Peritonitis in Horses: From Early Signs to Recovery

Peritonitis in horses is inflammation of the peritoneum — the thin serous membrane lining the abdominal cavity — and it is one of the most serious abdominal conditions a horse can develop. It can mimic colic in its early stages, kill within hours in catastrophic cases, and demand weeks or months of intensive antibiotic therapy in chronic ones. UK and South African equine vets see peritonitis regularly as a complication of colic surgery, foaling injuries, and primary infection by Actinobacillus equuli, and survival varies enormously depending on the underlying cause.

For owners, the message is simple: any horse with a persistent fever, dullness, reduced appetite, and signs that look like quiet, lingering colic needs urgent veterinary assessment. Abdominocentesis — the so-called “belly tap” — is the test that confirms or rules out peritonitis, and how quickly it is done is often the single biggest predictor of survival.

What Is Peritonitis in Horses?

The peritoneum is the thin, lubricated membrane that lines the inside of the abdominal cavity and covers the abdominal organs. It normally produces a small volume of clear, straw-coloured fluid (typically less than 100 mL in an adult horse) that lets the organs glide freely against one another during digestion and movement.

Peritonitis is inflammation of this membrane. The normal fluid balance is disrupted, white blood cells flood into the cavity, fibrin is laid down, and — where bacteria are involved — toxins are absorbed across a membrane that has an enormous surface area and a rich blood supply. This is precisely why bacterial peritonitis can tip a horse into systemic shock so rapidly. The Merck (MSD) Veterinary Manual classifies peritonitis by its cause, and that cause largely dictates both treatment and outlook.

Types of Peritonitis

Primary (Idiopathic) Peritonitis

Primary peritonitis develops without an obvious abdominal source — no rupture, no surgery, no penetrating wound. It is more common in horses than in many other species and is most often associated with Actinobacillus equuli, a normal inhabitant of the equine intestinal tract that, for reasons still not fully understood, sometimes invades the peritoneal cavity. Idiopathic peritonitis typically responds well to antibiotic therapy and carries a good prognosis.

Secondary Peritonitis

Secondary peritonitis is the more dangerous form. It develops when the peritoneum is contaminated by something — gut contents from a perforation, blood and bacteria from a uterine tear during foaling, surgical contamination during colic surgery, or external trauma. Survival rates are markedly lower than for primary peritonitis, particularly where the contamination involves intestinal contents.

Acute vs Chronic

Acute peritonitis develops over hours to days, with rapidly worsening pain, fever, and systemic illness. Chronic peritonitis develops over weeks, sometimes following an incompletely resolved acute episode, and tends to present as gradual weight loss, intermittent low-grade fever, recurrent mild colic, and reduced performance.

Septic vs Non-Septic

Septic peritonitis is caused by bacterial infection and dominates the clinical picture in horses. Non-septic (“chemical”) peritonitis is rarer and results from non-bacterial irritants such as bile, urine, gastric acid, or chemotherapeutic agents leaking into the abdomen.

Causes of Peritonitis in Horses

The most common causes of peritonitis seen in UK and South African equine practice are:

• Idiopathic / Actinobacillus equuli infection — the most common cause in adult horses presenting out of the blue
• Post-colic-surgery contamination — particularly after enterotomy, resection, or anastomosis procedures
• Intestinal rupture from severe colic — a catastrophic and almost always fatal cause
• Rectal tears — typically iatrogenic, occurring during rectal examination, particularly in stallions or anxious horses
• Uterine rupture or tear — during foaling or in late pregnancy
• Penetrating abdominal wound — kicks, or sharp objects in fencing and stables
• Migrating parasites — particularly large strongyles (Strongylus vulgaris) and heavy worm burdens that damage the gut wall and mesenteric arteries
• Bastard strangles — abscess rupture following strangles (Streptococcus equi) infection
• Tumour rupture — rare, and typically in older horses

Actinobacillus equuli and Idiopathic Peritonitis

The Swedish retrospective study by Odelros and colleagues (2019, 130 cases) remains the largest published series on idiopathic peritonitis. Actinobacillus species were the most commonly recovered bacteria, though only 41% of the cultures taken grew anything at all, and the study reported survival to discharge of 94% with appropriate antibiotic therapy.

Why does Actinobacillus sometimes cross from the gut into the abdomen? It is not fully understood. Theories include microscopic mucosal damage from parasite migration, transient mucosal weakening during episodes of mild colic, or haematogenous (blood-borne) seeding. The clinical picture is typically:

• Fever, though often only a mild one (the mean rectal temperature on admission in the Odelros series was 38.6°C, with a range of 37 to 40.8°C)
• Dullness, reduced appetite, mild colic
• No history of recent surgery or trauma
• Markedly elevated peritoneal fluid white cell count
• A good response to broad-spectrum antibiotic therapy

For clinicians, Actinobacillus equuli peritonitis is the form they hope for when peritonitis is on the differential list — it carries by far the best prognosis.

Post-Surgical Peritonitis After Colic

Peritonitis is one of the most feared complications of colic surgery. Despite scrupulous surgical technique, contamination of the peritoneal cavity during enterotomy or anastomosis can occur, and the risk is highest after procedures that open the gut lumen.

Post-surgical peritonitis carries a worse prognosis than idiopathic disease — partly because the bacterial contamination is often mixed (several gram-positive and gram-negative organisms together), and partly because the horse is already systemically compromised by the original colic. Reported survival is lower and more variable than for primary disease, which is why post-operative monitoring is so important.

Symptoms and Clinical Signs

Early Signs

• Mild fever (38.5–39.5°C)
• Dullness and depression
• Reduced appetite
• Mild, intermittent abdominal pain that looks like quiet colic
• Reluctance to move
• Stiff or guarded posture

The early presentation is often non-specific and easily missed. A horse that is “off colour” without an obvious reason — particularly after surgery or foaling — deserves a temperature check and prompt veterinary attention.

Progressive Signs

• Worsening abdominal pain — pawing, looking at the flanks, lying down
• Tucked-up appearance from abdominal guarding
• Bloated belly from accumulating fluid
• Sweating, elevated heart rate (over 60 bpm), increased respiratory rate
• Reduced or absent gut sounds
• Dehydration with a prolonged skin tent and slow capillary refill
• Diarrhoea in some cases

Severe / Septic Shock Signs

• Pale, dark red, or “muddy” gums
• A toxic line at the gum margin
• Weak, thready pulse
• Cold extremities
• Severe depression or recumbency
• A falling body temperature (a particularly poor prognostic sign)

Chronic Signs

• Gradual weight loss over weeks
• Intermittent low-grade fever
• Recurrent mild colic episodes
• A poor coat and dull eyes
• Reduced performance

Diagnosis and Abdominocentesis

Clinical Examination

The vet will take a full history — including any recent surgery, colic, foaling, or trauma — and perform a thorough physical examination. Findings typically include a raised temperature and heart rate, abdominal guarding, reduced gut sounds, and varying degrees of dehydration.

Abdominocentesis (“Belly Tap”)

The single most important diagnostic test is abdominocentesis. A small needle or teat cannula is introduced through the lowest point of the ventral abdomen into the peritoneal cavity, and a sample of peritoneal fluid is collected for analysis. The procedure is straightforward in the standing horse, takes minutes, and gives immediate information on:

• Colour — clear straw is normal; cloudy, yellow, red, or brown indicates pathology
• Total protein — a raised level suggests inflammation
• White blood cell count — a raised count confirms peritonitis
• Cytology — what the cells look like (degenerate neutrophils, intracellular bacteria) distinguishes septic from non-septic disease
• Gram stain and culture — identifies the organism for targeted antibiotic therapy

Blood Tests

Blood work typically shows:

• A raised white cell count (sometimes low initially in peracute septic cases — a bad sign)
• Increased serum amyloid A (SAA), a sensitive acute-phase protein
• Increased fibrinogen
• Raised lactate in shock states
• Electrolyte disturbances with prolonged loss of appetite

Ultrasound

Trans-abdominal ultrasound can reveal free fluid, fibrin tags, abscesses, and thickened bowel wall. It is now part of the standard work-up at referral centres and many first-opinion practices.

Reading the Peritoneal Fluid Results

What “normal” and “peritonitis” peritoneal fluid looks like in laboratory terms:

Plant material in the fluid is essentially diagnostic of intestinal rupture and almost always fatal.

Treatment of Peritonitis

Stabilisation and Fluid Therapy

Intravenous fluid therapy is started immediately to correct dehydration, support the circulation, and combat shock. Plasma may be given to replace protein losses in severely affected horses. Pain relief is essential, and anti-inflammatory medication forms part of most treatment plans. All medicines and doses are the responsibility of the attending vet — never medicate a suspected peritonitis case yourself.

Antibiotic Therapy

Broad-spectrum antimicrobial therapy is started before culture results return, then refined once the specific organisms and sensitivities are known. Treatment aims to cover the range of bacteria typically involved — gram-positives, gram-negatives, and, where intestinal compromise is suspected, anaerobes. Courses are long, frequently running to several weeks. Idiopathic Actinobacillus peritonitis often responds well; mixed infections from intestinal contamination are far harder to clear.

Peritoneal Lavage

In selected cases, particularly post-surgical peritonitis, the abdomen may be flushed (lavaged) with sterile fluid through indwelling drains. This removes bacteria, fibrin, and inflammatory toxins, and repeated lavage over several days can substantially reduce the bacterial load in the abdomen.

Surgical Treatment

Surgery is required when there is a physical lesion to deal with — a bowel perforation, an abscess to drain, or dead tissue to remove. Exploratory laparotomy is performed under general anaesthesia at a referral hospital. The surgical mortality rate for true peritonitis is significant, but in many scenarios surgery is the only option that offers any chance of recovery.

Long-Term Monitoring

Repeat abdominocentesis is used to track the response to treatment — falling white cell counts and clearing fluid indicate progress, and SAA trends are also useful. Stopping antibiotics too early is a common cause of relapse, so courses are usually continued beyond the point of apparent clinical recovery.

Prognosis and Survival Rates

Prognosis depends overwhelmingly on the underlying cause. Published case series and veterinary references consistently report the same pattern:

• Idiopathic / Actinobacillus peritonitis: the best outlook, with high survival on antibiotic therapy (94% survival to discharge in the Odelros 2019 series)
• Post-surgical peritonitis (no rupture): a guarded but genuine chance of recovery with intensive treatment
• Bowel rupture: grave, and almost always fatal
• Rectal-tear-associated peritonitis: highly variable, depending on the grade of tear and the speed of intervention
• Uterine rupture peritonitis: a realistic chance of survival with prompt, aggressive treatment

The factors that most strongly predict survival are the speed of diagnosis, identification of the underlying cause, and the intensity of treatment. Because exact figures vary between studies and depend heavily on the individual case, your vet is the right person to give a realistic prognosis for your horse.

Recovery Time and Aftercare

Recovery from peritonitis is rarely quick. Even a straightforward idiopathic case means a full antibiotic course (5 to 30 days, median 11, in the Odelros series), repeat belly taps to confirm the infection is clearing, and a gradual return to normal appetite and demeanour. Severe or post-surgical cases can take months and require a period of box rest and controlled exercise before the horse returns to full work.

Key elements of aftercare include:

• Completing the full antibiotic course, even after the horse looks well
• Monitoring temperature, appetite, and droppings daily during recovery
• Repeat blood tests or abdominocentesis to confirm resolution before treatment stops
• A staged return to exercise guided by your vet
• Awareness that intra-abdominal adhesions can form during healing and may predispose a horse to future colic

Because adhesions are a recognised long-term complication in survivors, any recurrence of mild colic after recovery should be taken seriously and reported to your vet.

Is Peritonitis in Horses Contagious?

No. Peritonitis itself is not contagious and does not spread from horse to horse. It is an inflammatory reaction within an individual animal’s abdomen, not a transmissible disease. That said, some of the underlying causes — such as strangles, which can rarely lead to abdominal abscessation and secondary peritonitis — are themselves highly contagious. In those situations the concern for the rest of the yard is the primary infection, not the peritonitis, and standard biosecurity and isolation advice from your vet applies.

Preventing Peritonitis

• Risk-based parasite control — test first with faecal worm egg counts and tapeworm saliva or blood tests, then treat the horses that need it, which reduces the risk of strongyle migration damage without burning through the drugs. Dosing every horse at fixed intervals is the older “strategic” model, and the British Horse Society is explicit that it drives resistance and “hasn’t been recommended for many years”
• Skilled rectal examinations — carried out by experienced vets with proper restraint and lubrication, ideally after sedation in twitchy horses or stallions
• Careful foaling supervision — quick recognition and management of dystocia reduces the risk of uterine trauma
• Post-surgical vigilance — twice-daily temperature checks for two weeks after colic surgery, with abdominocentesis at the first hint of fever or dullness
• Tetanus and strangles vaccination — reduces the risk of certain disease-related complications
• A safe stable and field environment — removing sharp objects, kicking hazards, and protruding fence elements reduces the risk of penetrating-wound peritonitis

When to Call the Vet

Phone your vet without delay if your horse:

• Has a temperature above 38.5°C (the top of the normal range for a healthy adult horse) together with reduced appetite or dullness
• Shows persistent or recurrent mild colic
• Is recovering from colic surgery and shows any fever, dullness, reluctance to move, or off-feed behaviour
• Is a recently foaled mare with a fever or signs of abdominal pain
• Has had a recent rectal examination and now seems unwell
• Has any kick or penetrating injury to the abdomen, however minor it looks

One point worth stating plainly, because an earlier version of this page had it the wrong way round: do not give bute or any other painkiller and then wait to see whether the colic settles. World Horse Welfare’s advice is to make an emergency call at the first signs of colic, and never to give pain relief yourself unless a vet has told you to, because medication makes it harder for the vet to judge whether the horse needs referral to a hospital. It also removes the most useful clue you have. Fever was the commonest sign in the Odelros series (83% of horses), and the authors note that figure is almost certainly an underestimate, because 42% of their cases had already been given NSAIDs before they arrived. Painkillers hide the fever that gets peritonitis diagnosed.

Peritonitis is a condition where the outcome depends heavily on how quickly treatment begins. A vet visit on day one is far cheaper and far more likely to succeed than a referral admission on day three. For related abdominal conditions, see our guides to colic surgery, equine colic, and spasmodic colic.

Frequently Asked Questions