New Study: Dual Antiviral Protocol Achieves 78.3% Remission in Previously Relapsed Feline Infectious Peritonitis (FIP) Cats

A 2024 field evaluation of 46 cats shows that combined oral GS-441524 and EIDD-1931 therapy delivers strong outcomes even in the most difficult Feline Infectious Peritonitis (FIP) cases, including cats that had already failed single-drug treatment.

The Hardest Cases to Treat

When a cat relapses after completing a full course of GS-441524 for Feline Infectious Peritonitis (FIP), it means the virus has found a way to survive the treatment. These cases are among the most difficult in veterinary antiviral therapy, because the virus has likely developed some degree of resistance to the drug. For cat owners who have already gone through 84 days of daily medication, watched their cat recover, and then seen the disease come back, it is devastating.

New Study: Dual Antiviral Protocol Achieves 78.3% Remission in Previously Relapsed Feline Infectious Peritonitis (FIP) Cats

A 2024 field evaluation conducted under the BasmiFIP initiative (BasmiFIP is a sister brand of CureFIP specialising in Feline Infectious Peritonitis (FIP) treatment across Southeast Asia) by researchers Li and Cheah specifically studied these challenging cases, and the results offer real hope for cats that need a second chance.

What the Study Did

Between June and September 2024, 46 domestic cats diagnosed with Feline Infectious Peritonitis (FIP) were enrolled in the study. The cats represented a broad range of breeds, ages, and Feline Infectious Peritonitis (FIP) types, including wet (effusive), dry (non-effusive), neurological, and ocular presentations. Approximately 90% were indoor-only cats.

Each cat received a standardized oral capsule containing both GS-441524 (at 10-15 mg/kg once daily) and EIDD-1931 (5 mg for cats under 4.5 kg, or 10 mg/kg for cats 4.5 kg and above). Treatment lasted 60 days, with one capsule per 2.5 kg of body weight every 24 hours.

Critically, a significant number of cats in this study had previously relapsed after completing GS-441524 monotherapy. This was not a cohort of easy first-time cases. Many of these cats had already been through a full treatment cycle, seen it fail, and needed a new approach.

Owners tracked their cat's progress daily, recording weight, appetite, clinical signs, and any side effects. Veterinary validation was obtained where possible. Cats were followed for 180 days after treatment completion to monitor for relapse.

The Results

78.3% achieved clinical remission. Of the 46 cats treated, 36 reached full clinical remission following the 60-day dual antiviral protocol. That means no remaining Feline Infectious Peritonitis (FIP) symptoms after treatment.

Only 6.5% relapsed. During the 180-day follow-up period, just 3 of the 46 cats experienced a return of symptoms. This is a notably low relapse rate, especially given that many of these cats had already relapsed once before on single-drug therapy. For comparison, published GS-441524 monotherapy studies report relapse rates of approximately 10-11%.

10.9% mortality. Five cats did not survive. The authors noted these deaths were likely due to advanced disease stages or secondary complications at the time treatment began, reinforcing the critical importance of starting treatment as early as possible.

91% showed weight gain and improved appetite. By mid-treatment, 42 of the 46 cats had documented weight gain and appetite improvement, tracked through individual daily logs. These are strong indicators that the treatment was working and the cats' bodies were recovering.

A:G ratio improved significantly. The albumin-to-globulin ratio, a key blood marker used to track Feline Infectious Peritonitis (FIP) progression, improved from approximately 0.35 at baseline to 0.62 by Day 60. This upward trend indicates that the inflammatory response driven by the Feline Infectious Peritonitis (FIP) virus was progressively resolving during treatment.

Minimal side effects. Only 2 of the 46 cats experienced adverse events, both minor gastrointestinal issues and brief lethargy during early dosing. No severe toxicity was observed, and no cat had to stop treatment due to side effects.

Why 78.3% Is Actually Impressive

The headline remission rate of 78.3% might appear lower than the approximately 92% typically reported for first-time GS-441524 treatment. But context is everything here.

Many of the cats in this study were not first-time treatment cases. They were cats that had already relapsed after completing GS-441524 alone, meaning the virus had likely developed some degree of resistance. Treating resistance cases is inherently harder than treating naive cases, and achieving 78.3% remission in this population is a meaningful and clinically significant result.

The very low relapse rate of 6.5% is particularly encouraging. For comparison, published GS-441524 monotherapy studies report relapse rates of approximately 10-11% in first-time treatment populations. The fact that the dual protocol achieved lower relapse rates in a harder-to-treat population suggests that the combination of GS-441524 and EIDD-1931 provides stronger and more durable viral suppression.

The mechanism behind this makes pharmacological sense. GS-441524 works by stopping the virus from copying itself through chain termination. EIDD-1931 works by introducing fatal errors into whatever copies the virus does manage to produce through lethal mutagenesis. Together, they attack the virus from two completely different angles, making it far more difficult for the virus to develop resistance to both simultaneously.

What This Means for Cat Owners in the United States

If your cat has been diagnosed with Feline Infectious Peritonitis (FIP), this study adds another layer of real-world evidence supporting dual antiviral therapy. The key takeaways are:

• Dual therapy works even in the hardest cases. Cats that had relapsed after GS-441524 monotherapy still showed strong recovery rates when treated with the GS-441524 + EIDD-1931 combination.

• The relapse rate is remarkably low. At just 6.5% over 180 days of follow-up, the dual protocol demonstrated strong durability against viral recurrence, even in a resistance-enriched population.

• It is well-tolerated and safe. With only 2 minor adverse events across 46 cats and no treatment discontinuations, the safety profile of the combined oral capsule is reassuring for cat owners concerned about side effects during a long treatment course.

• Early treatment still matters most. The cats that did not survive were generally those with the most advanced disease at the time of enrollment. Starting treatment promptly remains the single most important factor in Feline Infectious Peritonitis (FIP) outcomes, whether it is a first diagnosis or a relapse.

• The oral format supports treatment compliance. The study used a single daily capsule combining both antivirals, which simplifies the treatment routine and helps cat owners maintain the consistency that is critical for treatment success over the full course.

Study Limitations

The authors acknowledge that this was an observational field study, not a randomized controlled trial. There was no control group, and outcome data was partly based on owner reports. Pharmacokinetic studies and randomized comparative trials are needed to further validate these preliminary results. However, the consistency of outcomes across diverse Feline Infectious Peritonitis (FIP) phenotypes and the inclusion of previously relapsed cats make this a valuable contribution to the growing evidence base for combination antiviral therapy.

Read the Full Study

Want to dive deeper into the methodology, data, and clinical analysis? Download or read the complete research paper.

Download Full Study (PDF) | Read Full Study Online

"Enhancing FIP Therapeutic Outcomes: A Field-Based Evaluation of Combined Oral GS-441524 and EIDD-1931 Protocols" by Li, Y. & Cheah, B. (2024). BasmiFIP Initiative.

To learn more about dual antiviral treatment or to consult about your cat's Feline Infectious Peritonitis (FIP) treatment plan, visit curefipusa.com.

The CureFIP USA team offers free consultations. Because when every day matters, having the right support makes all the difference.

Reference: Li, Y. & Cheah, B. (2024). Enhancing FIP Therapeutic Outcomes: A Field-Based Evaluation of Combined Oral GS-441524 and EIDD-1931 Protocols. BasmiFIP Initiative.