Intrahepatic mucinous cholangiocarcinoma with recurrent colic in a horse: case report and literature review of cholangiocarcinoma in horses.

This study discusses a case of a 17-year-old Arabian mare diagnosed with a rare variant of cholangiocarcinoma, a type of cancer seldom observed in horses. The cancer was accompanied by long-term mild recurrent colic and other abnormal biochemical parameters. Postmortem examination revealed the presence of large cysts filled with mucin, leading to the classification of this cholangiocarcinoma as a mucinous variant.

Summary of the Case

• The horse in this study was a 17-year-old Arabian mare, brought in due to a long history of intermittent mild recurrent colic that typically responded well to medical treatment.
• Clinical findings included mild lymphopenia, increased gamma-glutamyl transferase and creatine kinase activities, and abnormal levels of certain biomarkers in the blood including elevated iron and glucose levels, and reduced magnesium and potassium concentrations. The results from abdominocentesis were compatible with low-protein transudate, a fluid often associated with conditions causing fluid leakage from blood vessels.

Postmortem Examination and Diagnostics

• Given the continuous and prolonged clinic signs, the owner of the horse chose euthanasia. After death, a postmortem examination was conducted, and histopathology revealed the presence of a bile duct cancer, known as cholangiocarcinoma.
• The cancer cells were arranged in large cysts filled with a gel-like substance known as mucin. Mucin accounted for 90% of the tumor’s volume, leading to the diagnosis of a rare variant of cholangiocarcinoma: mucinous cholangiocarcinoma.
• To confirm the bile duct origin of the cancer cells, immunolabeling tests were performed on the cells. The cancer cells showed strong cytoplasmic immunolabeling for cytokeratin 19, a protein typically found in bile duct cells, and lacked immunolabeling for hepatocyte paraffin 1, suggesting that they did not originate from liver cells. This further affirmed their classification as a cholangiocarcinoma.

Significance of the Case

• Cholangiocarcinomas are uncommon tumors in horses and often present with non-specific and slowly progressing clinical signs, such as recurrent colic, as observed in this case.
• The mucinous variant of cholangiocarcinoma is even rarer, with few reportings in veterinary medicine and no prior documented cases in horses, to the authors’ knowledge.