Equine peritonitis is the inflammation of the peritoneum — the inner lining of a horse’s abdominal cavity. Think of the peritoneal cavity as a balloon filled with a small amount of fluid, inside which the abdominal organs sit. This peritoneal fluid acts as a lubricant, reducing friction as the organs move during normal activity.
When the peritoneum becomes inflamed, this carefully maintained environment is disrupted. The result is a serious, often life-threatening condition that requires prompt veterinary attention. How a horse does depends heavily on the cause, and the published survival figures are a good deal better than the mortality range of “30% to 67%” that circulates online. In the largest series to date, 122 of 130 horses (94%) with idiopathic peritonitis survived to discharge (Odelros et al., Acta Veterinaria Scandinavica 2019;61:18); in a series of 72 horses that excluded gut rupture and post-operative cases, 60 (83%) survived (Arndt et al., Veterinary Surgery 2021;50:323-335). It is peritonitis following rupture of the gut that carries a grave prognosis, not the disease in general.
Peritonitis can be triggered by both infectious causes — such as bacterial, viral, fungal, or parasitic agents — and non-infectious causes, including physical trauma, chemical irritation, and cancer. Of these, bacterial infection is the most well-recognised cause in horses.
Table of Contents
Primary vs Secondary Equine Peritonitis
Primary Peritonitis
Primary peritonitis is a spontaneous disease process with no obvious identifiable cause. It typically occurs when the horse’s natural immune defences are compromised. The most commonly reported bacterial isolate in primary peritonitis cases is Actinobacillus. Because there is no obvious underlying injury or rupture, diagnosis usually requires a series of diagnostic tests rather than a single clear finding.
Secondary Peritonitis
Secondary peritonitis is by far the more common form in horses. It occurs when an identifiable cause allows bacteria or other contaminants to enter the normally sterile peritoneal cavity. Common causes include:
Penetrating injuries to the abdominal wall allow bacteria direct access to the cavity. Perforation or rupture of the gastrointestinal tract — for example, as a complication of severe colic — is one of the most serious triggers and carries a grave prognosis. Reproductive tract injuries, particularly uterine tears following a difficult foaling, can also result in peritonitis. Uterine tears tend to produce a slower, more progressive onset of clinical signs because the number of pathogenic organisms in the uterine lumen is relatively low compared to the intestine.
Other causes of secondary peritonitis include the spread of bacteria from the bloodstream (septicaemia), infection from the urinary tract or uterus, intra-abdominal abscesses, an infected umbilical cord in foals, complications following the gelding process, intestinal parasites, certain viral infections, and neoplasia (cancer).
Recognising the Signs of Equine Peritonitis
General Behaviour Changes
Horses with peritonitis rarely present with a dramatic, sudden onset of obvious symptoms. More often, owners notice subtle changes first. A horse that is quieter than usual, a little depressed, eating less, and producing fewer droppings than normal should raise concern — particularly if these signs persist for more than 24 to 48 hours.
Take Freddie, a 12-year-old grey gelding whose owner described him as “not quite right” for two days before he was referred for veterinary assessment. His case was written up by Leona Bramall MRCVS for Your Horse. His early signs — reduced appetite, fewer droppings, and a slightly subdued demeanour — are exactly what many peritonitis cases look like in the early stages.
Fever and Elevated Vital Signs
On examination, most horses with peritonitis will present with a fever above 38.5°C. Freddie had a temperature of 39.5°C. Heart and breathing rates are typically elevated, gut sounds are reduced, and the horse may show signs of dehydration. Toxaemia may also be evident in more severe cases.
Pain and Reluctance to Move
Affected horses may show signs of discomfort when the external abdominal wall is palpated. Some are reluctant to move at all. In cases where there is concurrent bowel obstruction or rupture, abdominal pain can be more severe.
Colic-Like Symptoms
Because the clinical signs of equine peritonitis overlap significantly with those of colic, misdiagnosis can occur. Signs of colic are especially likely in horses where peritonitis is secondary to a gastrointestinal cause. Mild abdominal pain, reduced gut sounds, and the characteristic “splinting” of the abdomen — where the horse holds its belly stiff to guard against movement — are all consistent with both conditions.
How Equine Peritonitis Is Diagnosed
Blood Work
Blood analysis is one of the first diagnostic steps. Horses with peritonitis often show an abnormal white blood cell count — either elevated or, in cases involving gram-negative bacterial infection, low. A low count results from white blood cells leaving the bloodstream to fight infection in the peritoneal cavity. Serum Amyloid A (SAA), an inflammatory marker, is typically elevated. Plasma fibrinogen concentrations are also frequently increased. Early in the disease, both packed cell volume and serum protein concentration may rise due to rapid fluid loss from plasma, though protein levels will drop as protein-rich fluid shifts into the peritoneal cavity.
Abdominal Ultrasonography
Transabdominal ultrasound is a key diagnostic tool. In Freddie’s case, ultrasound revealed an increased amount of free fluid in the peritoneal cavity, which was an important indicator that something was wrong. No other abnormalities were detected on his ultrasound, which helped narrow the diagnosis.
Abdominocentesis and Peritoneal Fluid Analysis
The definitive diagnostic step is abdominocentesis — the collection of a sample of peritoneal fluid via a sterile needle. The colour, turbidity, and cell count of this fluid provide critical information. A nucleated cell count exceeding 10,000 cells/µl is indicative of inflammation. When degenerative neutrophils with intracellular or extracellular bacteria are the dominant cell population, alongside fever and depression, this strongly suggests bacterial peritonitis.
A cell count above 40,000 cells/µl with healthy neutrophils as the predominant population also points to peritonitis. Counts above 100,000 cells/µl with degenerate neutrophils can appear within just three hours following accidental enterocentesis or surgical exploration. Peritoneal fluid protein levels are also assessed — high protein alongside a high white blood cell count confirms the diagnosis. Fluid samples should be submitted for both aerobic and anaerobic culture to identify the causative organism, though cultures may return negative in long-standing cases due to the dilution of bacteria by the large inflammatory fluid influx.
Treating Equine Peritonitis
Antibiotic Therapy
Antimicrobial therapy is central to treating peritonitis. In horses, the organism recovered most often is Actinobacillus spp., cultured from 21% of submitted samples in the largest published series – in which only 41% of cultures grew anything at all (Odelros et al., Acta Veterinaria Scandinavica 2019;61:18). Escherichia coli was the commonest isolate, at 37%, in a separate series of 55 horses (Nógrádi et al., Acta Veterinaria Hungarica 2011;59:181-193). Figures naming Staphylococcus epidermidis as a common isolate, or putting Bacteroides fragilis in 10% to 20% of culture-positive cases, come from human abdominal and dialysis peritonitis rather than from horses.
Broad-spectrum coverage is the standard approach, particularly in the early stages when multiple organisms may be involved. A typical protocol combines intravenous potassium penicillin with an aminoglycoside such as gentamicin. These are hospital drugs given by a vet rather than anything an owner administers, and the figures are given here so you can follow the reasoning: potassium penicillin at 22,000 IU/kg four times daily is the regimen used in the published equine work on abdominal disease (Sullins et al., Equine Veterinary Journal 2004;36:370-375), and gentamicin at 6.6 mg/kg once daily is the once-daily dose established by pharmacokinetic study in horses (Magdesian et al., Journal of the American Veterinary Medical Association 1998;213:1007-1011), with a later study in horses recovering from colic surgery computing a closely similar 6.8 mg/kg once daily for optimum concentrations against organisms with an MIC of 4.0 µg/ml or below (Tudor, Papich and Redding, Journal of the American Veterinary Medical Association 1999;215:503-506). Oral trimethoprim-sulphadiazine can provide broad-spectrum coverage when used alone or alongside penicillin. For chronic, localised peritonitis, drugs that penetrate fibrin, such as erythromycin and fluoroquinolones, are more effective, as aminoglycosides lose efficacy in the acidic environment of an abscess. Antibiotic therapy should continue for at least seven days after both systemic and peritoneal leucocyte counts have normalised and clinical signs have fully resolved.
Anti-Inflammatory Medication and Pain Management
NSAIDs (non-steroidal anti-inflammatory drugs) play an important supporting role, particularly in cases involving gram-negative organisms, by combating endotoxaemia and enhancing the host’s phagocytic response. Intravenous lidocaine, given as a loading dose of 1.3 mg/kg followed by a constant-rate infusion of 0.05 mg/kg/min, helps counter paralytic ileus and blunts the inflammatory response by reducing chemotactic factors for lysosomes, macrophages, and neutrophils. That is the regimen tested in the placebo-controlled equine trial (Malone et al., Veterinary Surgery 2006;35:60-66), in which 11 of 17 lidocaine-treated horses stopped refluxing within 30 hours against 4 of 15 on placebo, and surviving horses spent less time in hospital. That trial measured ileus rather than analgesia, so lidocaine is better described here as a prokinetic and anti-inflammatory than as a painkiller.
Intravenous Fluid Therapy
Dehydration is a common complication of equine peritonitis. Intravenous fluid therapy is used to correct and maintain hydration. In Freddie’s case, IV fluids were initiated immediately due to his mild dehydration and reluctance to drink. Care must be taken in hypoproteinaemic patients to avoid excessive fluid volumes; in these cases, colloids or plasma should be considered. A balanced polyionic electrolyte solution such as lactated Ringer’s is the standard choice.
Abdominal Lavage
When horses do not respond adequately to conservative treatment, abdominal lavage is indicated. This involves placing a catheter — such as a mushroom or Foley catheter — into the peritoneal cavity and flushing it with a warmed, balanced electrolyte solution to remove bacteria, toxins, inflammatory mediators, and cellular debris. Reported protocols are more modest than the 20 to 40 litres per flush sometimes quoted: a University of Edinburgh case series used 10 litres of warmed compound Ringer’s lactate twice daily for four days (Collins and Pirie, The Australian Equine Veterinarian 2012;30:47-51). Lavage is repeated until cytological improvement is confirmed in repeat peritoneal fluid samples.
Importantly, adding antibiotics or antiseptics (such as povidone-iodine or chlorhexidine) to the lavage fluid has not been shown to improve outcomes over systemic antibiotics alone and, in the case of antiseptics, may cause excessive peritoneal inflammation and even death.
Surgical Intervention
Surgery is necessary when peritonitis results from intestinal rupture, leakage of bowel contents, or vascular compromise of the intestine. Surgical lavage offers direct access to the peritoneal cavity, allowing thorough cleaning and visual confirmation of debris removal. The main disadvantage is that the procedure requires a full celiotomy and cannot easily be repeated. Lavage is significantly less effective in the later stages of peritonitis when bacteria are encased in fibrin. Surgical exploration also carries the risks of disease dissemination through abdominal manipulation and tissue ischaemia from reduced perfusion under general anaesthesia.
Controlling Adhesion Formation
Adhesion formation is a significant complication of peritonitis. Adhesions within the intestinal tract can lead to future bouts of colic. Heparin is sometimes used with the aim of inhibiting adhesion formation, but the controlled equine work does not support it. In a foal model of ischaemia-induced adhesions, heparin at 80 IU/kg given subcutaneously twice daily left adhesions in 3 of 4 foals, against none of the foals given flunixin with antibiotics and none of those given DMSO (Sullins et al., Equine Veterinary Journal 2004;36:370-375). Sodium carboxymethylcellulose, instilled into the abdominal cavity, was tested in the same study and left adhesions in 2 of 4 foals, so it too remains a cautious option pending further research.
Prognosis for Horses with Equine Peritonitis
Prognosis depends heavily on the cause, the severity of clinical signs, and how quickly treatment is started. Horses like Freddie — where a cause is identified early and treatment is initiated promptly — can make a full recovery. Within 24 hours of starting broad-spectrum antibiotics and IV fluids, Freddie’s temperature normalised and his demeanour, appetite, and faecal output all improved. He was discharged after seven days and returned to full work.
Cases secondary to gastrointestinal rupture carry a grave prognosis. Some horses simply do not respond to treatment regardless of how well it is managed, and the cause may never be identified. In these situations, euthanasia is often the kindest and most humane option.
Expect a longer stay than the “around five days” often quoted. Among the 122 survivors in the Swedish series, hospitalisation ran from 4 to 25 days, with a mean and median of 10 days, and the total course of treatment ran from 5 to 30 days (mean 13 days, median 11). Complications can extend this considerably. Treatment costs can be substantial, and horse owners are advised to factor in the potential for equine peritonitis when reviewing their horse health insurance cover.
When to Call the Vet
Early intervention makes a significant difference in outcomes. If your horse seems quieter than usual, is eating less, producing fewer droppings, or has a temperature above 38.5°C, contact your vet the same day rather than giving it 24 hours to settle first. A normal temperature for an adult horse is 37.5 to 38.5°C, so anything above that is a fever and a reason to pick up the phone. Peritonitis can be difficult to distinguish from colic on the basis of symptoms alone, and a proper examination, blood work, and ultrasound are needed to confirm the diagnosis.
The sooner equine peritonitis is identified and treated, the better the horse’s chances of recovery.
Frequently Asked Questions
How do I know if my horse has peritonitis or colic?
Equine peritonitis and colic share many of the same signs — abdominal discomfort, reduced gut sounds, and a horse that is clearly not itself. The key difference is that peritonitis usually brings a fever alongside the depression and reduced appetite. It does not always, though: 83% of the 130 horses in the largest published series were pyrexic on admission, at a mean of 38.6°C (range 37 to 40.8°C), so roughly one horse in six had no fever at all and a normal thermometer reading does not rule peritonitis out. Colic tends to present with more acute, obvious pain. Do not give these signs a day to declare themselves: a temperature above 38.5°C, or colic signs of any kind, is a same-day call to your vet, who can use blood work, ultrasound, and peritoneal fluid analysis to confirm the diagnosis. Half the horses in that series had been showing signs for only one day when they were admitted.
Can equine peritonitis be treated without surgery?
Yes, many cases of equine peritonitis are managed without surgery. Primary peritonitis, where no underlying injury or rupture is present, is typically treated with intravenous antibiotics, anti-inflammatory medication, and fluid therapy. In horses that do not respond to this approach, abdominal lavage using an indwelling catheter can be performed without going to surgery. Surgical intervention is generally reserved for cases where there is intestinal rupture, leakage of bowel contents, or vascular compromise to the gut — situations where conservative treatment alone will not resolve the underlying cause.
What is the survival rate for horses with equine peritonitis?
Better than the “30% to 67% mortality” figure still repeated across the web. Survival to discharge was 94% in the largest published series of 130 idiopathic cases (Odelros et al., 2019), 83% in a series of 72 horses that excluded gut rupture and post-operative cases (Arndt et al., 2021), and 78% in a series of 55 horses (Nógrádi et al., 2011). The much bleaker figures come from a 1985-1990 cohort of 67 horses that did include ruptured intestines and post-surgical cases, in which 40 did not survive (Hawkins et al., Journal of the American Veterinary Medical Association 1993;203:284-288). Horses where the cause is identified early and treatment is started promptly — such as primary bacterial peritonitis — have a significantly better chance of recovery. Cases linked to gastrointestinal rupture carry a grave prognosis. Early veterinary intervention is the single most important factor in improving a horse’s chances of survival.
How long does it take for a horse to recover from peritonitis?
Recovery time varies considerably depending on the severity of the case and whether complications arise. In the largest published series, surviving horses spent a mean and median of 10 days in hospital, with a range of 4 to 25 days, and were treated for 5 to 30 days in total. Freddie, the grey gelding described in this article, was discharged after seven days and made a full recovery, which puts him at the quicker end of that range. Horses with more severe peritonitis, or those that develop complications such as intestinal adhesions, may require a much longer recovery period and ongoing management at home following discharge.
Can equine peritonitis cause long-term complications?
Yes. One of the most significant long-term risks following peritonitis is the formation of intestinal adhesions — bands of fibrous tissue that develop as the abdomen heals. These adhesions can restrict normal gut movement and lead to recurrent bouts of colic, sometimes requiring further treatment or surgery down the line. Adhesion control is part of the treatment plan, but the evidence behind it is thinner than it is usually made to sound, and heparin is not given intravenously for this purpose. In the controlled equine study, heparin given under the skin at 80 IU/kg twice daily still left adhesions in 3 of 4 foals and sodium carboxymethylcellulose left them in 2 of 4, while flunixin with antibiotics, and DMSO, each left none (Sullins et al., Equine Veterinary Journal 2004;36:370-375). Horses that have had peritonitis should be monitored carefully for any signs of digestive discomfort after recovery.
For related conditions and care, read our companion guide to peritonitis in horses, our overview of colic surgery in horses, and learn to monitor the early warning signs with our guide to checking a horse’s temperature, pulse and respiration (TPR).
