What is your diagnosis? Muculent pleural effusion from a dog

: Pleural effusion was examined from a 5‐year‐old, female Brittany Spaniel with a 7‐day history of dyspnea, anorexia, and diarrhea. The fluid was yellow, cloudy, and slightly gelatinous, and had a total protein concentration of 2.8 g/dL, a total nucleated cell concentration of 1.1 × 103/μL, and a triglyceride concentration of 177 mg/dL. A cytocentrifuged preparation contained a mixed inflammatory cell population with a predominance of small lymphocytes and abundant mucinous material in the background. The dog died 3 days later and a mass was found within the lumen and wall of the right auricle of the heart at necropsy. Histopathologic sections of the mass contained a population of anaplastic spindle cells diffusely suspended in a pale basophilic matrix, consistent with myxosarcoma. The cells were positive for vimentin and negative for cytokeratin, desmin, and von Willebrand factor VIII‐related antigen. A myxoid matrix was confirmed by positive staining with Alcian blue. Myxosarcoma is a rare cardiac tumor in dogs that should be considered, along with mucus‐producing carcinomas and bile, as a cause of muculent effusion.