Hydroallantois in a mare associated with schistosomus and unilateral ovarian agenesis in the fetus.

Hydropsical conditions in domestic animals are characterized by abnormal fluid accumulation. While their complete pathophysiology remains unclear, factors such as changes in chorioallantoic membranes, placentitis, or fetal abnormalities may contribute. Hydrops present as a medical emergency that can cause complications for the mare. Early diagnosis and treatment are crucial for the mare's prognosis, as fetal survival is often poor. This report describes a case of hydroallantois in a 9-year-old pregnant Quarter mare at 271 days of gestation, leading to dystocia of a non-viable foal with multiple congenital abnormalities. Given the poor prognosis for fetal survival, transcervical gradual fluid drainage and pregnancy termination were performed. The malformed fetus was subsequently delivered and humanely euthanized. Necropsy findings revealed a schistosomus fetus with palatoschisis, a wry nose, prognathism, and unilateral ovarian agenesia. The case underscores the importance of timely intervention in managing hydroallantois. The report contributes to understanding horses' clinical presentation, diagnosis, and management of hydroallantois.