Wet (effusive) feline infectious peritonitis is a viral disease in which a mutated form of feline coronavirus causes blood vessels in the abdomen or chest to leak protein-rich fluid. A young cat — usually under 2 years — with a distended belly, persistent fever, and weight loss is the classic picture. Until 2019, wet FIP was nearly always fatal; today, antiviral treatment with GS-441524 cures roughly 80 to 90 percent of cases (Pedersen et al., 2019, JFMS).
Last reviewed: May 2026
What Wet FIP Actually Is
Wet FIP is the "effusive" form of feline infectious peritonitis — a systemic vasculitis (inflammation of small blood vessels) triggered by a mutated feline coronavirus. Most cats are exposed to feline enteric coronavirus as kittens through litter-box contact and most clear it without issue, but in a small subset the virus mutates inside the cat's own macrophages and switches from a gut infection to a disseminated immune-driven disease. When the vasculitis affects vessels in the abdomen or chest, protein-rich yellow fluid leaks out and pools — producing the swollen, fluid-filled belly that owners typically see first. Cats under 2 years and cats from multi-cat or shelter environments are most at risk, with roughly 70 percent of FIP cases occurring before 2 years of age in published cohorts.
What Owners Notice First
The early signs are vague: a kitten or young cat who is not growing the way the littermates are, who skips meals, runs a stubborn low-grade fever, and seems quieter than usual. Over days to a few weeks, the belly starts to enlarge — not bloated and tense, but soft and pendulous, like a water balloon. The cat may still be playful early on, which is part of why FIP is missed. Other signs include weight loss across the spine and ribs while the belly grows, jaundice (yellow gums, ears, or eyes), pale gums, and labored breathing if the fluid is in the chest instead of (or in addition to) the abdomen.
The Fever That Will Not Quit
A persistent fever that does not respond to antibiotics is one of the most useful clues. The fever is typically 103.5 to 105°F (39.7 to 40.6°C), waxes and wanes through the day, and recurs each time antibiotics are stopped. This is because FIP is not bacterial — antibiotics do nothing to the underlying viral process. The AAFP-AAHA Feline Life Stage Guidelines, 2021 emphasize that any kitten with a recurrent unexplained fever should have FIP on the differential list, especially with a recent shelter, rescue, or breeder history.
How Vets Diagnose Wet FIP
There is no single perfect test. Workup includes a full blood panel, abdominal or chest ultrasound to confirm fluid, and a tap of the fluid for analysis. The classic fluid findings — high protein (>3.5 g/dL), low cellularity, an albumin-to-globulin ratio below 0.4 to 0.5, and a viscous straw-yellow appearance — together make wet FIP very likely. Bloodwork commonly shows anemia, high total protein with very high globulins, mild liver enzyme elevation, and sometimes high bilirubin. PCR testing on the effusion for feline coronavirus, immunostaining of macrophages within the fluid, or biopsy can confirm. A 2009 review reported that the combination of effusion analysis plus serum protein patterns has high diagnostic accuracy when interpreted together, but no test alone is 100 percent specific (Hartmann, 2005, JFMS).
Treatment — The Real Revolution
For decades wet FIP was nearly 100 percent fatal within weeks. Since 2019, oral or injectable GS-441524 (the nucleoside analog also known as the active form of remdesivir) has fundamentally changed the disease. The original 2019 trial of GS-441524 in 31 cats with naturally occurring FIP reported sustained remission in 25 of 31 cats (about 81 percent) after a 12-week treatment course (Pedersen et al., 2019, JFMS). Subsequent real-world cohorts have reported response rates between 80 and 95 percent for wet FIP when treated for the full 84 days. The protocol involves daily dosing, weekly to biweekly weight checks, and rechecks of blood work and protein ratios. Pain control follows the AAHA Pain Management Guidelines, 2022, with attention to comfort during the recovery period.
Cost, Access, and What Has Changed
Because the FDA only approved an oral GS-441524 product for cats in 2024, families had spent years buying medication through informal networks. Now veterinarians can prescribe FDA-conditionally approved oral GS through the usual pharmacy channels. Typical treatment costs in the US run roughly $3,000 to $8,000 for a full 84-day course (including rechecks and bloodwork), depending on the cat's weight, route of administration, and specialist involvement. This is dramatically less expensive than informal-channel costs of the early 2020s but is still a major financial commitment.
After Treatment: What Recovery Looks Like
Cats who respond typically feel better within 1 to 2 weeks — fever drops, appetite returns, and fluid stops re-accumulating. Belly size shrinks. Bloodwork improves over the next month: protein ratio normalizes, anemia resolves, liver values come down. The 84-day course is treated as a fixed minimum, with a 12-week post-treatment observation window. Relapse during the observation window prompts a second course, often at a higher dose. Long-term survival data show that most cats who complete treatment without relapse during the observation window go on to live a normal life span.
Wet Versus Dry FIP — Why It Matters
Wet FIP is the effusive form (fluid in abdomen or chest); dry FIP is the non-effusive form, in which immune granulomas form in organs like the eyes, brain, kidneys, or intestinal lymph nodes without large fluid pockets. Wet and dry can overlap (some cats develop both, or evolve from one to the other), and the antiviral treatment is the same. Dry FIP is harder to diagnose — there is no easy fluid to tap — and historically had a slightly lower response rate, though newer cohorts show similar response if the disease is caught and treated early.
When to See a Vet
Call your vet today if:
- A kitten or young cat with a belly that is enlarging without weight gain
- Persistent fever for more than 48 hours, especially if antibiotics aren't helping
- Yellow tinge to gums, ears, or whites of the eyes
- New labored or rapid breathing in a young cat
- A recent rescue, shelter, or breeder cat with poor appetite and weight loss
Go to the ER immediately if:
- Open-mouth breathing or severe respiratory distress
- Collapse, unresponsiveness, or pale-to-white gums
- Seizures or sudden neurological changes (head tilt, circling, blindness)
- A massively distended abdomen with vomiting and refusal to eat for more than 24 hours
- A known FIP cat on treatment who suddenly worsens
What's going on with your pet?
Describe symptoms or snap a photo. Voyage tells you urgency, home care, and whether you need a vet.
First, tell us about your pet
Breed and age make a real difference in how Voyage interprets symptoms.
Describe the symptoms
🏆 Outperforms ChatGPT & Gemini · 🩺 Vet-grounded · 🔒 Private
Love it? See everything Voyage can do
Frequently Asked Questions
Is FIP contagious to other cats or to people?
FIP itself is not contagious. The everyday gut coronavirus that some cats carry is contagious cat-to-cat through litter-box contact, but the mutation that turns it into FIP happens inside each individual cat. Most exposed cats never develop FIP. FIP cannot infect humans, dogs, or other species.
How much does FIP treatment cost?
A full 84-day course of oral GS-441524 plus diagnostic workup (ultrasound, bloodwork, fluid analysis) and rechecks typically runs $3,000 to $8,000 in the US. Severely ill cats needing initial hospitalization, oxygen, and IV fluids may add $1,000 to $3,000. Diagnostic-only costs (ultrasound, bloodwork, fluid analysis) are around $600 to $1,200 if you are deciding whether to treat. Some specialty hospitals offer payment plans.
Can wet FIP really be cured now?
For most cats, yes. Real-world response rates with oral or injectable GS-441524 sit in the 80 to 95 percent range when the full 84-day course is completed and the cat is followed through the 12-week post-treatment observation period. Cats who relapse during observation usually respond to a second course at a higher dose. "Cure" is defined as remission with no relapse for at least 12 weeks after stopping treatment.
Will my other cats get FIP?
Almost certainly not. Even in multi-cat households where the gut coronavirus is circulating, only a small percentage of exposed cats — usually under 10 percent — go on to develop FIP. Stress reduction, low-density housing, separate litter boxes, and good kitten nutrition all reduce the risk in young cats. Quarantining a sick cat from littermates is reasonable but rarely changes outcomes for the others.
How is wet FIP different from heart or liver disease in a young cat?
Heart failure and liver disease can both cause belly fluid, but the fluid composition is different. FIP fluid is high-protein, low-cellularity, and viscous yellow; heart failure fluid is usually low-protein clear fluid; liver disease fluid varies but is rarely as protein-dense as FIP fluid. Bloodwork patterns also differ — high globulins and a low albumin-to-globulin ratio strongly favor FIP in a young cat.
Still Not Sure if Your Cat Needs a Vet?
When you're not sure if this is wait-and-see or call-tonight, Voyage AI Vet triages in under 2 minutes. Describe what you're seeing in chat, share photos of a swollen belly, gum color, or eye changes, or hop on a live video call if you want a second pair of eyes. Every answer comes with citations to the actual veterinary literature it's pulling from — so you see exactly where the guidance comes from, not just a chatbot's word.