Multiple intra-abdominal serosal myxosarcomas in two koalas (Phascolarctos cinereus)

Two adult koalas (Phascolarctos cinereus) were presented for necropsy examination without any clinical history. A diffuse, severe gelatinous effusion was found in the abdominal cavity, while numerous 1-3 mm diameter, round, well-demarcated, multifocal to coalescing, raised, firm nodules containing a clear gelatinous fluid were scattered on the surface of the parietal peritoneum, diaphragm, liver, gastrointestinal tract and mesentery. Microscopically, the nodular lesions consisted of spindle-shaped to stellate neoplastic cells, with scant eosinophilic cytoplasm and moderate anisocytosis and anisokaryosis. The neoplastic cells were admixed with and surrounded by abundant acellular pale eosinophilic material that was stained by Alcian blue and only weakly by the periodic acid-Schiff reaction. Serosal proliferations are described rarely in koalas and to the authors' knowledge this is the first reported case of serosal myxosarcoma. The significance and pathogenesis of this condition in the koala population is unknown.