Clinical Features and Treatment Response to Differentiate Idiopathic Peritonitis From Non-strangulating Intestinal Infarction Associated With Strongylus Vulgaris Infection in the Horse

Abstract Background: Peritonitis in horses secondary to non-strangulating infarction (NSII) has a grave prognosis, even after intestinal resection. In contrast, horses with idiopathic peritonitis respond well to medical treatment. Affected horses in both cases often show signs of both colic and systemic inflammation, but early diagnosis is crucial for optimal treatment and an accurate prognosis. One cause of NSII is thrombus formation secondary to Strongylus vulgaris larval migration. There has been a documented increase in S. vulgaris prevalence in Sweden since the implementation of selective anthelmintic treatment in 2007, which subsequently could result in a rise in NSII cases. In a retrospective clinical study, medical records from cases diagnosed with NSII or idiopathic peritonitis from three equine referral hospitals in Sweden during 2017-2020 were reviewed. Information including demographic data, relevant medical history, and clinical- and laboratory parameters were obtained from patient records. To facilitate the differentiation between cases of idiopathic peritonitis and cases with confirmed NSII, the aim of the study was to compare clinical and laboratory parameters, clinical progression and initial response to antimicrobial treatment as well as survival-rates.Results: Horses with NSII (n = 20) were significantly more likely to present during the winter months with a poorer response to medical treatment within 48 hours. Cases of idiopathic peritonitis (n = 115) had a 99.1 % survival rate with medical treatment. In comparison, all confirmed NSII cases were non-responsive to antimicrobial treatment and had a low survival rate (25%) with surgical treatment. Specific rectal findings and peripheral blood neutropenia were strongly associated with NSII. Conclusions: In Sweden, idiopathic peritonitis cases still predominate over S. vulgaris associated NSII cases and have an excellent survival rate with antimicrobial treatment. However, horses presenting with septic peritonitis during the winter months with a palpable rectal mass and continuing fever and colic signs beyond 48 hours of medical treatment are likely to suffer from NSII and should be considered for abdominal surgery.