Bilateral testicle teratoma in an equine fetus: A rare case.

Fetal loss in mares can result from a range of causes, including infectious agents or noninfectious conditions such as neoplasia, which may originate from either the fetus or the dam. Among neoplastic conditions, teratomas are rarely reported and, to date, have not been described in the equine fetal testis. Teratomas arise from the partial differentiation of pluripotent germ cells and may occur in young and adult horses, typically in cryptorchid testes. Their gross and histopathological features vary widely between cases. This report describes a rare case of bilateral fetal testicular teratoma in a 310-day pregnant mare, which resulted in dystocia due to fetal abdominal distension. The diagnosis was based on gross and histopathological findings, which revealed small-caliber blood vessels, cystic structures, and a variety of differentiated tissues in both testes, including bone, cartilage, and skin with hair follicles, connective tissue, and adipose tissue.