overwhelming clinical evidence.
A physical examination, including ultrasound, detects abdominal or thoracic swelling, masses is organs within the abdomen or chest cavity, and lesions within the eye, or neurological signs such as posterior weakness/incoordination.
Blood tests may not always point to FIP. Blood abnormalities commonly observed in both wet and dry FIP include:
- unresponsive anaemia
- elevated white blood cell count
- absolute lymphopenia
- elevated total protein and globulin
- low albumin
- low A:G ratio
- of most significance is high globulin, indicating viral diseases, including FIV and cats testing FeLV negative
Neurological and/or ocular forms of FIP may be confused with feline systemic toxoplasmosis with vets testing for this and treating with antibiotics. Yet systemic toxoplasmosis is a rare disease of cats compared to FIP.A veterinarian can also check Corona titres (but as mentioned there is possible cross-reactivity with the enteric form) and carry out immunofluorescence on the macrophages of the fluid collected. This is most accurate if present but is not always detected. Histopathology on abdominal lymph nodes is also an available diagnostic tool
 The sphynx breed is hairless and so do not have full cilia present on their cells. The cilia may be broken, sparse or even not these at all. Therefore the immune system of a kitten takes longer to develop – up to 16 weeks.
 Pesteanu-Somogyl, Radzai and pressler 2006; https://www.vcahospitals.com/know-your-pet/feline-infectious-peritonis,2020)