Visceral disseminated hemangiosarcoma causing severe bicytopenia in a mare.

Hemangiosarcoma is an uncommon but highly aggressive malignant vascular neoplasm in horses, most often identified as a disseminated process at postmortem examination. This report describes an adult Warmblood mare presenting with acute hemorrhagic diarrhea, severe anemia, and thrombocytopenia associated with disseminated hemangiosarcoma likely originating in the transverse colon with metastasis to the liver and lungs. This case highlights gastrointestinal hemangiosarcoma as a potential differential diagnosis in horses with unexplained bicytopenia and hemorrhagic enteropathy.