Understanding idiopathic peritonitis

Inflammation of the peritoneum is a medical condition occasionally seen in horses

Peritonitis (inflammation of the peritoneum) is a medical condition occasionally seen in horses. It is most commonly caused by a bacterial infection following events such as rupture of an abdominal organ, trauma to the peritoneal cavity with secondary infection, or abdominal surgery as a secondary complication. Treatment usually involves long courses of broad-spectrum antibiotics or antibacterial agents, and the outcomes are often unrewarding.

Perhaps less well-known is that peritonitis can be seen in horses where there is no obvious underlying cause (idiopathic peritonitis). A recent retrospective study from Sweden documented 130 cases of idiopathic peritonitis and described its clinical signs, laboratory findings, results of bacterial culture, treatment protocols, and survival rates.

Idiopathic peritonitis has not been described previously as a separate condition. Instead, cases were included in other reports on peritonitis. The following have been postulated as causes of idiopathic peritonitis:

  • parasite migration or foreign bodies, with associated leakage of microorganisms from the gastrointestinal tract;
  • ulcerations of the viscous caused by treatment with non-steroidal anti-inflammatory medications; and
  • nonstrangulating infarctions of the intestines.

Other cases have been associated with infection by Actinobacillus equuli. In general, better outcomes have been associated with idiopathic peritonitis than with peritonitis associated with other causes.

Clinical signs

The final study group consisted of 130 horses. Of these, 54 percent were geldings, 37 percent were mares, and nine percent were stallions. The age of the studied horses ranged from six months to 30 years, with a mean and median of 11 and 10 years, respectively. A variety of breeds were represented, including warmbloods, Icelandic horses, and ponies.

The primary presenting clinical signs were lethargy, fever, anorexia, and abdominal pain (colic). Clinical signs were usually acute, and there was no previous history of illness. Horses were typically referred for evaluation and treatment within a day of the onset of clinical signs. Almost half the horses in the study had large colon impactions (diagnosed by rectal examination). While it has been suggested large colon impactions are secondary complications to dehydration and fever that may occur with the condition, it also is possible impactions compromise the intestinal mucosa and cause secondary leakage. As such, the combination of fever and large intestinal impaction should spur the attending clinician to look deeper, as most uncomplicated cases of colic do not also have pyrexia.

Peripheral blood tests were often normal in affected horses, including complete blood counts and plasma fibrinogen; however, serum amyloid A (SAA) was often markedly elevated. The most reliable diagnostic tool for idiopathic peritonitis was abdominocentesis. Thus, in horses presenting with the vague signs described previously, abdominocentesis should be considered as a potentially valuable diagnostic tool.

Analysis of abdominal fluid is normally diagnostic in cases of idiopathic peritonitis. The peritoneal fluid of affected horses is usually turbid, although there may not be a change in color. Nucleated cell counts are typically elevated (> 10 × 109 cells/L), as is total protein (> 25 g/L).

While useful in helping to determine antimicrobial choices, microbial culture was positive in only 41 percent of the cases. Of those positive cultures, Actinobacillus spp. was cultured in 21 percent of the cases. The study’s results suggested the number of positive cultures might have been higher if blood-culture media had been used routinely.


All horses identified with idiopathic peritonitis received antimicrobial therapy. Many of the treated horses received a combination of penicillin and gentamycin. The survival of affected horses until discharge was 94 percent.

Interestingly, in the middle of the study, attending veterinarians adopted a new treatment protocol. Instead of combination therapy, horses were treated with penicillin alone. Based on the apparent insensitivity of the fibrinogen assay, SSA levels were monitored to see when treatment could be discontinued. This protocol reduced the use of broad-spectrum antimicrobial agents. It also reduced the length of time horses stayed in the hospital, since they could be discharged when SAA levels fell and treated at home with penicillin by their owners.

Caveats are in order, however. The use of the SAA test was not standardized among clinicians. Some followed horses with SAA alone, while others obtained confirming peritoneal samples. Since SAA was not monitored according to a standardized protocol, the study was unable to provide treatment recommendations based on specific SAA levels. Further, as horses were discharged before treatment was stopped, the total number of treatment days was not determined for all those in the study.

Long-term follow up

The survival rate of affected horses was excellent. Ninety-seven percent of horses diagnosed with idiopathic peritonitis survived for more than one year after discharge, which is higher than survival rates reported in studies where idiopathic cases were evaluated concomitant with cases of known origin of the condition. (In such studies, survival rates of 58 to 84 percent have been reported previously.)

The recurrence rate also was very low. Only one horse reported a second case of idiopathic peritonitis after discharge. Recurrent colic was seen in 17 percent of the cases discharged; however, this rate is lower than has been reported for recurrence rates in horses treated medically for other types of colic.


Medical treatment of idiopathic peritonitis is generally successful; in studied horses, penicillin was often the only antibiotic agent given. Finally, horses with idiopathic peritonitis tend to survive with treatment and do not typically suffer long-term adverse effects.

David W. Ramey, DVM, is a Southern California equine practitioner who limits his practice to the care of performance and pleasure horses. Visit his website at doctorramey.com. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.


See bit.ly/2MGxIZP to read the full text of the article, Oldedros, E, et al. Idiopathic peritonitis in horses:
A retrospective study of 130 cases in Sweden (2002 – 2017). Acta Vet Scand, 2019 Apr 25; 61(1):18.

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