Anomalous Left Atrial Drainage of the Vena Cava in an Adult French Bulldog

•Aberrant insertion of both vena cava is previously unreported in dogs.•The dog presented with severe polycythemia and severe exercise intolerance.•It is caused by flow abnormalities in-utero associated with a sinus venosus ASD.•A systemic to pulmonary shunt provided clinical benefit and normalisation of PCV.