Feline Infectious Peritonitis (FIP) is a serious and often progressive disease that can develop in some cats infected with feline coronavirus (FCoV). FCoV is common, particularly in multi-cat environments, and usually causes either no signs or only mild gastrointestinal illness. In a small proportion of cats, however, the virus undergoes mutation within the body and triggers an abnormal immune response. This immune-mediated reaction leads to the systemic inflammation associated with FIP. The disease may present in an effusive (fluid-accumulating) form or a non-effusive form affecting organs such as the eyes, brain, liver, or kidneys.
Clinical Signs of FIP
FIP can affect cats in different ways, and early signs are often subtle. Common indicators may include:
- Persistent fever unresponsive to antibiotics
- Lethargy or reduced activity
- Loss of appetite or weight
- Poor growth in kittens
- Abdominal swelling or breathing difficulty
- Neurological changes such as wobbliness, tremors, or seizures
- Eye inflammation or vision changes
- Jaundice or enlarged lymph nodes
Because these signs can overlap with other conditions, veterinary assessment is essential for diagnosis.
Diagnosis
Diagnosis is typically based on a combination of:
- Clinical examination findings
- Blood test abnormalities (including albumin-to-globulin ratio and acute phase proteins)
- Imaging (ultrasound or radiography)
- Analysis of effusion (if present), including the Rivalta test
- PCR or immunohistochemistry in selected cases
Veterinarians use clinical judgement and available diagnostics to determine the likelihood of FIP. No single test can definitively confirm FIP in all cases, so diagnosis often relies on the pattern of findings taken together.
Antiviral Medicines Reported in Scientific Literature
Scientific studies and case reports have explored several antiviral compounds in cats with FIP. These drugs, called nucleoside analogues, work by interfering with the virus's ability to replicate its RNA. The most widely discussed compounds in the literature include GS‑441524 and molnupiravir.
Important: No antiviral medicine is currently registered by the Australian Pesticides and Veterinary Medicines Authority (APVMA) specifically for FIP. Any use of these medicines occurs under veterinary prescribing frameworks and legal compounding pathways. Outcomes reported in studies vary between cats and should not be interpreted as guaranteed results.
The Role of Compounding Pharmacies
In the absence of registered antiviral products for FIP in Australia, veterinarians may prescribe compounded formulations prepared by licensed pharmacies under applicable state and federal legislation. Compounding pharmacies can work collaboratively with veterinary professionals to prepare individualised medications tailored to each patient's needs, as directed by the prescribing veterinarian.
Frequently Asked Questions
FIP itself is not considered directly contagious. It develops when the common feline coronavirus (FCoV), which is widespread in multi-cat environments, mutates inside a cat. While FCoV can spread between cats, the mutated form that causes FIP usually does not transmit to other cats.
The prognosis for FIP has traditionally been poor, but with the emergence of antiviral treatments reported in recent studies, outcomes for some cats have improved significantly compared to historical expectations. Some cats may deteriorate rapidly, while others can live weeks to months after diagnosis. Prognosis depends on the form of the disease, the cat's overall health, and the level of veterinary care, and is best discussed with a veterinarian.
There is currently no guaranteed way to prevent FIP. Because it arises from mutations of the common feline coronavirus, risk can sometimes be reduced in multi-cat households by:
- Practising good hygiene and litter box management
- Minimising stress and overcrowding
- Separating infected cats where possible
Even with careful management, FIP can still occur, and prevention strategies cannot completely eliminate the risk.
Yes. FIP is not limited to outdoor or free-roaming cats. Even indoor-only cats can develop FIP if they are exposed to feline coronavirus, for example from other cats in the household, or if they were exposed as kittens. The disease arises from a mutation of the virus within the cat itself, rather than being directly transmitted in its mutated form.
FIP is most commonly seen in young cats, particularly those under two years of age. Kittens are more susceptible because their immune systems are still developing, making them more vulnerable to the effects of the virus and the abnormal immune response that leads to FIP. However, FIP can also occur in older cats, although it is less common.
In clinical studies of antiviral use, some cats may relapse or experience recurrence after initial improvement, while others maintain remission. Recurrence is influenced by factors such as the type of FIP, the individual cat's health, and the veterinary management of the case. Each situation should be assessed by a veterinarian.
Supportive care focuses on maintaining comfort and quality of life. This can include:
- Providing a quiet, low-stress environment
- Ensuring easy access to food, water, and litter
- Monitoring weight, appetite, hydration and energy levels
- Regular veterinary check-ups to monitor clinical signs and overall wellbeing
Supportive care does not treat FIP itself but can help cats remain comfortable and maintain strength throughout the course of the disease.
Speak With Your Veterinarian
If your cat has been diagnosed with FIP, speak with your veterinarian about the available treatment options. Veterinary professionals seeking information about compounding services are welcome to contact us or register for a professional account.
References
- Murphy BG, Perron M, Murakami E, et al. The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies. Veterinary Microbiology. 2018;219:226–233.
- Pedersen NC, Perron M, Bannasch M, et al. Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. Journal of Feline Medicine and Surgery. 2019;21(4):271–281.
- Dickinson PJ, Bannasch M, Thomasy SM, et al. Antiviral treatment using the adenosine nucleoside analogue GS-441524 in cats with clinically diagnosed neurological feline infectious peritonitis. Journal of Veterinary Internal Medicine. 2020;34(4):1587–1593.
- Pedersen NC. A review of feline infectious peritonitis virus infection: 1963–2008. Journal of Feline Medicine and Surgery. 2009;11(4):225–258.
- Hartmann K, Ritz S. Treatment of cats with feline infectious peritonitis. Veterinary Immunology and Immunopathology. 2008;123(1–2):172–175.
Disclaimer: This page is intended for general educational purposes only and does not constitute veterinary advice, diagnosis, or treatment recommendations. The information provided is based on published scientific literature and should not be used as a substitute for professional veterinary consultation. Always consult a qualified veterinarian regarding your cat's health and treatment options. Compounding 4 Vet does not make any therapeutic claims about specific compounded products on this page.