Thymic teratoma in a chicken and a literature review of gonadal and extragonadal teratomas in veterinary species.

A 3-y-old rooster was submitted for postmortem evaluation following a 4-mo history of progressive ulcerative dermatitis affecting the right hock. Autopsy revealed 6 bilateral, discrete globoid masses along the caudoventral cervical musculature and extending into the proximal celomic cavity. Histologically, a neoplasm effaced and/or compressed residual thymic parenchyma and consisted of ill-defined aggregates of 2-3 primordial germ cell lineages (ectoderm, mesoderm, endoderm). Immunohistochemical and histochemical stains, including desmin, glial fibrillary acidic protein, pancytokeratin, and phosphotungstic acid hematoxylin, confirmed the presence of heterogeneous germ cell tissues, consistent with an extragonadal teratoma. Extragonadal teratomas are rare in non-human species, particularly in avian species, and reports of mediastinal involvement are limited. In our case, the teratoma was an incidental finding; the primary reason for submission was progressive necroulcerative bacterial dermatitis affecting the right leg.