Peritonitis is inflammation of the peritoneum, the thin membrane lining your dog’s abdominal cavity and covering its organs. It ranges from a mild, localized reaction to a life-threatening emergency, particularly when bacteria are involved. Septic (infected) peritonitis carries a mortality rate that can exceed 35% even with aggressive surgical treatment, making it one of the more dangerous abdominal conditions in veterinary medicine.
How Peritonitis Develops
The peritoneum normally keeps abdominal organs protected and lubricated. When it becomes irritated or contaminated, it triggers an intense inflammatory response. Fluid rushes into the abdominal cavity, white blood cells flood the area, and if bacteria are present, toxins can spill into the bloodstream and cause body-wide infection.
Veterinarians classify peritonitis in two main ways. Primary peritonitis develops spontaneously, without an obvious abdominal injury or disease. It’s thought to occur when bacteria from elsewhere in the body travel through the bloodstream to the peritoneum, typically in dogs with weakened immune systems. This form is uncommon. Secondary peritonitis is far more frequent and results from an existing problem inside the abdomen, such as a ruptured organ or a surgical complication. It can also be classified as septic (caused by infection) or nonseptic (caused by chemical irritation from substances like bile or urine leaking into the cavity).
Most Common Causes
Leakage from the gastrointestinal tract is the leading cause of septic peritonitis in dogs. In one study of dogs requiring emergency surgery for the condition, the specific triggers included:
- Foreign body perforation (the most common single cause), where a swallowed object punctures through the stomach or intestinal wall
- Ulcer perforation, often linked to the use of anti-inflammatory or steroid medications
- Surgical site breakdown, where a previous abdominal incision in the gut wall opens after surgery
- Tumor perforation, where a cancerous growth erodes through the intestinal wall
- Stomach torsion (GDV), which can cut off blood supply and cause tissue death
Less commonly, peritonitis develops when bile leaks from the gallbladder or bile ducts, or when urine escapes from a ruptured bladder or damaged urinary tract. Penetrating wounds, including bite injuries and gunshot wounds, can also introduce bacteria directly into the abdomen.
Signs to Watch For
Peritonitis typically makes dogs visibly and rapidly ill. The hallmark sign is severe abdominal pain. Your dog may stand with a hunched posture, cry or flinch when touched around the belly, or refuse to lie down. Vomiting is common, and many dogs stop eating entirely.
As the condition progresses, you may notice abdominal swelling from fluid accumulating inside the cavity. Fever often develops early, but as the body becomes overwhelmed, temperature can actually drop. Reduced or absent bowel movements occur because the intestines stop moving normally, a condition called paralytic ileus. In advanced cases, dogs go into septic shock: their gums turn pale or muddy, their heart rate climbs, their blood pressure drops, and they become weak or collapse. When large volumes of intestinal contents spill into the abdomen, death from the resulting toxin load can happen quickly.
How Veterinarians Diagnose It
A veterinarian suspecting peritonitis will typically start with blood work and imaging. X-rays or ultrasound can reveal free fluid in the abdomen or signs of a ruptured organ. But the most definitive step is abdominocentesis, a procedure where a needle is inserted into the abdominal cavity to collect a sample of the fluid.
That fluid sample tells the story. Under a microscope, the presence of bacteria inside white blood cells is the gold standard for confirming septic peritonitis. The fluid can also be cultured to identify exactly which bacteria are involved. Another useful test compares glucose levels in the abdominal fluid to glucose in the blood. In septic peritonitis, bacteria consume the sugar in the abdominal fluid, so levels drop significantly. A difference of more than 20 mg/dL between blood glucose and fluid glucose is highly reliable for confirming infection. Fluid glucose alone is less accurate, but a reading below 50 mg/dL in the abdominal fluid raises strong suspicion.
Treatment Requires Surgery
Septic peritonitis is a surgical emergency. The core goal is to find and fix the source of contamination, whether that means removing a foreign body, repairing a perforated intestine, or resecting damaged tissue. Before a dog goes to surgery, the veterinary team works to stabilize it with intravenous fluids and antibiotics, since most dogs arrive dehydrated, in pain, and fighting infection.
During surgery, the abdomen is thoroughly flushed with sterile fluid to remove as much contaminated material as possible. In some cases, a drain is placed so fluid can continue to exit the abdomen after surgery. Nonseptic peritonitis, such as cases caused by a sterile bile leak, may sometimes be managed with less aggressive intervention depending on the underlying cause, but surgery is still frequently needed.
After the procedure, dogs remain hospitalized for intensive monitoring. The veterinary team watches closely for signs of recurring infection, manages pain, provides nutritional support, and adjusts antibiotics based on culture results. Hospitalization typically lasts several days to over a week, depending on severity and complications.
Survival Rates and Complications
Survival depends heavily on the cause, how quickly the dog reaches surgery, and how sick it is at the time of treatment. For biliary peritonitis (caused by bile leaking into the abdomen), one study of 33 dogs found an overall mortality rate of 36%. Dogs that needed blood pressure medications to maintain circulation or developed kidney problems fared significantly worse. When a specific liver enzyme marker exceeded a certain threshold, mortality climbed to 50%.
The most dangerous complications after surgery include organ failure affecting multiple systems at once, blood clotting disorders, recurrent infection requiring additional surgery, and infections acquired during the hospital stay. Fluid overload from aggressive resuscitation is another recognized risk. Low urine output can occur shortly after surgery due to the combined effects of sepsis and dehydration, though it often improves with continued fluid therapy. Multi-resistant bacterial infections are an ongoing concern in dogs that require prolonged hospitalization and multiple rounds of antibiotics.
What Recovery Looks Like
Dogs that survive the initial surgery and first few days of hospitalization generally have a reasonable chance of full recovery, but the road is not short. Most dogs need close veterinary follow-up for weeks after discharge, including monitoring the surgical site, rechecking blood work, and ensuring appetite and energy return to normal. Activity is usually restricted for two to three weeks while the abdomen heals internally.
The financial reality of treating peritonitis reflects its severity. Emergency abdominal surgery, multiple days of ICU-level care, lab work, imaging, and medications add up quickly. Costs vary widely depending on location, the complexity of surgery, and the length of hospitalization, but owners should expect a bill in the range of several thousand dollars. Peritonitis caused by a straightforward foreign body removal with a smooth recovery will cost considerably less than a case requiring repeat surgery, extended ICU time, or treatment for organ failure.