Complete transposition of the great arteries with ventricular septal defect and pulmonary stenosis in a Thoroughbred foal.

This research discusses the case of a Thoroughbred foal with complete transposition of great arteries, ventricular septal defect and pulmonary stenosis. This is a rare and complex congenital heart defect, and this study provides a detailed examination of the foal’s physical, clinical, radiographic and pathological manifestations of the condition.

Clinical Presentation and Laboratory Findings

• The Thoroughbred foal was seven months old, female and suffered from a significant heart murmur and respiratory tract infection upon evaluation.
• The foal presented with severe depression, reluctance to movement, an elevated body temperature of 413°C, heart rate of 76/min and a complex systolic heart murmur. Mucous membranes were cyanotic indicating poor oxygenation and petechial hemorrhages were present on the oral mucous membranes.
• Lab tests indicated polycythaemia (abnormally high concentration of hemoglobin in the blood through an increase in red blood cell numbers), neutrophilic leucocytosis (increase in neutrophils, a type of white blood cell), high fibrinogen levels (clotting factor), and metabolic acidosis (excessive acidity in the body fluids).

Radiographic and Anatomic Findings

• An electrocardiogram revealed sinus tachycardia, a rapid heartbeat resulting from an abnormally high rate of impulses in the sinus node, along with the observation of other abnormal electrical patterns in the heart.
• X-rays revealed prominent interstitial infiltrates in the lungs with increased size of pulmonary vessels and an enlarged heart.
• Cardiac catheterisation showed abnormalities in ventricular connections and contrast material passage, suggesting a complex heart defect structure.
• Necropsy findings revealed abnormalities such as dilated right atrium, thickened ventricular wall, enlarged right ventricle, and normal mitral valve among others.

Microsopic and Necropsy Findings

• At a molecular level the heart had a thick covering of fibrin consisting variable neutrophils and considerable detritic debris. The right ventricular muscle fibers were hypertrophied and vessels had irregular walls with varying structures.
• Small abscesses were found in the kidney interstitium and small widely scattered foci of lymphocytic infiltration were observed, suggesting septic embolism or septicaemia.

Discussion and Implications

• The authors discuss that this case falls into the category of complete transposition of great arteries, a complex condition where the oxygen-rich and oxygen-poor blood do not mix properly.
• Various clinical manifestations depend on the level of intercirculatory mixing and can include symptoms such as cyanosis, polycythaemia, stunted growth and chronic hypoxia.
• The pulmonary stenosis in the foal seems to have protected the pulmonic bed from developing severe vascular disease by decreasing blood flow thus allowing better intercirculatory mixing.
• The complications of such cases can include a potential increase in pulmonary stenosis or the natural closure of the communication between the two circulations leading to aggravated clinical signs.