Retrobulbar rhabdomyosarcoma in a budgerigar (Melopsittacus undulatus).

This report describes a retrobulbar rhabdomyosarcoma in a 7-year-old male budgerigar (Melopsittacus undulatus). The bird was presented with orbital distension and exophthalmia of the left eye and died during surgery. Necropsy revealed a tan to grey retrobulbar mass compressing all components of the left eye with obscured vision. In histopathological examination, the mass was composed of sheets of spindle-shaped cells with pleomorphic nuclei, numerous bizarre mitotic figures and mononucleated or multinucleated giant cells and also typically strap cells. Neoplastic cells had no cross-striations in sections stained by phosphotungstic acid haematoxylin. The tumour cells did not invade surrounding tissues, including the retina, and there was no metastasis to other organs. Immunohistochemically, tumour cells were positive for desmin, vimentin and alpha-smooth muscle actin, but were negative for S-100 protein and pancytokeratin.