Feline infectious peritonitis (FIP) was nearly always fatal until the antiviral GS-441524 changed everything. Most cats now achieve clinical remission with an 84-day course of GS-441524, given as either daily injections or oral tablets, with cure rates above 85 percent across wet, dry, ocular, and neurological forms. Diagnosis still depends on signs, fluid analysis, and PCR testing; treatment is increasingly available through veterinarians as the legal landscape evolves.
Last reviewed: May 2026
What Is FIP and How Has Treatment Changed?
Feline infectious peritonitis is caused by a mutated form of feline coronavirus that infects macrophages and triggers widespread vasculitis. Until 2019, FIP carried a near-100 percent mortality rate; the introduction of the nucleoside analog GS-441524 transformed it into a treatable disease. Cure rates of 85 percent or higher with an 84-day GS-441524 protocol have been demonstrated across multiple international studies (Pedersen et al., 2019, JFMS).
In the United States, GS-441524 has historically been obtained from compounding pharmacies and overseas suppliers. The regulatory landscape is shifting — as of 2024 and 2025, several FDA-approved compounded options for veterinary use have become available, and most exotic and internal medicine specialists now feel comfortable prescribing and supervising treatment.
Wet vs Dry vs Ocular vs Neurological FIP
Wet (effusive) FIP is the most common form and presents with fluid in the abdomen or chest, lethargy, weight loss, and persistent fever. About 60 to 70 percent of FIP cases are effusive. Dry (non-effusive) FIP causes pyogranulomatous lesions in organs (kidneys, liver, lymph nodes) and presents as weight loss, fever, and poor appetite without obvious fluid. Ocular FIP causes uveitis, color changes in the iris, or KP precipitates on the cornea. Neurological FIP causes ataxia, seizures, head tremor, or behavior change.
The form matters because dosing protocols differ — neurological and ocular FIP require higher GS-441524 doses (typically 10 to 15 mg/kg daily) because the drug must cross the blood-brain or blood-eye barrier.
How Vets Diagnose FIP
There is no single diagnostic test for FIP — diagnosis is built from compatible clinical signs, fluid analysis (effusion that is yellow, sticky, and high-protein with a low cell count), albumin-to-globulin ratio below 0.4, elevated alpha-1 acid glycoprotein, and PCR detection of feline coronavirus in effusion or tissue. Abdominal ultrasound often shows lymph node enlargement or kidney lesions. The Rivalta test on abdominal fluid is a useful, inexpensive in-clinic screening tool.
A definitive diagnosis traditionally requires immunohistochemistry on tissue, but in practice most cats are treated empirically when the clinical and lab picture is strongly consistent — the response to GS-441524 within 2 to 3 days is itself supportive of the diagnosis.
The 84-Day GS-441524 Protocol
The standard protocol is 84 days of GS-441524 at 6 to 15 mg/kg daily, depending on FIP form and severity. Dosing is given either by daily subcutaneous injection (more painful, but more reliable absorption) or by oral tablets (better tolerated, but absorption can vary). Fever usually breaks within 24 to 72 hours; appetite returns in 3 to 7 days; effusion typically resolves within 2 to 4 weeks.
Weekly weight checks and dose adjustments are critical — many cats gain 30 to 50 percent of body weight during treatment and the dose (in mg) should be increased accordingly. Bloodwork every 2 to 4 weeks tracks albumin-to-globulin recovery and rules out complications. After 84 days, treatment is stopped and the cat enters a 12-week observation period; relapse within that window occurs in roughly 5 to 10 percent of cats and typically responds to a repeat course.
Cost and Practical Considerations
GS-441524 treatment is expensive. A typical 84-day course for a 4 kg cat costs $2,500 to $6,000 in total when sourced from veterinary compounding pharmacies, with neurological cases running higher because of the increased dose. Diagnostic workup adds $400 to $1,200 (exam $50 to $150, bloodwork $150 to $300, ultrasound $300 to $600, fluid PCR $150 to $300). Hospitalization for unstable cats can add $500 to $1,500 per day for the first few days.
Despite the cost, success rates and quality of life after treatment justify the investment for most owners, as outlined in current AAFP feline care guidance (AAFP-AAHA Feline Life Stage Guidelines, 2021).
When to See a Vet
Not every symptom is a midnight emergency, but some warrant same-day attention and a few are true ERs. Use the lists below to sort which bucket you're in.
Call your vet today if:
- Persistent fever above 103 F for more than 48 hours
- Progressive weight loss in a young cat (under 2 years) or shelter-adopted cat
- Yellowing of the eyes or gums (jaundice)
- Round, distended belly that has come on over weeks
- Color change in one eye, cloudiness, or new uveitis
Go to the ER immediately if:
- Severe difficulty breathing from chest fluid
- Seizures, sudden ataxia, or rapid neurological decline
- Collapse, unresponsiveness, or pale gums
- Complete refusal to eat or drink for more than 48 hours
- Acute jaundice with vomiting
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Frequently Asked Questions
Can FIP really be cured now?
Yes — with an 84-day course of GS-441524, more than 85 percent of cats achieve sustained remission across wet, dry, ocular, and neurological forms. Neurological FIP has somewhat lower cure rates (around 75 to 80 percent) and requires higher doses. This is a dramatic change from the pre-2019 era when FIP was nearly always fatal within weeks to months.
How much does FIP treatment cost?
A full 84-day GS-441524 course costs roughly $2,500 to $6,000 for a typical 4 kg cat from a US veterinary compounding pharmacy, with neurological cases on the higher end because of larger doses. Diagnostic workup adds $400 to $1,200 (exam $50 to $150, bloodwork $150 to $300, ultrasound $300 to $600, fluid PCR $150 to $300). Initial hospitalization, if needed, can add $500 to $1,500 per day. Total typical out-of-pocket: $3,000 to $8,000.
Is GS-441524 legally available in the US?
The legal landscape is evolving rapidly. As of 2024 and 2025, several FDA-approved compounded options for veterinary use are available through veterinary compounding pharmacies, and many internal medicine and exotic specialists now prescribe and supervise GS-441524 treatment directly. Owners no longer need to source the drug from unregulated overseas suppliers in most US states — ask a feline internal medicine specialist or experienced exotic vet for current guidance in your area.
What is the difference between wet and dry FIP?
Wet (effusive) FIP causes fluid accumulation in the abdomen, chest, or both, with lethargy, weight loss, and fever — about 60 to 70 percent of cases. Dry (non-effusive) FIP causes pyogranulomatous lesions in solid organs without obvious fluid, and presents as weight loss, persistent fever, and poor appetite. Wet FIP usually shows clinical signs faster, while dry FIP often takes longer to diagnose because the picture is more subtle.
Can FIP relapse after treatment?
Relapse occurs in about 5 to 10 percent of treated cats, usually within the 12-week observation window after the 84-day course ends. Relapse typically responds to a repeat treatment course, often shorter than the original. Cats that go 12 weeks symptom-free after treatment have an excellent long-term prognosis and the disease rarely recurs after that point.
Still Not Sure if Your Cat Needs a Vet?
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