Peritonitis should be considered in horses that show a fever, signs of colic and lethargy, according to researchers.
Peritonitis is inflammation of the peritoneum, the tissue that lines the inner wall of the abdomen and covers and supports most of the abdominal organs.
In horses, peritonitis is most often seen as a secondary complication to injuries involving the abdominal cavity, rupture of the bowel or abdominal surgery.
Left untreated, it can lead to blood poisoning, organ failure and death.
It is often associated with prolonged antibiotic treatment and there is a guarded prognosis for survival.
However, not all cases of peritonitis in horses have a known underlying cause.
Swedish researchers say idiopathic peritonitis – that is, peritonitis of unknown cause – is commonly identified in their country.
In contrast to peritonitis linked to traumatic incidents, affected horses appear to respond well to medical treatment.
Emma Odelros and her colleagues set out to describe the clinical signs, laboratory findings, bacterial culture results, treatment programs and survival rates for horses diagnosed with idiopathic peritonitis.
The study team, whose findings are reported in the journal Acta Veterinaria Scandinavica, got the medical records of 130 horses diagnosed with idiopathic peritonitis. Diagnosis was based on abnormal peritoneal fluid and other tests.
Eighty-three percent of the horses had presented with a fever, 80% were lethargic, 68% had lost their appetite and 51% were showing signs of abdominal pain.
Microbial cultures were performed in 84% of the cases, of which 41% were positive – a result not too far removed from similar testing in humans with septic peritonitis.
The most commonly recovered bacteria were species of Actinobacillus, cultured from 21% of the submitted samples.
All horses received antimicrobial therapy.
In the middle of the study period, a new treatment protocol for idiopathic peritonitis was implemented at the contributing hospitals, using only penicillin.
If the horse showed signs of high levels of endotoxins in the blood, or did not respond to treatment within 24 hours, the treatment regimen was extended to include gentamicin or trimethoprim-sulphadiazine.
Treatment with penicillin as the only antibiotic was used in 36% of the horses, with 91% of them responding very well.
In the end, 43 were treated with penicillin alone, 76 with a combination of penicillin and gentamicin, 5 with penicillin and trimethoprim-sulphadiazine, 4 with penicillin, gentamicin and trimethoprim-sulphadiazine, and 2 with a combination of penicillin, gentamicin and metronidazole.
Survival until discharge was 94%. Long-term follow-up was available for 93 of the 122 horses discharged from hospital. It was found that 90 of them – 97% – had survived more than 12 months after discharge.
The discharged horses did not seem to be at increased risk of suffering from future colic episodes, compared to horses previously treated medically for various other colic types.
Discussing their findings, the authors say the way bacteria manage to infect the peritoneum in such cases remains unknown, but the range of bacteria identified in cultures pointed to the gut as being the likely source.
They suspect the mucosal barrier in the intestine is likely to be temporarily compromised to some extent, allowing temporary translocation of bacteria. Another possibility is parasite migration.
Peritonitis has also been diagnosed in horses subsequently found to have underlying non-strangulating intestinal infarctions caused by the parasite Strongylus vulgaris.
“Idiopathic peritonitis is a disease that should be considered in horses presented with fever, signs of colic and lethargy,” they concluded.
Examination of the peritoneal fluid can confirm the diagnosis and a bacterial culture is helpful if the horse is not responding to initial treatment.
“Medical treatment of idiopathic cases in this study was successful and, in Sweden, most infections respond to treatment with penicillin as the only antimicrobial.”
The study team comprised Odelros and Anna Kendall, both from Mälaren Equine Clinic; and Ylva Hedberg-Alm and John Pringle, both with the Swedish University of Agricultural Sciences.
Idiopathic peritonitis in horses: a retrospective study of 130 cases in Sweden (2002–2017)
Emma Odelros, Anna Kendall, Ylva Hedberg-Alm and John Pringle
Acta Veterinaria Scandinavica 2019 61:18 https://doi.org/10.1186/s13028-019-0456-2