Peritonitis in cats is inflammation of the peritoneum, the thin membrane lining the abdomen and covering the internal organs. It comes in two very different forms: feline infectious peritonitis (FIP), caused by a mutated coronavirus, and septic peritonitis, caused by bacteria leaking into the abdominal cavity from a ruptured organ or wound. Both are serious and potentially fatal, but they have different causes, different symptoms, and very different treatment paths.
Two Types With Very Different Causes
FIP starts when a common and usually harmless feline coronavirus mutates inside a cat’s body. About 10 percent of cats infected with feline coronavirus develop this mutation, which allows the virus to infect white blood cells and spread throughout the body. The resulting immune response triggers intense inflammation around blood vessels in the abdomen, kidneys, brain, and other organs. FIP is not caused by bacteria and cannot be treated with antibiotics.
Septic peritonitis, on the other hand, happens when bacteria contaminate the abdominal cavity. The most common cause is a perforation somewhere in the gastrointestinal tract, essentially a hole in the stomach or intestines that lets gut bacteria spill into a space that should be sterile. This can result from intestinal tumors (lymphoma is the most common in cats), inflammatory bowel disease, certain medications like anti-inflammatory drugs or corticosteroids, kidney disease, or severe stress and shock. In many cases, no clear cause is ever identified. Bacteria commonly found in these infections include E. coli, Enterobacter, Klebsiella, and Streptococcus species.
Symptoms to Watch For
Both types of peritonitis can look vague in the early stages, which makes them easy to miss. Common signs across both forms include lethargy, loss of appetite, weight loss, and fever. Many cats with peritonitis run a persistent low-grade fever that does not respond to antibiotics.
The hallmark of the “wet” form of FIP is fluid accumulation in the abdomen or chest. You might notice your cat’s belly looks visibly swollen or distended, or your cat may develop labored breathing if fluid builds up around the lungs or heart. About one-third of cats with the wet form have lung involvement. Some cats develop yellowing of the skin, gums, or eyes. In the “dry” form of FIP, there is no obvious fluid buildup. Instead, inflammatory cells accumulate in organs like the liver, kidneys, eyes, and brain. Cats with dry FIP often have vague, recurring illness, and eye problems or neurological signs like wobbliness or seizures can develop.
Septic peritonitis tends to come on more acutely. Cats are often in obvious pain, reluctant to be touched around the belly, and may stand hunched or avoid lying down. Rapid decline, dehydration, and signs of shock (pale gums, rapid heart rate, weakness) can follow quickly because the bacterial infection triggers a systemic inflammatory response.
How Vets Diagnose Peritonitis
Abdominal ultrasound is typically the first step. In cats with FIP, ultrasound detects free abdominal fluid in about 88 percent of cases. It can also reveal intestinal wall thickening (seen in roughly 68 percent of cats) and changes to the mesentery, the tissue connecting the intestines to the abdominal wall.
If fluid is present, your vet will likely draw a sample with a needle. The appearance and composition of that fluid tells a lot. FIP fluid is typically thick, sticky, and straw-colored with a high protein content. One simple test used to evaluate it is the Rivalta test, which involves dropping a small amount of the fluid into a dilute acid solution. If a visible clot forms and sinks, the test is positive. This test catches about 91 percent of FIP cases and is especially accurate in cats under two years old, where sensitivity reaches nearly 95 percent.
For septic peritonitis, the fluid sample is examined under a microscope for bacteria and inflammatory cells. Bacterial culture identifies the specific organisms involved, though results can sometimes come back negative even when infection is present. Blood work, imaging, and sometimes exploratory surgery help pinpoint the source of the leak.
Treatment for FIP
FIP was considered uniformly fatal until recently. The antiviral compound GS-441524 has transformed the outlook dramatically. A large study tracking 633 cats treated between 2020 and 2024 reported a survival rate of 94.12 percent, with only a 0.63 percent relapse rate. Most deaths occurred within the first 20 days of treatment, suggesting that cats who survive the initial phase have an excellent chance of full recovery. Treatment typically lasts at least 12 weeks, during which the cat receives daily oral or injectable antiviral medication.
These numbers represent a remarkable shift. Earlier studies reported relapse rates ranging from 3 to 26 percent. The current generation of treatment protocols appears to be more effective and more consistent.
Treatment for Septic Peritonitis
Septic peritonitis is a surgical emergency. The source of contamination, whether a perforated intestine, ruptured abscess, or other defect, needs to be found and repaired. During surgery, the abdominal cavity is thoroughly flushed with large volumes of sterile fluid (the standard in veterinary medicine is 200 to 300 milliliters per kilogram of body weight) to remove bacteria and debris.
After surgery, some cats need ongoing drainage of the abdomen. Options include closed-suction drains or, in severe cases, leaving the abdomen partially open under sterile bandages to allow continued drainage. Cats treated this way require intensive hospital care for several days. Antibiotics are started before surgery and continued afterward, though research has shown that adding antibiotics directly to the abdominal rinse fluid can actually damage tissue and is not recommended.
Pain management is a critical part of recovery. Peritonitis causes significant abdominal pain, and cats in the hospital typically receive injectable pain medications on a scheduled basis to keep them comfortable during healing.
Survival Rates and What Affects Them
For FIP treated with antivirals, the prognosis has become genuinely hopeful. A 94 percent survival rate means most cats with FIP can now be expected to recover, provided treatment starts before the disease becomes too advanced.
Septic peritonitis carries a more guarded outlook. Survival depends heavily on how quickly the cat receives surgery, the underlying cause of the perforation, and how sick the cat is at presentation. One significant prognostic marker is liver function: cats with elevated bilirubin levels (a sign of liver stress) at the time of diagnosis have a mortality rate around 48 percent, compared to just 9 percent in cats with normal bilirubin. Cats with intestinal lymphoma as the underlying cause face a more complicated recovery because the cancer itself requires ongoing management even after the perforation is repaired.
Which Cats Are Most at Risk
FIP disproportionately affects young cats. It is most common in cats under two years old, particularly those from shelters, catteries, or multi-cat households where feline coronavirus circulates easily. Stress from rehoming, surgery, or other illness can trigger the viral mutation that leads to FIP.
Septic peritonitis can happen at any age but is more common in older cats, partly because the conditions that cause gastrointestinal perforation, such as intestinal lymphoma and inflammatory bowel disease, are diseases of aging. Cats on long-term anti-inflammatory or steroid medications are also at increased risk because these drugs can weaken the protective lining of the stomach and intestines, making perforation more likely.