Malignant ovarian teratoma in a Lusitano filly: Diagnostic challenges and clinical significance.

An 8-month-old Lusitano filly (a young female horse) was referred with a five-day history of abdominal distension, respiratory distress, and hyporexia. Clinical and ultrasonographic examinations revealed ventral edema, anemia, leukocytosis, hypoalbuminemia, and bilateral pleural effusion. The filly subsequently developed progressive clinical deterioration, including persistent hyporexia, scant fecal output, and acute colic with nasogastric reflux, followed by sudden respiratory decompensation, collapse, and death. Post-mortem examination revealed a large, multilobulated abdominal mass involving the mesentery and the broad ligament of the uterus, along with markedly enlarged mediastinal lymph nodes and pleural involvement. Histopathology revealed a heterogeneous ovarian neoplasm composed of well-differentiated tissues derived from all three embryonic germ layers, intermixed with extensive areas of highly anaplastic germ cells exhibiting a high mitotic activity. A diagnosis of malignant ovarian teratoma with metastatic involvement of mediastinal lymph nodes and extensive transcoelomic dissemination was established. This case highlights the highly aggressive biological behavior and the diagnostic challenges associated with malignant ovarian teratomas in young equids.