Malignant Mesothelioma of the Vaginal Tunic and Peritoneum in a Young Stallion.
Authors: MacMillan Kathleen M, Martinson Shannon A, Smith Melissa, Lofstedt Jeanne
Journal: Journal of equine veterinary science
Summary
Malignant mesothelioma remains exceptionally rare in equine medicine, making this first documented case of vaginal tunic-origin mesothelioma with peritoneal metastasis in a 2-year-old Standardbred stallion a significant clinical contribution. The stallion presented with marked hemoperitoneum, moderate anaemia and dependent oedema; post-mortem examination revealed approximately 40 litres of serosanguineous fluid, tumour infiltration of the right pampiniform plexus, and multifocal nodular masses across parietal and visceral peritoneal surfaces, with histopathological and immunohistochemical findings (cytokeratin and vimentin positive, factor VIII-related antigen negative) confirming mesothelioma diagnosis. This case expands the limited understanding of how mesothelioma behaves when arising from the vaginal tunic rather than other peritoneal surfaces, revealing an aggressive clinical course characterised by rapid development of severe abdominal effusion and widespread metastatic disease. For practitioners, recognition that mesothelioma can originate from the tunics of the testicular cord is clinically important when evaluating young stallions presenting with unexplained hemoperitoneum and abdominal distension, as the non-specific signs may delay diagnosis until disease is advanced. Further documentation of similar cases would help establish diagnostic protocols and inform prognostic expectations for this otherwise poorly understood malignancy in horses.
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Practical Takeaways
- •Mesothelioma should be considered in the differential diagnosis for young stallions presenting with hemoperitoneum, anemia, and abdominal distension, though it remains rare
- •Immunohistochemistry panels (cytokeratin, vimentin, factor VIII-related antigen) are useful for confirming mesothelioma diagnosis in equine cases
- •Vaginal tunic neoplasia can metastasize to peritoneal surfaces; necropsy and histopathology are essential for definitive diagnosis and understanding disease progression
Key Findings
- •A 2-year-old Standardbred stallion presented with marked hemoperitoneum (approximately 40 L of dark red fluid), moderate anemia, and moderate dependent edema due to mesothelioma of the vaginal tunic
- •Histopathology revealed malignant spindle cells with sarcomatoid appearance that were cytokeratin and vimentin positive and factor VIII-related antigen negative
- •This is the first documented case of mesothelioma originating from the vaginal tunic in a horse with subsequent spread to the peritoneum