Peritonitis in cats — inflammation of the abdominal cavity lining — is a life-threatening emergency whether it stems from a ruptured bowel, a bile leak, or the notorious feline infectious peritonitis virus. A cat with a tense, painful abdomen, fever, and rapid decline needs emergency care measured in hours, not days.
Last reviewed: June 2026
What Is Peritonitis in Cats?
Peritonitis is inflammation of the peritoneum — the thin membrane lining the abdominal cavity and covering the organs within it. In cats it can be septic (bacterial contamination from a ruptured hollow organ), bile peritonitis (from gallbladder or bile duct rupture), uroperitoneum (urine leakage from a bladder or urethral tear), or the effusive form of feline infectious peritonitis (FIP), which causes a protein-rich abdominal effusion through vasculitis rather than true infection, as described in Nelson and Couto's Small Animal Internal Medicine.
Causes of septic peritonitis include:
- Gastrointestinal perforation — foreign body penetration, severe ulceration, or tumors eroding through the bowel wall
- Bite wound penetration — cat fight wounds that pierce through the body wall into the abdomen
- Surgical dehiscence — leakage from a GI anastomosis or intestinal closure after abdominal surgery
- Ruptured pyometra — infected uterus that ruptures and spills septic contents into the abdomen
FIP-related effusive peritonitis is distinct — it produces a characteristic straw-yellow, high-protein, low-cell-count effusion from systemic vasculitis caused by feline coronavirus mutation.
Signs of Peritonitis in Cats
- Abdominal pain — hunched posture, guarding (flinching when touched around the belly), refusal to jump or be picked up
- Tense, rigid, or distended abdomen — the belly feels tight or board-like; or a visible fluid wave if effusion is present
- High fever (103–106°F / 39.4–41.1°C) — septic peritonitis drives a strong febrile response; FIP may cause fluctuating fever
- Vomiting — often repeated, sometimes projectile
- Anorexia — complete loss of interest in food
- Rapid breathing — from pain and pressure of abdominal contents on the diaphragm
- Lethargy and collapse — septic shock develops quickly without treatment
- Jaundice (yellow skin or eyes) — if bile peritonitis is present
- Weight loss — especially pronounced in the subacute FIP effusion form that builds over days to weeks
Diagnosis
Peritonitis is a clinical-laboratory diagnosis confirmed by:
- Abdominal ultrasound — detects free fluid, gas (pathognomonic for GI perforation), and organ changes; also guides abdominocentesis
- Abdominocentesis (belly tap) — analysis of the fluid is critical: septic fluid shows degenerate neutrophils with intracellular bacteria; FIP effusion shows high total protein (>35 g/L), high albumin-to-globulin ratio inversion, and positive FCoV antibody or RT-PCR
- CBC and chemistry panel — leukocytosis with a left shift (immature neutrophils) in septic peritonitis; hyperbilirubinemia in bile peritonitis; elevated globulins and low albumin in FIP
- Lactate measurement — elevated blood lactate confirms tissue hypoperfusion and guides resuscitation urgency
- Diagnostic imaging — radiographs or CT if foreign body or mass perforation is suspected
The AAFP-AAHA Feline Life Stage Guidelines, 2021 support rapid fluid analysis in any cat presenting with abdominal effusion and systemic illness.
Treatment
Septic peritonitis requires emergency surgery to identify and correct the source, lavage the abdominal cavity, and place temporary abdominal drains. Preoperative resuscitation with IV crystalloids and broad-spectrum antibiotics is initiated immediately. Without surgery, survival is minimal.
FIP effusion is now treatable with GS-441524 (an antiviral nucleoside analogue) and/or nirmatrelvir combinations. The AAFP Senior Care Guidelines, 2021 acknowledge that antiviral therapy has transformed FIP management from uniformly fatal to potentially curable.
Bile peritonitis: Emergency surgery to repair or remove the bile duct source, followed by intensive supportive care.
Postoperative care for all forms includes IV fluids, analgesics, nutritional support via feeding tube, and antibiotic therapy tailored to culture results.
When to See a Vet
Call your vet today if:
- Your cat has a tense or painful abdomen and is not eating
- Fever persists beyond 24 hours with lethargy
- Your cat recently had abdominal surgery and the incision looks red, swollen, or is leaking
- Your cat has had repeated bouts of vomiting and cannot keep food down
Go to the ER immediately if:
- Your cat is in obvious abdominal pain and cannot settle
- Your cat is collapsed, barely responsive, or breathing rapidly
- You can feel or see a distended, fluid-filled abdomen
- Rapid deterioration within hours after any surgery or trauma
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Frequently Asked Questions
What is the most common cause of peritonitis in cats? Septic peritonitis most commonly results from gastrointestinal perforation (foreign body, tumor, severe ulceration) or uterine rupture (in intact females with pyometra). FIP-associated effusive peritonitis is actually the most common form of abdominal effusion diagnosed in younger cats and involves the feline coronavirus, not bacteria. Knowing which type is present changes treatment completely — only abdominocentesis fluid analysis can definitively distinguish them.
Is peritonitis always an emergency? Septic peritonitis and bile peritonitis are absolute emergencies — without surgery within hours, survival rates drop dramatically with each hour of delay. FIP effusion can progress more subacutely over days to weeks before the cat decompensates. However, any cat with abdominal pain, fever, and lethargy should be evaluated the same day rather than observed at home.
Can cats survive peritonitis? Survival depends entirely on the type and how quickly treatment begins. Septic peritonitis treated aggressively with surgery and intensive care has survival rates of 50–70% in cats who make it to surgery. FIP peritonitis, historically fatal, now achieves sustained remission in 80–90% of treated cats with GS-441524 antiviral therapy.
How much does peritonitis treatment cost in cats? Emergency workup — ultrasound ($300–600), abdominocentesis and fluid analysis ($150–300), bloodwork ($100–250) — runs $500–1,150. Septic peritonitis surgery with abdominal lavage costs $2,000–5,000, plus intensive care hospitalization ($500–1,500/day) for 3–7 days. Total for septic peritonitis: $4,000–12,000. FIP antiviral therapy (GS-441524) costs $100–250 per month for a 12-week course, making it far more affordable than surgery-based treatment.
How long does recovery from peritonitis take in cats? Recovery from surgical peritonitis is 2–4 weeks for the wound, with intensive care for the first 3–5 days postoperatively. Cats treated successfully often return to normal activity within 4–6 weeks. FIP antiviral therapy requires 12 weeks of treatment with monitoring every 4 weeks; cats often feel dramatically better within the first 2 weeks of treatment.
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 your cat's abdomen, posture, or any visible swelling, 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.