Valvular Dysplasia in Horses: Congenital Mitral or Tricuspid Valve Defects

Valvular dysplasia in horses is a congenital malformation of a heart valve, meaning the valve did not develop normally before birth. In this article, that usually means the mitral valve on the left side of the heart or the tricuspid valve on the right side. The valve leaflets, chordae tendineae, papillary muscles, or supporting ring can be misshapen, thickened, shortened, or poorly attached. As a result, the valve may not close tightly, and blood can leak backward with each heartbeat.

In horses, congenital heart disease is considered uncommon, and valvular dysplasia is one of several defects that can be reported. Some affected horses are identified as foals, but others are not recognized until later, especially if the murmur is mild and the horse seems normal at rest. A horse may live comfortably for a long time with a mild defect, while a more severe defect can lead to chamber enlargement, reduced athletic ability, arrhythmias, or congestive heart failure.

For pet parents, the hardest part is that a murmur alone does not tell the whole story. Horses can have audible murmurs without severe disease, and the loudness of a murmur does not always match how serious the defect is. That is why your vet may recommend an echocardiogram rather than making decisions based on auscultation alone.

The outlook depends on which valve is affected, how malformed it is, whether the heart has enlarged, and whether rhythm problems or heart failure are present. Mild cases may be managed with monitoring and workload adjustments. More serious cases may need referral-level cardiology care and may not be safe for strenuous riding or breeding.

Valvular dysplasia is considered a developmental defect present at birth. The mitral or tricuspid valve and its supporting structures do not form normally during fetal development. The exact cause is often not identified in an individual horse. In many cases, there is no clear event, exposure, or management factor that a pet parent could have prevented.

The abnormality may involve malformed valve leaflets, shortened or fused chordae tendineae, abnormal papillary muscles, or a distorted valve annulus. These changes can cause regurgitation because the valve cannot seal properly, and in some horses they may also interfere with normal forward blood flow. Over time, the heart chambers handling that extra volume may enlarge.

A genetic contribution is suspected for some congenital cardiac defects across species, but for horses, the evidence is limited and the condition is considered rare. Because congenital heart disease can have implications for breeding populations, your vet may advise caution about using an affected horse for breeding, especially if the defect is clearly structural and clinically important.

This condition is different from acquired valvular disease, which develops later in life from degeneration, infection, or other cardiac problems. That distinction matters because a congenital defect may be present long before symptoms appear, and the long-term plan depends on the horse's anatomy and current heart function rather than age alone.

Diagnosis starts with a careful physical exam. Your vet may hear a systolic murmur over the mitral or tricuspid valve area, note an abnormal rhythm, or find signs such as poor recovery after exercise, jugular changes, or fluid accumulation. Because horses can have murmurs that are not severe, auscultation is only the first step.

The key test is echocardiography with Doppler. This allows your vet or a veterinary cardiologist to look at the valve leaflets and supporting structures, identify regurgitant blood flow, and measure whether the atria or ventricles have enlarged. Echocardiography is the best way to confirm that the problem is a congenital valve malformation rather than a more common acquired valve disorder.

Additional testing may include an electrocardiogram (ECG) at rest, ambulatory or exercise ECG if arrhythmias are suspected, thoracic imaging in selected cases, and bloodwork to assess overall health before making exercise or treatment decisions. In referral settings, exercise testing can help determine whether rhythm disturbances appear during work.

Diagnosis is not only about naming the defect. Your vet is also trying to answer practical questions: How severe is the leakage? Is the heart enlarging? Is the horse safe to ride? Does the horse need workload restriction, repeat imaging, or treatment for complications? Those answers shape the care plan much more than the murmur grade alone.

Because valvular dysplasia is a congenital developmental defect, there is no guaranteed way to prevent it in an individual foal through routine feeding, turnout, supplements, or training changes. Pet parents should not blame themselves if a horse is diagnosed with this condition later in life.

The most practical prevention strategy is really risk reduction at the population level. Horses with confirmed, clinically important congenital heart defects are often poor breeding candidates, because congenital cardiovascular disease may have heritable implications even when the exact genetic basis is unknown. Your vet can help you think through breeding decisions on a case-by-case basis.

Early detection also matters. A careful prepurchase exam, routine wellness exams, and follow-up of any newly detected murmur can identify affected horses before strenuous work exposes them to more risk. If your horse has a murmur, reduced stamina, or an irregular rhythm, asking your vet whether an echocardiogram is appropriate is one of the most useful steps you can take.

While you cannot prevent the malformation itself, you can help prevent avoidable complications by following exercise recommendations, keeping recheck appointments, and seeking prompt veterinary care if your horse develops swelling, breathing changes, weakness, or collapse.