Mitral regurgitation and decompensated heart failure in a young pregnant pony mare: An aetiological challenge

Summary A 4‐year‐old, 6‐month pregnant, Welsh pony mare presented with severe acute clinical signs including depression, anorexia, pale mucous membranes, tachycardia with a grade 4/6 holosystolic murmur and moderate expiratory dyspnoea with diffuse wheezes. There was no evidence of placentitis or fetal distress. Electrocardiography showed sinus tachycardia. Echocardiography revealed atrial and ventricular dilatation, pulmonary artery enlargement, irregular hyperechoic thickening of the mitral valve (MV) leaflets, with moderate to severe regurgitation and severely impaired systolic function. Thoracic radiographs were consistent with pulmonary oedema. Congenital MV dysplasia or early degenerative valve disease was suspected. Left heart failure did not respond to emergency treatment, and the pony developed severe jugular venous distension and pulsation. Due to the poor prognosis, euthanasia was elected. Post‐mortem examination revealed thickening of the mitral leaflets, predominantly the septal cusp, and severe four‐chamber dilatation, with no other cardiac lesions. Histological examination showed myxoid valvular degeneration but did not allow differentiation between congenital dysplasia and early degenerative disease. In this case, physiological effects of gestation may have exacerbated the deleterious haemodynamic effects on mitral insufficiency.