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* This is the Veterinary Version. *

Congenital and Inherited Disorders of the Cardiovascular System in Horses

By Davin Borde, DVM, DACVIM, Staff Cardiologist, Veterinary Heart Institute ; Benjamin J. Darien, DVM, MS, DACVIM, Associate Professor of Internal Medicine, School of Veterinary Medicine, University of Wisconsin ; Ase Risberg, VMD, Large Animal Internal Medicine, School of Veterinary Medicine, University of Wisconsin

Congenital abnormalities of the cardiovascular system are defects that are present at birth. They can occur as a result of genetic defects, environmental conditions, infections, poisoning, medication taken by the dam, poor maternal nutrition, or a combination of factors. For several defects, an inherited basis is suspected based on breed and breeding studies. Congenital heart defects are significant not only for the effects they produce but also for their potential to be transmitted to offspring through breeding.

The most common congenital defects in horses are ventricular septal defect, patent ductus arteriosus, tetralogy of Fallot, and tricuspid dysplasia. Arabian horses have a relatively higher rate of congenital defects than other breeds; a variety of defects have been reported for this breed. However, it is important to understand that the overall number of horses with congenital defects is very low.

Detecting Congenital Heart Defects

It is important to detect a congenital heart defect as early as possible. Certain defects can be corrected with surgery, and treatment should be performed before the defect leads to congestive heart failure or irreversible heart damage. If the defect is discovered in a recently purchased horse, you may be able to return it for a refund. Horses with congenital heart defects are likely to die prematurely, causing emotional distress. Animals purchased for performance have limited potential and will likely be unsatisfactory. Early detection also prevents continuing genetic defects into breeding lines.

The evaluation of most animals with a congenital heart defect may include a physical examination, electrocardiography (recording electrical activity of the heart), x-rays, and echocardiography (ultrasonography). These steps allow diagnosis and an assessment of the severity of the defect. Once the diagnosis has been made and the severity determined, treatment options can be developed and a medical opinion given as to the likely outcome of the disease.

Congenital heart defects produce signs that vary depending on the type of heart failure involved and the severity of the defect. Left- or right-sided heart failure may develop. Possible signs include shortness of breath or difficulty breathing, coughing, fatigue, or an accumulation of fluid in the lungs or abdomen.

General Treatment and Outlook

The medical importance of congenital heart disease depends on the particular defect and its severity. Mildly affected horses may show no ill effects and live a normal life span. Defects causing significant circulatory disturbances will likely cause death in newborn (or unborn) foals. Medical or surgical treatments are most likely to benefit horses with congenital heart defects of moderate severity. However, it is very important to understand what level of work or competition the horse will be physically able to carry out following treatment, in order to make the best decision.

Common Congenital Heart Defects

Although a wide variety of congenital defects may occur in horses, few of these occur often enough to warrant concern. The defects discussed below are those that occur with the greatest frequency in horses. However, these defects are still rare.

Ventricular Septal Defects

Ventricular septal defects (openings between the left and right ventricles) vary in both size and effects on blood circulation. Ventricular septal defects may occur with other abnormalities of the heart present at birth. They are the most common congenital defects in horses.

Shunting of blood from the left ventricle into the right ventricle is the most common result of this defect, due to the higher pressures of the left ventricle. Blood shunted into the right ventricle is recirculated through the blood vessels in the lungs and left heart chambers, which causes enlargement of these structures. The right ventricle may enlarge as well. Significant shunting through the pulmonary arteries can induce narrowing of these vessels, leading to reduced blood flow or increased blood pressure. As resistance rises, the shunt may reverse (that is, become a right-to-left shunting of blood).

Signs depend on the severity of the defect and the shunt direction. A small defect usually causes minimal or no signs, and affected horses may be able to engage in moderate levels of physical activity. Larger defects may result in severe left-sided congestive heart failure. The development of a right-to-left shunt is indicated by a bluish tinge, fatigue, and exercise intolerance. Most affected animals have a loud murmur; however, this murmur is absent or faint when a very large defect is present or when shunting is right to left. Chest x‑rays, echocardiography (ultrasonography), and other more specialized techniques may be used to confirm the defect.

Treatment also depends on the severity of signs and direction of the shunt. Horses with small ventricular septal defects do not typically require treatment and the outlook is good. Animals with a moderate to severe defect more commonly develop signs, and treatment should be considered. Horses with a ventricular septal defect should not be bred.

Patent Ductus Arteriosus

The ductus arteriosus is a short, broad vessel in the unborn foal that connects the pulmonary artery with the aorta. It allows most of the blood to flow directly from the right ventricle to the aorta. In an unborn foal, oxygenated blood within the main pulmonary artery is forced into the descending aorta through the ductus arteriosus, bypassing the nonfunctional lungs. In most species, the ductus closes at birth when the animal begins to breathe, allowing blood to flow to the lungs. In foals, however, the complete closure of the ductus may be delayed for up to a week after birth, causing a heart murmur.

If the ductus does not close within the first week, the blood flow is forced from chambers of the left side of the heart to those of the right side; these defects are called left-to-right shunts. They result in overcirculation of the lungs and enlargement of the heart chambers, which may result in arrhythmias. Over time, signs of left-sided congestive heart failure develop.

Tetralogy of Fallot

Tetralogy of Fallot is a defect that produces a bluish tinge to skin and membranes because there is not enough oxygen in the blood. It is caused by a combination of pulmonic stenosis (an obstruction of blood flow from the right ventricle), a ventricular septal defect see Congenital and Inherited Disorders of the Cardiovascular System in Horses : Ventricular Septal Defects, thickening of the muscle fibers of the right ventricle, and varying degrees of the aorta rotating to the right.

The effect of this grouping of defects depends primarily on the severity of the pulmonic stenosis, the size of the ventricular septal defect, and the amount of resistance to blood flow provided by the blood vessels. Consequences may include reduced blood flow to the lungs (resulting in fatigue and shortness of breath) and generalized lack of oxygen in the blood causing a bluish tinge to skin and membranes. Red blood cells may be abnormally increased, leading to the development of blood clots and poor circulation of blood.

Electrocardiographs, x-rays, and echocardiography (ultrasonography) can help confirm the diagnosis. Treatment options include surgery and medical management, but the outlook is guarded to poor.

Tricuspid Dysplasia (Atresia)

Tricuspid dysplasia refers to abnormal development or malformation of the tricuspid valve of the heart, allowing regurgitation of blood back into the right atrium. This defect is seen occasionally in horses at birth. Arabian horses are more likely to have tricuspid dysplasia, suggesting that there may be a genetic basis for the defect in some cases.

Longterm tricuspid regurgitation leads to volume overload of the right heart, enlarging the right ventricle and atrium. Blood flow to the lungs may be decreased, leading to fatigue and an increased rate of respiration. As the pressure in the right atrium increases, blood pools in the veins returning to the heart, causing an accumulation of fluid in the abdomen.

The more severe the defect, the more obvious the signs will be in affected horses. Signs of right-sided congestive heart failure, such as accumulation of fluid in the abdomen and lungs, may be seen. A loud heart murmur is very noticeable. Arrhythmias, especially the sudden onset of a very high heart rate, are common and may cause death. In a severe form, called tricuspid atresia, the entire valve may be undeveloped or absent.

Electrocardiography and x-rays may show enlargement of the right ventricle and atrium, while the malformed tricuspid valve can sometimes be seen using echocardiography (ultrasonography). The outlook for horses with these signs is guarded, although mild defects may pose few problems.

* This is the Veterinary Version. *