Mitral Valve Regurgitation

What is mitral valve regurgitation?

Heart murmurs are caused by fast and/or turbulent blood flow. Mitral valve regurgitation, a relatively common condition in horses, is a leak at the valve made of flexible tissue flaps or leaflets that form a seal between the left ventricle and atrium. (This valve that separates the left atrium and ventricle is called the mitral valve on the and the valve that separates the right atrium and ventricle is called the tricuspid valve.) Since the valve does not close properly, blood flows back into the chamber (atrium) that it came from (regurgitation). In severe cases, this can lead to an increase in chamber pressure and enlargement of the heart. Enlargement of the left atrium increases the risk of developing an irregular heart rhythm known as atrial fibrillation. 

Mitral valve regurgitation is a common cardiac finding in horses. Mitral valve regurgitation can be caused by malformation of the valve, age-related changes of the valve’s connective tissue, as a result of physical trauma during intensive training, due to scarring from infection of the valve, or a combination of these factors.

What are the clinical signs of mitral valve regurgitation?

Horses with mild or moderate mitral valve regurgitation may not exhibit any significant clinical signs. In more severe cases, clinical signs of mitral valve regurgitation can include a cardiac murmur, exercise intolerance, reduced performance and in some cases congestive heart failure, which can lead to increased heart rate, trouble breathing, coughing, and abnormal heart rhythms.

How is mitral valve regurgitation diagnosed?

Diagnosis of mitral valve regurgitation is based on a physical examination and ultrasound of the heart known as echocardiography. Additional tests such as exercise testing, bloodwork, and repeated imaging over time may be helpful in evaluating safety of exercise and the progression of the disease. The veterinarian may identify a heart murmur after listening to the horse’s heart with a stethoscope.

How is mitral valve regurgitation treated?

Horses that do not exhibit clinical signs (asymptomatic) are usually not treated for mitral valve regurgitation. Medication and regular monitoring by echocardiogram may be necessary for more severe cases.

What is the prognosis for mitral valve regurgitation?

The prognosis for mitral valve regurgitation is generally excellent for mild cases. They are often able to exercise and have a normal life expectancy. Moderate to severe cases can result in heart enlargement, which can eventually lead to congestive heart failure. Horses in heart failure can develop increased heart rates, coughs, pronounced jugular pulses and irregular heart rhythms. Horses in congestive heart failure are at risk of sudden death due to pulmonary artery rupture, arrhythmia development, and respiratory distress.  Moderate cases should be monitored regularly (at least once a year) and often may participate in low-level exercise until they begin to develop structural changes in the heart consistent with heart failure. Owners and handlers should monitor these horses for exercise intolerance, increased respiratory and/or heart rate, trouble breathing, or prolonged recovery after exercise. Severe cases are usually not safe to ride.

How can mitral valve regurgitation be prevented?

The factors that lead to the degeneration of the mitral valve are not fully understood, so there is currently no way to prevent mitral valve regurgitation.

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.