Ovarian hemorrhage as the primary cause of hemoperitoneum in cycling and pregnant Miniature Horse mares.

Objective: To determine whether Miniature Horse mares are predisposed to hemoperitoneum from nonneoplastic ovarian hemorrhage and report the clinical characteristics of this disorder.

Animals: 51 equines with hemoperitoneum, including 22 nonminiature mares and 9 Miniature Horse mares, identified by searching medical records of a tertiary large animal hospital for confirmed hemoperitoneum cases between 2012 and 2023.

Clinical Presentation: Signs of hemoperitoneum in miniature mares included lethargy, inappetence, colic, tachycardia, and moderate to severe anemia (PCV ranging from 11% to 19%). Five miniature mares did not survive and were necropsied.

Results: Miniature Horse mares were markedly overrepresented at 29% of hemoperitoneum mares, and no miniature stallions or geldings with hemoperitoneum were identified. In nonminiature mares, hemoperitoneum was attributed to intestinal strangulation (n = 4), suspected or confirmed neoplasia (11), peripartum uterine artery rupture (2), splenic trauma (3), ovarian hematoma (1), and spontaneous broad ligament hematoma (1). In miniature mares, ruptured corpus hemorrhagicum (n = 4) and ovarian hemorrhage (1) were confirmed at necropsy. The other 4 miniature mares survived with no definitive cause of hemoperitoneum identified on extensive diagnostics; 1 was treated for hemoperitoneum again 6 years later, at which time a corpus hemorrhagicum was suspected on ovarian ultrasound. Two miniature mares in first-trimester gestation during treatment for hemoperitoneum subsequently produced normal foals.

Clinical Relevance: Ovarian hemorrhage is an uncommon cause of hemoperitoneum in equines; however, it should be considered a likely differential diagnosis for hemoperitoneum in miniature mares, including during pregnancy. Reproductive tract assessment should be performed in equine hemoperitoneum cases, particularly for miniature mares.