Current Vet

In this episode of Current Vet, Dr Lottie talks about feline infectious peritonitis (FIP)

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Timeline:
00:00 Intro
00:29 Case
03:10 Aetiology & Pathogenesis
04:35 Clinical Signs
07:07 Diagnosis
11:47 Treatment
17:19 Prevention
18:15 Key Points
19:26 Outro


Recommended Reading
  • Hartmann, K. (2005) ‘Feline infectious peritonitis’, Veterinary Clinics of North America: Small Animal Practice, 35(1), pp. 39–79. doi: 10.1016/j.cvsm.2004.10.011.
While this is a great overview of FIP, it was written in 2005 when the latest treatment options and antivirals were not yet available

References
  • Fischer, Y., Sauter-Louis, C. and Hartmann, K. (2012) ‘Diagnostic accuracy of the Rivalta test for feline infectious peritonitis’, Veterinary Clinical Pathology, 41(4), pp. 558–567. doi: 10.1111/j.1939-165X.2012.00464.x.
  • Tasker, S. (2018) ‘Diagnosis of feline infectious peritonitis: update on evidence supporting available tests’, Journal of Feline Medicine and Surgery, 20(3), pp. 228–243. doi: 10.1177/1098612X18758592.
  • Felten, S. and Hartmann, K. (2019) ‘Diagnosis of feline infectious peritonitis: a review of the current literature’, Viruses, 11(11), p. 1068. doi: 10.3390/v11111068.
  • Tekes, G. and Thiel, H.-J. (2016) ‘Feline coronaviruses: pathogenesis of feline infectious peritonitis’, Advances in Virus Research, 96, pp. 193–218. doi: 10.1016/bs.aivir.2016.08.002.
  • Taylor, S.S., Coggins, S., Barker, E.N., Gunn-Moore, D., Jeevaratnam, K., Norris, J.M., et al. (2023)‘Retrospective study and outcome of 307 cats with feline infectious peritonitis treated with legally sourced veterinary compounded preparations of remdesivir and GS-441524 (2020–2022)’, Journal of Feline Medicine and Surgery, 25(9). doi: 10.1177/1098612X231194460.
  • Thayer, V., Gogolski, S., and Olah, G.A. et al. (2022) ‘2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines’, Journal of Feline Medicine and Surgery, 24(9), pp. 905-933. doi: 10.1177/1098612X221118761.

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.
While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host
Dr. Lottie Wilkinson
Creator and host of the Current Vet podcast

What is Current Vet?

The podcast that makes veterinary medicine simple.

In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

Whether you’re cramming for exams or looking for a soundtrack for your dog walk, Current Vet will make veterinary medicine simple

Intro

Hello, hello and welcome back to Current Vet. This is the podcast that makes veterinary medicine simple. I'm Dr. Lottie, and today we're talking about a condition that can be really difficult for both owners and vets, but that we've actually made a lot of progress in in the last few years, and that is feline infectious peritonitis or FIP.

So let's get straight into our case.

Case

It is late afternoon and your last consult of the day is Luna, a 10 month old female domestic short hair. She was adopted from a shelter three months ago. Her owner, Sarah says Luna hasn't been herself for a couple of weeks. Now she's become quieter, sleeping more, only picking at her food and has lost some weight.

She also mentions that Luna's belly looks a little bit round, but she's unsure whether that's just because Luna's getting older. On exam, Luna is quiet but alert. Her temperature is 39.8 degrees Celsius. Her coat is dull and her body condition is about a three out of nine. Her abdomen feels soft but distended.

It's not obviously painful, but there's a fluidity shifting sensation when you palpate her gently. Her mucus membranes are slightly icteric, but you can't palpate any abnormalities in her lymph nodes. You make note of the fact that she's a young cat, has a shelter background, low grade fever, weight loss, and abdominal distension, so your differentials are gonna include ascites, possibly secondary to hepatic disease, neoplasia, although she's young, so.Maybe that isn't so high on your list. Right-sided heart failure, especially with that ascites, hypoproteinaemia, and then infectious causes such as FIP. So you discuss with Sarah that Luna needs further investigation, starting with some basic bloods and imaging. So you admit her for a workup the next day.

The following morning, Luna's hematology shows a mild non regenerative anemia and lymphopenia. Biochemistry shows high globulin levels, hyperbilirubinaemia, and an albumin globulin ratio of 0.4. So these results are immediately pushing FIP higher on your list. An abdominal ultrasound shows free fluid with echogenic fibrous strands swirling through it.

You decide to collect a small sample of the effusion for analysis. As you draw it up, the fluid is thick, straw colored and sticky. After you've tested it, it has a total protein of 41 grams per liter and a low cell count dominated by macrophages. At this point, you have plenty of evidence to make a presumptive diagnosis of effusive, FIP.

Aetiology & Pathogenesis

But let's talk about what FIP actually is. . Feline infectious peritonitis develops from a mutation in feline coronavirus inside the cat's body. Feline coronavirus itself is actually very common in cats, especially in density populated environments. So like multi cat households, breeding colonies and shelters where the prevalence can be as high as 90%.

Most infected cats stay healthy or only get a mild enteritis, but in less than around five to 10% of the cat population, the virus mutates and the new mutated form known as feline infectious peritonitis virus gains the ability to infect and replicate inside macrophages. So once infected these macrophages spread the virus throughout the bloodstream, trigger systemic inflammation and vasculitis.

So FIP is caused by a mutation in an existing coronavirus infection, not an entirely separate infection in itself. Wild felids, like cheetahs have also been known to develop FIP, particularly those that are captive or in high density environments. But today we're just gonna focus on domestic cats.

Clinical Signs

So once this mutation has occurred, there are two main clinical presentations that can develop. The first is the wet or effusive form, and the second is the dry or non effusive form. So this wet form develops when the inflammation and vasculitis cause protein-rich fluid to leak from the blood vessels into body cavities like the pleural cavity, abdominal cavity, the pericardium, and actually it can even go into the scrotum in males.

The clinical signs for this form depends on where the fluid is accumulating, but you might see things like dyspnea and tachypnea if there's pleural effusion, ascites with peritoneal effusion, which is why Luna had that rounded belly or cardiac tamponade, and a muffled heart sounds with pericardial effusion.

In males, if they have scrotal effusion, it might just appear quite swollen. This form can progress very quickly over a couple of days or weeks. The second dry form causes granulomatous lesions to develop in organs like the kidneys, liver, central nervous system, or eyes. This has a more chronic presentation and progressives over weeks and months rather than days, like the wet form.

Again, the signs seen depend on where these granulomas are developing, but ocular and neurological granulomas are most common.

Ocular granulomas cause inflammation of the iris, chorioretinitis, and uveitis. So that's gonna cause signs like corneal edema, hypopyon, hyphema, dyscoria, and aqueous flare in the eyes. Neurological signs can include ataxia, a head tilt, nystagmus, and seizures. Of course, there are also gonna be some non-specific signs that can be seen in both forms or in mixed presentations, even
things like lethargy inappetence, progressive weight loss, pyrexia and icterus. And like I just mentioned, to make it more complicated for you as a vet, cats can develop mixed presentations or one form could actually develop into the other and vice versa, in which case you're gonna see a mixture of those two clinical presentations.

Diagnosis

So for diagnosis, you'll typically use a combination of tests and evidence rather than one single diagnostic test. So first we have clinical suspicion to start with. There are a few features in a history that would make you suspicious of FIP. So a young cat from a shelter or a multi cat household with a persistent fever, weight loss, and either effusion or neurological or ocular signs would definitely raise your suspicion.

if fluid is present, you should always try and obtain a sample so you can do an effusion analysis. FIP effusions are usually straw colored and viscous with a high protein content of generally more than 35 grams per liter. A low albumin globulin ratio, so less than 0.4, is gonna be suggestive of FIP, but if it's more than 0.8 FIP is unlikely. this effusion typically has a low cellularity, but the majority of cells that are present are macrophages and neutrophils. We can also do a Rivalta's test with this effusion, and this is a really cheap and cheerful screening test that vets can do in practice.

It was developed to differentiate transudate from exudates. So isn't actually specific for FIP. It's performed by placing a drop of the effusion sample into a solution of acetic acid, which is basically vinegar. And if the sample is an exudate, the drop will retain its shape within the solution, whereas Transudate will disperse and the solution will stay clear.

It actually has a really high sensitivity. So a low false negative rate, which means that if the test is negative, it's unlikely to be FIP. However, as we said, it is not specific for FIP. It's just differentiating exudates from Transudate. So while it's definitely useful as a quick screening test, septic peritonitis and effusions from lymphomas can also give a positive result.

The third thing we're gonna do is hematology and biochemistry. So hematology usually shows lymphopenia and a mild non regenerative anemia. Biochemistry typically shows hyper globulin, anemia from that infection and hypoalbuminemia. So again, that's gonna cause that low albumin globulin ratio. We can also see hyper bilirubin anemia, which is consistent with the fact that we're seeing icterus in our clinical signs. Other results on the biochem may actually give clues as to which organs are affected. So , for example, if we had a granuloma in the liver, we might see a high ALT and GGT.

A fourth diagnostic option is imaging, so things like ultrasound or radiographs can actually be really useful for finding effusions or granulomas within organs. And finally, we have molecular and immunological tests, and these are the tests to detect the virus itself or any immune responses to the virus.

So reverse transcriptase, PCR can detect. The coronavirus, RNA in samples of effusion, blood or tissues. But because it's detecting the coronavirus, a positive result does not confirm feline infectious peritonitis. It just tells you that the cat is infected with coronavirus, which as we said before, could be as high as 90% of the CAT population.

So this is really not a useful diagnostic test for FIP.

Secondly, we can do a histopathology, immunostaining, or immunofluorescence for feline coronavirus antigens in tissue and effusion samples, and this is considered the gold standard test for FIP because it not only shows that coronavirus is present, but we are visualizing the virus in an effusion or in a granuloma, and if they are found inside macrophages, which only happens once the mutation to FIP virus has occurred in Luna's case, that fluid analysis, hyper Glo anemia and her history all support a diagnosis of effusive. FIP.

Treatment

Let's come on to talking about treatment. So until very recently, FIP was considered fatal, full stop. But in recent years, this has changed a lot as antivirals, which target the virus, have become available. So first we have GS441524. is not very catchy, but it's a nucleoside analog and the active metabolite of rem desir and it's given orally.

And then we have remdesivir, which is the pro-drug form, which can be given as an injection either intravenously or subcutaneously or orally as a tablet. Both of these drugs inhibit viral, RNA polymerase, which blocks viral replication inside the host macrophages.

So while it's wonderful to now have treatment options available for FIP, it hasn't made treating it easy. The antivirals are firstly very expensive and require a minimum of a 12 week course. Clinical improvement can start as early as a week into treatment, but it's so important that the full 12 week course is fulfilled.
The costs depend on the dose required for each cat, which varies depending on the form of FIP and the weight of the cat, so it can actually get more and more expensive as the cat improves over the course of treatment and they start gaining weight again. Just to give you a couple of ballpark costs. GS oral therapy costs around three to 6,000 US dollars or two to 5,000 British pounds for a standard 12 week course.

And that kind of varies depending on supplier and dose, but in that range. Remdesivir IV treatment has to be done under vet supervision and it's more expensive because of these hospitalization fees. Remdesivir can also be administered subcutaneously, which can actually be done by owners in the home, but these subcut injections are really painful for the cats.

And doing this daily for 12 weeks can ruin an owner's relationship with their pet. There are also side effects of these antivirals to consider, and that includes nephrotoxicity, UIs, and gastrointestinal upset.

In addition to these antivirals, we are also having to do supportive care. So fluids, nutrition, analgesia, anti-inflammatories, and these are all gonna further add to the cost, especially if our patients are hospitalized.

There are a couple of other possible antiviral therapies, so GC376, which is a protease inhibitor, a Molnupiravir, which is another nucleoside analog. And while these options are shown to be clinically effective, they aren't as thoroughly researched as GS or remdesivir. The other thing to consider is that in many regions, GS remains unlicensed and owners may resort to sorting it through unofficial channels or from black market sellers.

This can be really dangerous as formulations and doses are not being regulated, and owners can't actually be certain that these unlicensed meds won't do more harm than good. In addition to antivirals and supportive care, immunostimulants, like feline interferon and poly paral can be used, but they're not cures for the virus. So they have to be used in addition to antivirals. So while these new antivirals have achieved amazing survival rates, reported up to 80 to 90% in treated cats.

The access to the drug legalities and cost of treatment still remain massive. Barriers to therapy being available to a lot of families. It's really important that we explain to owners that euthanasia is always an option for Luna's owner. Cost became the deciding factor. After discussing the prognosis and the cost, she sadly had to choose euthanasia because she was unable to afford months of treatment and hospitalization. It's an incredibly hard conversation, but one that a lot of vets are facing.

So FIP is a real mixed bag of exciting new medicine, new treatments. But also those emotional consults and financial considerations. So as a vet, your role is to present accurate information, explaining that while effective treatments exist, it may not be feasible for everyone, and we have to be able to discuss the prognosis costs, and quality of life early on in the disease process.

Vets also really need to be aware of the emotional toll this condition can have on owners. Many feel incredibly guilty for choosing euthanasia, so empathy, clarity, and reassurance that they are making a kind considered decision are essential. At the end of the day, this is an incredibly horrible disease for cats to have, and while treatment options are available, it is still a long and difficult process to put them through, and at the end of it all recurrence can still happen.

So please, please make owners aware that there is no right decision. It is just what is best for them and their pet in their situation.

Prevention

Let's talk about prevention. So since feline coronavirus is ubiquitous, prevention focuses on good management. So minimizing stress and overcrowding in multi-cat environments, maintaining really good litter hygiene. So separate trays for groups of cats and daily cleaning, isolate possibly infected cats and breeders can consider coronavirus screening tests to make sure that they have seronegative animals. Vaccinations do exist, but they're not recommended for general use. Shelters need to be focusing on hygiene, biosecurity, and reducing animal density. And remember that the goal is not to eliminate feline coronavirus, but to prevent the stressful conditions where it's more likely to result in a mutation to FIP.

Key Points

So what are the key takeaways to know about FIP? Feline infectious peritonitis is caused by a mutation in feline coronavirus that occurs within the host, which allows the virus to infect and replicate inside macro fixtures. There are two main forms, the effusive or wet form, and non effusive or dry form, and the clinical signs associated with each depend on where these effusions and granulomas are developing.

Diagnosis in practice generally relies on a combination of clinical signs, fluid analysis, imaging, and lab findings. GS441524 and Remdesivir have transformed the treatment landscape for FIP. But access and cost of these drugs remain major limitations. And finally, early recognition, client communication and compassionate case management are key when dealing with these cases.

Outro

Thank you so much for listening to this episode of Current Vet. If you found it helpful, hit follow or subscribe wherever you get your podcasts and share it with a friend who needs help with this topic. If you found it helpful, remember to hit, follow or subscribe and share it with a friend who needs help with this topic.
You will find links to further reading and resources on FIP in the show notes below. So check those out if you want to learn more. As always, you can find us on Instagram and TikTok at Veterinary Vista and let us know if there's a topic that you want to hear about next.

See you next time.