FIP in 2026: New Guidelines, a Better Diagnostic Test, and a Disease That's No Longer a Death Sentence

The European Advisory Board on Cat Diseases (ABCD) has updated its FIP treatment guidelines. CSU has developed the most accurate FIP diagnostic test to date. And researchers are just beginning to understand what happens to cats who survive. Here's everything that's changed — and what it means for your practice right now.

By Vet Candy Editorial  |  June 2026  |  Feline Medicine & Infectious Disease

Five years ago, an FIP diagnosis in a cat was effectively a death sentence. You had the conversation with the client, you managed the timeline, and you provided supportive care until quality of life could no longer be maintained. That was the standard.

In 2026, that's no longer the case and the clinical evidence base is moving fast enough that what you knew about FIP two years ago may already be out of date.

Three significant developments dropped in recent weeks: updated ABCD treatment guidelines confirming a shorter, more accessible treatment protocol; a new immunofluorescent diagnostic assay from Colorado State University that outperforms existing tests; and emerging data on post-FIP complications in cats who survive. This is the complete update.

The ABCD Guidelines Update — Treatment Has Changed

The European Advisory Board on Cat Diseases (ABCD) published updated FIP treatment guidelines in April 2026 in the journal Viruses. The Feline Veterinary Medical Association (FelineVMA) has also endorsed a companion guide updated in 2025. Together, these represent the current consensus on best-practice FIP management.

The headline finding: a 42-day course of oral GS-441524 can be as effective as the standard 84-day course for many FIP presentations.

"This has important implications for access to care for cats with FIP as a shorter course reduces costs, minimises potential compliance difficulties or side effects, decreases stress associated with treatment administration and causes fewer adverse events," the ABCD authors wrote.

For clients who were already managing the financial and logistical demands of a 12-week antiviral course, cutting that in half is clinically and practically significant.

First-Line Treatment: Oral GS-441524

The guidelines consolidate what the clinical evidence has been pointing toward for several years: oral GS-441524 is the preferred first-line therapy. Unless a cat is unable to swallow, extremely dehydrated, or severely unwell — in which case injectable or remdesivir may be more appropriate — the oral route is now the standard approach.

Why oral over injectable? Tablet and liquid formulations are easier to administer, allow for more accurate dosing, and are associated with fewer adverse effects. Injectable GS-441524 is associated with pain at the subcutaneous injection site, which affects both patient welfare and owner compliance.

GS-441524 is not FDA-approved for use in cats — it is available through compounding pharmacies under the Animal Medicinal Drug Use Clarification Act. In 2024, the FDA indicated it would apply regulatory discretion to compounded GS-441524 provided compounders follow GFI #256, requiring a veterinary prescription for a specific patient for FIP treatment. That framework is now established, though sourcing remains through compounding pharmacies rather than commercial manufacturers.

Second-Line Options: Molnupiravir, Protease Inhibitors, and Combination Approaches

About 10–15% of cats treated with GS-441524 relapse. For those cats, the current evidence supports:

Increasing the GS-441524 dose as a first step, before switching agents.

Molnupiravir as a second-line option with demonstrated efficacy comparable to GS-441524 — a 2025 study in JFMS showed 77% survival at six months in a 73-cat cohort — but with more side effects to monitor including GI upset, neutropenia, and a clinically notable one: floppy ear syndrome.

Molnupiravir affects cartilage and bone growth and can cause ears to fold in treated cats. It is also teratogenic. It's a mutagenizing agent — it works by introducing errors into viral replication — which makes its mechanism of action less safe than nucleoside analogues from a resistance and safety standpoint. It's not a first-line drug.

3C-like protease inhibitors (GC-376, nirmatrelvir) are less commonly used and are not recommended as monotherapy but may serve as adjuncts in refractory, relapsed, or severe cases that haven't responded to adequate doses of nucleoside analogues.

Glucocorticoids: When They Help

The updated guidelines include specific guidance on glucocorticoid use alongside antiviral therapy. Anti-inflammatory or immunosuppressant doses can be helpful in specific situations — the most notable being immune-mediated hemolytic anemia in association with FIP. This is not routine adjunctive use; it's targeted to situations where the inflammatory component of the disease process requires direct management.

The New Diagnostic Test — 78% Accuracy, Better Than Anything Available

FIP has always been notoriously difficult to diagnose. The underlying feline coronavirus is ubiquitous — up to 90% of domestic cats are infected with it at some point — and the mutation that produces FIP occurs in a small subset of those cats. Distinguishing FIP from the benign coronavirus, and distinguishing FIP from other diseases that present similarly (lymphoma, pancreatitis, toxoplasmosis, epilepsy), has been one of the persistent clinical challenges.

"FIP is still a diagnosis of assumption for many patients," said Dr. Samantha Evans, assistant professor at CSU's Department of Microbiology, Immunology, and Pathology.

CSU researchers, working with colleagues at University of Georgia, Ohio State, and University of Michigan, have developed a new immunofluorescent assay that achieved 78% accuracy in a clinical trial involving 84 cats enrolled at CSU and OSU Veterinary Medical Centers.

"This is better than anything we have currently," Evans said. "Importantly, we also found that our test is complementary to the other tests we looked at, meaning that it can correctly identify some cases that a PCR test cannot, and vice versa. We think this will be a useful add-on as part of a diagnostic panel for FIP."

The test uses FIP-specific antibodies to detect the virus within cells from the patient. It will soon be available through CSU Veterinary Diagnostic Laboratories as part of their commercial diagnostic offerings — meaning it will be accessible to practitioners who send samples to CSU's lab.

Why Accurate Diagnosis Is Now More Critical Than Ever

With effective treatments available, the stakes of a diagnostic error have changed in both directions.

Under-diagnose FIP and a treatable cat doesn't get treated. That's the obvious concern. But the emerging concern is equally real: if cats are treated for FIP when they don't have it, you're potentially selecting for antiviral-resistant coronaviruses, exposing cats to unnecessary adverse effects, and directing client resources toward the wrong disease while the actual diagnosis is delayed.

"The more we treat patients that don't have this disease and don't need treatment, the more we are going to be breeding for resistant viruses. That's a big concern," Evans said.

The new immunofluorescent assay, used alongside PCR and clinical presentation, gives practitioners a stronger diagnostic foundation before initiating treatment — particularly in cases where the presentation is ambiguous.

Life After FIP — The Next Frontier

Here's the part of the FIP story that's just beginning to be told: what happens to cats who survive?

As the survivor population grows, researchers are identifying conditions appearing during and after FIP treatment that weren't previously documented — because previously, cats didn't survive long enough for post-treatment sequelae to manifest.

Myocarditis. A May 2026 study in the Journal of Veterinary Internal Medicine by Dr. Petra Cerna and colleagues showed that cats with FIP can present with elevated markers of myocardial injury and cardiac inflammation. Whether this is a direct consequence of the viral disease, the treatment, or the inflammatory response is still being characterized.

Large-cell lymphoma post-FIP. A March 2026 study in JFMS described four cases of large-cell lymphoma developing within two years of FIP diagnosis and successful treatment in a 202-cat cohort. The researchers noted that immune overstimulation associated with FIP might have negative long-term consequences — a hypothesis that requires further investigation but warrants monitoring in FIP survivors.

Persistent lymphadenomegaly, lymphocytosis, eosinophilia. Researchers have reported ongoing laboratory and imaging findings in some FIP survivors that persist beyond treatment — not fully resolved even when the cat is clinically well.

The clinical takeaway: FIP survival is the new baseline, and post-FIP monitoring protocols for surviving cats are going to need to be developed as the evidence base matures. This is an active area of research, not a settled one.

What to Do Right Now in Your Practice

Update your FIP treatment protocols. If you're still defaulting to 84-day GS-441524 courses without evaluating whether 42 days is appropriate for your patient, review the ABCD guidelines.

Transition to oral GS-441524 as your default route unless clinical presentation requires otherwise.

Know your compounding pharmacy sources. GS-441524 is still only available through compounding, and quality control varies. The FDA guidance requires a prescription for a specific patient for FIP treatment — make sure your prescribing documentation reflects that.

Add the CSU immunofluorescent assay to your diagnostic toolkit when it becomes available through CSU Veterinary Diagnostic Laboratories — particularly for ambiguous presentations where PCR alone may miss cases.

Start thinking about post-FIP monitoring. Cats who complete treatment successfully need a follow-up framework. Cardiology screening, lymph node assessment, and CBC monitoring at six and twelve months post-treatment are reasonable starting points as the data develops.

AVMA Convention 2026 — FIP Sessions

Dr. Petra Cerna (University of Georgia) is presenting two FIP-focused sessions at AVMA Convention 2026 (July 10–14, Anaheim, CA):

"The Mysterious FIP: Diagnosis, Treatment and Management of Complicated FIP Cases" — July 11, 3 p.m. Pacific, CE Theater 2, Anaheim Convention Center

"Felines First: Cats are Shaping the Future of Veterinary Medicine" — July 12, 8 a.m. Pacific, Room 260ABC, Anaheim Convention Center