Successful intra-abdominal resection of a 24 kg ovarian granulosa cell tumour in a Warmblood mare

Background: Granulosa cell tumours (GCT) are the most common ovarian tumours in mares. The classical presentation of a GCT is a unilaterally enlarged ovary appearing as a multicystic honeycomb mass. In rare cases, GCTs cause haemoperitoneum as a result of rapid growth of the tumour. The clinical diagnosis of GCT is usually based on history, rectal examination, ultrasonographic examination and serum hormone analysis, and surgical removal of the affected ovary is the treatment of choice. The different surgical approaches are based on the dimension of the GCT. Case description: A 7-year-old mare was referred to the department for horses due to suspicion of a large colon impaction. The mare presented with clinical signs of colic, fever and signs of hypovolemic shock. Rectal and ultrasonographic examination showed haemoperitoneum and a honey-comb mass within the abdomen, and a granulosa cell tumour (GCT) as the cause of an acute haemoperitoneum was diagnosed based on the serum level of Anti-Müllerian hormone. After stabilization of the mare, the GCT was removed through a ventral midline incision. Because of the enormous dimensions of the GCT, intra-abdominal partial resection of the tumour using a tenotomy knife was necessary to exteriorize the ovarian pedicle. At 3 months follow-up, the mare was ridden for her intended use. Conclusion: This report provides an approach to an uncommon case of a very large and heavy GCT.