Degenerative valvular disease in older horses

About 7.5 percent to 20 percent of U.S. horses are geriatric horses, those greater than 20 years of age. In a survey of geriatric horses, it was noted that 7 percent are affected by some form of cardiovascular system disease.

Degenerative valve disease (DVD) is seen in geriatric horses and also horses as young as 10 years of age.

As horses age, something triggers the valves to leak blood backwards, usually affecting the aortic or mitral valves. The heart murmur typically is heard when blood starts to leak to the point where it generates noise.

"The literature would say that horses begin to be at risk in their teens for all valves, says Abby Sage, VMD, Dipl. ACVIM, University of Minnesota College of Veterinary Medicine.

The general consensus is that DVD is one of the more common cardiac diseases seen in horses, says Jerry Woodfield, DVM, Dipl. ACVIM, of Northwest Cardiology Consultants in Seattle.

"Fortunately when the murmurs are heard, many times they are not to a point that they are impacting the horse that dramatically. The concern is trying to get an idea of how risky any compromise of heart function is on anyone that is using the horse for athletic reasons," Woodfield says.

Photo 1: This image of regurgitation through the aortic valve was obtained from the left parasternal window in longitudinal plane in the left 5th ICS. The aortic valve is just above the base of the color-flow window.

"The significance of heart murmurs in horses is often difficult to determine if the horse is not exhibiting clinical signs or if the signs, such as poor performance, are non-specific," says Virginia Reef, VMD, professor and director of large animal cardiology at the University of Pennsylvania New Bolton Center.

Sources agree that valve disease is commonly an acquired disease. Throughout most of its life, the horse has a normal heart. As it ages, something triggers the valves to leak blood backwards, usually affecting the aortic or mitral valves.

"We don't know what the trigger is, whether it is mechanical force on the valve or a failure of some of the tissues. Outside of the rare case where you get an infected valve, otherwise the cause of the origination of the 'leaky valve' is unknown," Woodfield says.

Photo 3: This ultrasound image of a leaking mitral valve was imaged from the left side of the horse. Color-flow doppler was used to identify regurgitation or abnormal backflow (indicated by the arrows) of blood from the left ventricle to the left atrium, across a closed but "leaking" degenerative mitral valve. The size of the regurgitant jet and its effect on the size of the left atrium are useful in determination of a prognosis. LA is left atrium. LV is the left ventricle.

The heart murmur is heard usually when the blood starts to leak to the point that it generates some noise. Commonly in horses, the murmurs of the defective valves are detected but have yet to result in significant clinical problems. From there, a decision is made to do a further work up by ultrasound/echocardiogram to try to determine what impact it might be having on cardiac function. In many cases, the DVD progresses slowly during many years. Echocardiography is necessary to diagnose the specifics of the condition and to assess its severity.

"Valvular lesions and regurgitation occur frequently and depending on their severity may impair the horse's performance, compromise him, or have no affects on his life expectancy," Reefs says.

Getting specific

Timing, duration, intensity, quality, pitch, shape, point of maximal intensity and radiation define murmers.

"Timing refers to the phase of the cardiac cycle in which it occurs (systolic, diastolic or continuous)," Reef explains. "The duration is the period during which the murmur is detectable during the cardiac cycle (i.e. early, mid, late, or holo, or pansystolic, or diastolic). The intensity grade 1-6/6 is determined by the quantity and velocity of blood flow through the murmur's origin, the distance of this blood flow from the stethoscope and the acoustic properties of the interposed tissue. The quality is harsh, coarse, rumbling, scratchy, musical, honking, or blowing, and may be high, medium or low-pitched. The shape auscultated is determined by the phonocardiographic depletion of the intensity of the murmur over time. The radiation is usually from the murmur's origin in the direction of the abnormal blood flow, and is determined by the murmur's intensity and physical characteristics of the chest."

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Heart murmurs are detected on a six-point grading system. "It's not totally subjective; it has objective criteria, though the grading system is not correlated with severity of disease," Woodfield says. "It's how loud the murmur is more than anything. Subjectively the way I tend to look at it, which applies a little bit better to owners, is a soft, moderate or loud murmur, grades 1 and 2 as soft, 3 and 4 as moderate, 5 and 6 as loud." A Grade 1 murmur by definition is the first audible sound you can hear that you call a murmur in an ideal environment, where you can barely detect a murmur with your stethoscope. Generally speaking, a Grade 1 murmur is not that much of a health concern, but you can't always go by that. A grade 2 murmur can be heard fairly easily, but one has to concentrate to make sure that it's there all the time. Three's and four's are fairly easily heard murmurs. "I differentiate a 4 from a 5 is by definition," Woodfield explains. "A grade 5 murmur has what we call a 'palpable thrill.' When you put your hand on the chest wall, the murmur is vibrating the chest wall, and it is a very loud murmur. To me, a grade 4 is a loud murmur, which I think is probably a grade 5, but it is not vibrating the chest wall."

A Grade 6 murmur is one that you can actually hear even with your stethoscope off of the chest wall by about an inch. Though they are loud, they are not necessarily indicative of severe disease. For example, one can have loud, buzzing harmonic murmurs of aortic regurgitation in a horse that might be graded 5-6/6, but they may be almost incidental, minor, trivial disease. Therefore a loud murmur does not necessarily reflect a severe condition. The grading system is mainly a communications scale, so that different doctors can communicate what they are hearing.

DVD in dogs versus DVD in horses

Mitral and aortic insufficiency

From human medicine, a normal mitral valve consists of two thin leaflets, located between the left atrium and the left ventricle of the heart. These leaflets, shaped like parachutes, are attached to the inner wall of the left ventricle by a series of strings called chordae. When the ventricles contract, the mitral valve leaflets close snugly, preventing backflow of blood from the left ventricle into the left atrium. When the ventricles relax, the valves open to allow oxygenated blood from the lungs to fill the left ventricle. In human patients with mitral valve prolapse, the leaflets and chordae are affected by a process called myxomatous degeneration. In myxomatous degeneration, the structural protein, collagen, forms abnormally and causes thickening, enlargement and redundancy of the leaflets and chordae. When the ventricles contract, the redundant leaflets prolapse (flop backwards) into the left atrium, sometimes allowing leakage of blood through the valve opening, i.e. mitral regurgitation.

Although its etiology is unknown, degenerative lesions resulting in regurgitation of the atrioventricular (mitral, tricuspid) and semilunar (aortic, pulmonary) valves are the most common cause of valvular disease in older horses, though the mitral and aortic valves are affected more frequently than the tricuspid and pulmonary valves, Sage says. Lesions involve "degeneration of the collagen in the fibrosa of the valve and proliferation of loose fibroelastic tissue in the spongiosa of the valve. The lesions also have histiocyte, lymphocyte and fibroblast infiltration of the superficial layers of the endocardium. Degenerative lesions appear as generalized or discrete fibrous thickening and rounding of the free borders of the valve leaflets that may form nodules."

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Mitral regurgitation

Reef and Sage note that murmurs of mitral regurgitation (MR) are the most commonly acquired valve lesions associated with underlying cardiac disease in horses that result in clinical signs. They are usually holosystolic or pansystolic, band shaped, grade 3/6 or louder; coarse or honking; with their point of maximal intensity at the left AV valve area radiating dorsally and caudally. The clinical signs are increased respiratory rate and effort during exercise, excessive sweating, prolonged recovery time after exercise, coughing, and atrial fibrillation — exercise intolerance. Post exercise, these signs may take longer to recover to normal levels. At rest, horses with MR may not show clinical signs other than the murmur.

Echocardiography is used to further diagnose mitral regurgitation, quantify its severity and determine a prognosis. Spectral and color flow Doppler are used to map the size and location of the regurgitant jet. According to Sage, if the jet is small, of brief duration and limited to a small area behind the valve leaflets, then it is thought to be a normal physiological finding and does not represent a manifestation of subclinical valvular disease. The regurgitant jet seen in horses with cardiac murmurs compatible with mitral regurgitation seen on color flow Doppler is longer in duration than normal horses but similar in length, width, area and time of onset.

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"Quantification of the regurgitant jet on echo is based on the velocity of flow, distance it travels in the atria or into the ventricle and the percent of cross section of the valve that is involved," Sage explains.

"Prognosis for horses with MR is based on the duration, etiology and severity of the regurgitation, the extent of chamber enlargement, if any, myocardial contractility and clinical signs at the time of presentation," Sage says. "If the left atrium is normal size or the degree of left atrial and left ventricular enlargement is small and the regurgitation is mild to moderate, the regurgitation is likely to progress slowly over years, some without clinical significance."

If it progresses, horses with MR have a poor prognosis as the mitral regurgitation can lead to left heart failure.

Aortic regurgitation

Aortic regurgitation (AR) is most common in horses over 10 years of age. Reef states that "they range from 1-6/6". Both state that it is a holodiastolic decrescendo murmur with a point of maximal intensity in the left 4th intercostal space or "in the aortic valve area and radiate toward the cardiac apex." Aortic regurgitation murmurs often have a musical quality due to vibration of the valve leaflets.

According to Sage, most horses tolerate even severe aortic regurgitation well without clinical signs. Because most horses do not develop AR until they are middle aged (10+) and it usually progresses slowly, it rarely leads to congestive heart failure or death. Reef notes that in older horses, they may progress slowly, and they rarely cause significant concern for the health of the horse. Aortic valve prolapse also occurs in older horses, associated with regurgitation, but likewise remains unchanged or slowly progresses over several years.

Tricuspid regurgitation

Tricuspid regurgitation (TR) is defined by a holosystolic, band-shaped Grade 2/6 or louder, coarse with a point of maximal intensity at the right 4th intercostal space. Reef says they may be pansystolic, are coarse or blowing in quality with their point of maximal intensity at the right AV valve area radiating dorsally. Horses with tricuspid regurgitation rarely show clinical signs, and if it is mild to moderate, they do not show exercise intolerance, can perform maximally, and only display the cardiac murmur. Many of them do not have valvular lesions that can be detected by echocardiography. The majority of horses with tricuspid insufficiency have clinically insignificant to moderate tricuspid regurgitation and normal exercise tolerance, Reef says. With TR, most horses perform up to their owner's expectations without much concern.

Pulmonary regurgitation

Though more rarely to occur, pulmonary regurgitation murmurs are usually holodiastolic, decrescendo and are usually less intense than aortic regurgitation murmurs.

"They may range from grade 1-6/6, but most horses have a murmur that is grade 1-3/6, if it can be heard," Reef says. "It is more likely to be associated with congestive heart failure, with a poor to grave prognosis for life, about 3-6 months with supportive therapy."

How does DVD affect horse performance?

"A lot of them (murmurs) in their early findings are found to be trivial to mild," says Woodfield. They progress very slowly to the point where they need to be checked every couple of years or every several years. This is especially true if they have not changed much to where the owners are still getting good use from the horse without any consequence. As they start to progress, dependent on the intended use of the horse, any horse that has some symptoms associated with its heart disease are probably taken out of athletic use. Valve disease in an endurance horse is probably going to show up sooner than in the backyard horse, especially one that is ridden on a limited basis. When the exercise regimen is more intense, you're more apt to pick up the signs, or the horse is more apt to show exercise intolerance than a horse that's mildly worked. The more strenuous the exercise, the more apt the horse will be unable to perform at its peak. "The biggest question for the horse rider is: What risk is it to me to be on a horse with cardiac disease?" Woodfield says.

Horse DVD treatment options

During the early stages, it is not necessary to treat horses with DVD. If DVD is acquired in a 15 year-old horse, and it lives to 35, it might catch up to him as a geriatric. At an advanced age, by the time the horse gets symptoms necessary to treat, it is probably already retired and unable to perform. It becomes a decision for the owner to treat a horse that is now a pet or euthanize it. In addition, many of the drugs that can be used for dogs are difficult to use in horses just because of the cost and the formulations. The alternative is the case of the stallion, where an owner wants to get another year or two of breeding from it. It can be treated with an annual treatment of medication to carry it through a few more seasons so it doesn't go into heart failure.

Dr. Kane earned his doctorate in equine nutrition and physiology from the University of Kentucky in 1978. He works within the animal-feed industry with a specialty in horses.