Malignant Mesothelioma of the Vaginal Tunic and Peritoneum in a Young Stallion.

This research explores a rare case of mesothelioma, a type of cancer, in a young horse. The cancer originated in the horse’s vaginal tunic and spread to the peritoneum. The study provides new insights into how this type of cancer presents and behaves.

Understanding the Clinical Findings

• The affected horse was a 2-year-old Standardbred stallion showing signs of severe internal bleeding (hemoperitoneum), moderate anemia (a decline in red blood cells), and dependent edema (swelling caused by fluid accumulation).
• Upon examination after death (necropsy), around 40 liters of dark red fluid was found in the horse’s abdomen, and a dark red mass was discovered within and effacing the right pampiniform plexus, an important component of the horse’s reproductive system.
• Additionally, red nodules indicated the presence of cancerous masses on both the parietal (outer) and visceral (inner) surfaces of the horse’s peritoneum, the thin layer of tissue lining the abdomen.

Identifying the Disease

• The cancerous masses comprised malignant spindle cells, which often mark aggressive forms of cancer. These cells had a sarcomatoid appearance, replicating the form of certain bone and muscle cancers.
• The researchers used immunohistochemistry, a technique that uses antibodies to detect specific proteins in tissue samples, to identify the nature of the tumor cells. The cells tested positive for both cytokeratin and vimentin, proteins found in a variety of cell types, and showed no presence of factor VIII-related antigen, ruling out certain other types of cancer.
• The presence of cytokeratin, vimentin and the absence of the antigen factor VIII, along with the location and appearance of the tumors, led to the diagnosis of mesothelioma, a rare form of cancer, often linked with asbestos exposure in humans.

Unveiling Novel Insights

• This case is the first of its kind involving a horse with mesothelioma originating in the vaginal tunic, a membrane surrounding the testes, and subsequently spreading to the peritoneum.
• The research sheds light on potential signals of this form of mesothelioma in horses, such as severe internal bleeding, anemia, and edema.
• Moreover, it contributes to the understanding of the clinical behaviour of mesothelioma that originates from the vaginal tunic, expanding knowledge of how the disease can reveal itself and spread in horses.