Mediastinal lymphoma with complete atrioventricular block in a horse.

A neutered-male pony showed symptoms of severe weight loss which was later found to be due to an uncommon case of mediastinal lymphoma, a type of cancer which led to a complete function failure (block) in the unit responsible for regulating cardiac rhythms (the atrioventricular node).

Case presentation

• A fifteen-year-old neutered-male pony had been experiencing weight loss for a period of four months.
• Upon the clinical evaluation, the pony was discovered to possess a considerably decreased heart rate, or severe bradycardia, and a complete blockage of signals from the atrium to the ventricle of the heart, a condition known as complete atrioventricular block.

Post-mortem findings

• At necropsy, a mass with multiple lobes was found in the anterior mediastinum, the central part of the chest.
• Additionally, an abnormal increase in size or moderate enlargement of the superficial cervical lymph nodes, the glands located in the neck region, was observed.
• Two crucial body structures, the vagus nerve (which indicates how the body should respond in calm or restful situations) and the brachiocephalic trunk (a major artery supplying blood to the head and neck), were found to be embedded in the anterior mediastinal tumor.

Microscopic Examination and Diagnosis

• Upon histological examination, meaning the process of investigating the tissue under a microscope, the mass was found to be composed of sheets of abnormal, cancerous lymphocytes – a type of white blood cell that forms part of the body’s immune system. These were expressing CD3, a marker that is used to identify a certain class of cells known as T cells.
• The tumor had a low mitotic rate, meaning its cells were dividing at a slower rate than normal.
• This confirmed the mass to be a lymphoma – a group of blood cancers that develop from lymphocytes – positioned within the mediastinum, making it a mediastinal lymphoma.

Significance of findings

• This is the first recognized and reported case where a mediastinal lymphoma is associated with a complete atrioventricular block in horses, thereby adding to the limited extent of information and literature in this area of pathology.