What Is Feline Infectious Peritonitis (FIP)?

Feline infectious peritonitis (FIP) is a serious disease that can affect domestic cats and other members of the cat family. FIP is associated with a feline coronavirus called feline coronavirus (FCoV).

Feline coronavirus is relatively common among cats, particularly in environments where many cats live together. Most cats infected with FCoV do not develop FIP and may have few or no noticeable symptoms. In some cats, however, changes in the virus and the cat's immune response can lead to FIP.

FIP can affect cats of any age, but it is more frequently diagnosed in younger cats. Early recognition of concerning symptoms and prompt veterinary evaluation are important because FIP can progress quickly.

How Does FIP Develop?

Feline coronavirus is primarily associated with transmission through fecal-oral exposure, particularly through contaminated litter boxes and shared environments.

Importantly, having feline coronavirus does not automatically mean that a cat has FIP.

FIP develops when a coronavirus infection is associated with disease involving the immune system and tissues throughout the body. The exact mechanisms are complex, and researchers continue to study why some cats develop FIP while most infected cats do not.

Stress, overcrowding, age, genetics, and other factors may influence a cat's risk of developing FIP.

Common FIP Symptoms

FIP symptoms can vary significantly depending on which organs and body systems are affected.

Common signs may include:

• Loss of appetite

• Weight loss

• Persistent or recurring fever

• Lethargy and reduced activity

• Poor growth in kittens

• Abdominal enlargement

• Difficulty breathing

• Eye abnormalities

• Changes in vision

• Neurological abnormalities

• Jaundice in some cases

Because these symptoms can occur with many other feline diseases, symptoms alone cannot confirm FIP.

If a cat has persistent fever, unexplained weight loss, abdominal fluid, neurological changes, or eye problems, a veterinarian should evaluate the cat as soon as possible.

Wet FIP vs. Dry FIP

FIP is commonly described as wet FIP or dry FIP. These terms describe different patterns of disease, although cats can sometimes show features of both.

Wet FIP

Wet FIP is characterized by the accumulation of fluid in body cavities.

Depending on where fluid accumulates, cats may develop:

• A swollen or distended abdomen due to abdominal fluid

• Difficulty breathing due to fluid around the lungs

• Lethargy

• Reduced appetite

• Weight loss

• Fever

The fluid may be described as an effusion. Veterinarians may analyze this fluid as part of the diagnostic process.

Dry FIP

Dry FIP does not typically produce large amounts of fluid in the abdomen or chest. Instead, inflammatory lesions can develop in different organs.

Possible signs include:

• Weight loss

• Persistent fever

• Poor appetite

• Lethargy

• Enlarged lymph nodes

• Eye problems

• Neurological problems

• Changes affecting the kidneys, liver, or other organs

Dry FIP can sometimes be more difficult to recognize because its symptoms may resemble other illnesses.

Ocular and Neurological FIP

Some cats develop primarily ocular or neurological manifestations of FIP.

Ocular FIP can cause inflammation and changes within the eye, potentially affecting vision.

Neurological FIP may cause signs such as:

• Unsteady walking

• Balance problems

• Weakness

• Tremors

• Seizures

• Behavioral or coordination changes

These symptoms require prompt veterinary assessment because several other diseases can cause similar neurological signs.

How Is FIP Diagnosed?

There is no single symptom that proves a cat has FIP, and diagnosis can be challenging.

Veterinarians may use a combination of:

• Physical examination

• Medical history

• Complete blood count (CBC)

• Blood chemistry testing

• Albumin and globulin measurements

• Albumin-to-globulin (A:G) ratio

• Bilirubin measurements

• Ultrasound or X-rays

• Analysis of abdominal or chest fluid

• Cytology

• Feline coronavirus testing

• PCR testing

• Eye examinations

• Neurological examinations

A positive feline coronavirus antibody test does not by itself mean that a cat has FIP. Many cats may have been exposed to feline coronavirus without developing FIP.

For this reason, veterinarians generally interpret laboratory results, imaging findings, clinical signs, and the overall history together.

Is FIP Treatable?

For many years, FIP was considered almost universally fatal. Research into antiviral medications has significantly changed the treatment landscape.

Antiviral approaches involving GS-441524, the active metabolite of remdesivir, have shown substantial promise for cats with FIP. Remdesivir and GS-441524 are closely related antiviral compounds, and treatment protocols vary depending on the medication, formulation, route of administration, and the cat's clinical condition.

Treatment should be planned with appropriate veterinary guidance.

The required antiviral dose can vary depending on factors such as:

• FIP presentation

• Body weight

• Disease severity

• Ocular involvement

• Neurological involvement

• Response to treatment

• Specific medication and formulation

Because dosing is weight-based and may need adjustment during treatment, regular veterinary monitoring is important.

How Long Does FIP Treatment Take?

FIP treatment protocols can vary. Historically, 84 days (12 weeks) has been a commonly referenced treatment duration for antiviral therapy, although newer research and veterinary protocols may use different approaches for selected cats.

The veterinarian may monitor:

• Body weight

• Appetite

• Activity level

• Temperature

• Bloodwork

• Eye findings

• Neurological signs

• Abdominal or chest fluid

• Overall clinical response

A cat's weight can change significantly during recovery, especially in kittens. Because many antiviral treatments are calculated according to body weight, regular weight checks are particularly important.

What Improvement Can Owners Expect?

Some cats receiving effective antiviral treatment may show noticeable clinical improvement relatively quickly. Appetite, activity, and fever may improve early in treatment, although the response varies from cat to cat.

A rapid improvement does not necessarily mean that treatment should be stopped.

Cats with ocular or neurological disease may require more careful monitoring and may recover more gradually than cats with uncomplicated disease.

Always follow the treatment plan provided by your veterinarian and report any worsening or new symptoms.

Supportive Care During FIP Treatment

Antiviral therapy is only one part of caring for a sick cat.

Supportive care may include:

• Encouraging adequate food and water intake

• Maintaining hydration

• Managing nausea when appropriate

• Providing a quiet, comfortable environment

• Monitoring body weight regularly

• Monitoring temperature when recommended

• Treating secondary or concurrent medical problems

• Following scheduled veterinary examinations and blood tests

A cat that stops eating, becomes severely weak, has difficulty breathing, experiences seizures, or develops other emergency signs should receive prompt veterinary care.

Can FIP Be Prevented?

Because feline coronavirus is widespread and can spread between cats, completely preventing exposure is difficult.

However, good hygiene and environmental management can reduce the risk of infectious disease transmission.

Helpful practices include:

• Keeping litter boxes clean

• Providing enough litter boxes for the number of cats

• Cleaning food and water bowls regularly

• Avoiding overcrowding

• Reducing chronic stress

• Separating sick cats when appropriate

• Quarantining and evaluating newly introduced cats when advised

• Maintaining good overall nutrition and veterinary care

There is no guaranteed way to prevent every case of FIP.

Is FIP Contagious Between Cats?

Feline coronavirus can spread between cats, particularly through fecal contamination. However, FIP itself is generally not considered to spread between cats in the same way as the original feline coronavirus infection.

The development of FIP is associated with complex interactions between the virus and the individual cat's immune response.

If one cat in a multi-cat household is diagnosed with FIP, discuss hygiene, litter-box management, and the health of other cats with your veterinarian.

FIP in Kittens

Kittens can be particularly vulnerable to feline coronavirus infections and are among the cats in which FIP is commonly diagnosed.

Warning signs in a kitten may include:

• Failure to gain weight

• Poor appetite

• Persistent fever

• Lethargy

• Abdominal swelling

• Breathing difficulties

• Eye abnormalities

• Neurological changes

Because kittens can deteriorate quickly, unexplained illness should not be ignored.

Frequently Asked Questions About FIP

Can a cat recover from FIP?

Yes. The outlook for cats diagnosed with FIP has changed considerably with the development of effective antiviral treatment approaches. However, treatment success depends on factors such as disease presentation, timing, medication, dosing, monitoring, and the individual cat's response.

Can a blood test confirm FIP?

Usually, no single routine blood test can definitively confirm FIP. Veterinarians generally combine clinical findings, laboratory results, imaging, fluid analysis, and other diagnostic tests.

Does a positive coronavirus test mean my cat has FIP?

No. A positive feline coronavirus test indicates exposure or infection with feline coronavirus, but it does not automatically mean the cat has FIP.

What are the earliest signs of FIP?

Early signs may be nonspecific and can include reduced appetite, lethargy, weight loss, fever, or poor growth. More specific signs may develop as the disease progresses.

Is wet FIP worse than dry FIP?

Both forms can be serious. The severity depends on which organs are affected, how advanced the disease is, and the cat's overall condition. Ocular and neurological involvement can require particularly careful treatment and monitoring.

How quickly can FIP progress?

FIP can progress rapidly in some cats, while the clinical course varies in others. A cat showing persistent fever, severe lethargy, breathing difficulty, neurological signs, or significant appetite loss should be evaluated promptly.

Final Thoughts

Feline infectious peritonitis is a complex disease, but it is no longer the hopeless diagnosis it was once considered to be. Advances in antiviral treatment have provided new options for many cats with FIP.

The most important steps are early veterinary evaluation, an accurate assessment of the cat's condition, appropriate antiviral treatment when indicated, and ongoing monitoring.

If you suspect your cat may have FIP, do not rely on symptoms or an online diagnosis alone. Work with a veterinarian to evaluate the cat and determine the most appropriate treatment plan.

Important: This article is for educational purposes only and is not a substitute for veterinary diagnosis or medical advice. Treatment decisions, including medication selection and dosing, should be made with a qualified veterinarian.