Acquired Pulmonic Stenosis Due to a Compressing Intrathoracic Abscess in a Thoroughbred Yearling

• Pulmonic stenosis can be acquired in the horse.
• This is a case of extrinsic pulmonic stenosis due to a cranial thoracic abscess.
• Streptococcus equi spp. zooepidemicus infection can lead to important complications seen in this case including large thoracic abscesses.

A 12-month-old Thoroughbred filly was admitted for poor weight gain, intermittent fever, and distal limb edema. Initial clinical examination revealed tachycardia, tachypnea, and pyrexia. A loud holosystolic murmur was identified by cardiac auscultation, and thoracic auscultation identified bilateral diffuse crackles. Echocardiography demonstrated severe extrinsic pulmonic stenosis, enlargement of the right ventricle and compression of the left ventricle. Mediastinal ultrasound revealed a large, encapsulated, cavernous mass in the cranial thorax, compressing the heart. Bacterial culture of tracheal aspirates and aspirations of the intrathoracic mass yielded a pure culture of Streptococcus equi spp. zooepidemicus. Acquired pulmonic stenosis has not previously been reported in horses, whereas congenital pulmonic stenosis has. This case report demonstrates that in line with human medicine, pulmonic stenosis can be acquired because of thoracic masses compressing the pulmonic artery.