Capsule Reports

An 8-year-old freshwater common musk turtle (Sternotherus odoratus) presented with a solid, reddened, nonmobile mass on the right forelimb that was reported to develop one week prior to presentation. The 6 3 4 3 3.5 cm mass was surgically excised and found to have a thin membranous lining. Histopathology revealed a completely excised mass with some infiltrative growth and up to 18 mitotic figures in 10 high-power fields. Immunohistochemistry was used to confirm a diagnosis of rhabdomyosarcoma using the criteria developed for humans and dogs. This mass was not positive for a-smooth muscle actin, which supports the diagnosis of rhabdomyosarcoma, but failed to show presence of one or more skeletal muscle specific markers. Transmission electron microscopy was used to demonstrate the presence of myofilament tangles and attenuated Z-lines, which helped affirm a diagnosis of rhabdomyosarcoma. These findings, along with positive staining on Azan stain and indistinct cytoplasmic cross-striations on Masson trichrome staining, added to the diagnosis. This is the first known reported case of rhabdomyosarcoma in a freshwater turtle.