Acquired Heart and Blood Vessel Disorders in Horses

Degenerative valve disease is characterized by thickening of the heart valves. In horses, degenerative valve disease most often affects the aortic, mitral, and tricuspid valves. The mitral valve is most commonly affected. The condition generally occurs slowly over time and is most common in older horses. Failure of the valve to close properly results in turbulent blood flow. Blood flows back into the previous chamber (called regurgitation), leading to an increase in its blood volume and size. When regurgitation is severe, the chamber's pressure may also increase. The body uses compensatory mechanisms to counteract the decreased blood flow. This results in further enlargement of the heart.

Signs are not usually present unless the valvular disease is severe enough to lead to congestive heart failure. The signs of failure vary depending on the affected valve. Severe mitral valve degeneration can lead to signs of left-side heart failure, such as trouble breathing or coughing. Severe tricuspid valve disease can lead to signs of right-side heart failure, such as fluid accumulation under the skin of the limbs and on the underside of the chest and abdomen. In many horses, signs of aortic valve degeneration are uncommon because the aortic regurgitation is rarely severe enough to cause heart failure.

A veterinarian can often diagnose degenerative valve disease based on physical examination findings and appropriate imaging procedures, which may include chest x-rays and echocardiography (ultrasonography). A heart murmur may be heard. Your veterinarian will prescribe medications to control the signs of congestive heart failure, if present. Give all medications as directed by your veterinarian.

Horses with mild or moderate valve disease and no signs have a good outlook and can usually still be used for physical activity that is not overly strenuous. Horses with more severe disease should not be ridden or exercised and have a poor outlook.

Myocarditis is a local or widespread inflammation of the heart muscle, with degeneration or death of the heart muscle cells. There are numerous causes, including several viruses and bacteria, particularly Streptococcus species. Other bacteria that can cause myocarditis include Salmonella, Clostridium, and Borrelia burgdorferi (the bacteria that causes Lyme disease). Viral causes include equine infectious anemia virus and equine influenza. Strongylosis (a parasitic infection) can also cause myocarditis. Less common causes include severe deficiencies of vitamin E or selenium (white muscle disease) and ingestion of cardiac toxins, such as monensin (a feed additive found in some cattle feeds), blister beetles, and poisonous plants (such as rubber vine or white snakeroot). Mineral deficiencies (such as iron and copper) can also cause degeneration of the heart muscle cells.

Signs of right-side heart failure are common and include accumulation of fluid under the skin on the lower half of the body and in the abdomen, congestion in veins, and large pulsations in the jugular vein. A murmur or arrhythmia may be noted on examination. Your veterinarian may also perform blood tests and an echocardiogram (an ultrasound of the heart) to confirm the diagnosis.

Treatment is directed at improving the ability of the heart to contract, relieving fluid buildup, and controlling the narrowing of blood vessels (which can increase blood pressure) through the use of appropriate medication. The underlying cause also needs to be treated, if possible.

The endocardium is the thin membrane that lines the heart cavity. Infective endocarditis is an infection of the endocardium and typically involves one of the cardiac valves, although endocarditis of the cavity’s wall may occur. Infection is caused by bacteria carried in the blood. The infection gradually destroys the valve and keeps it from working properly. Preexisting damage to the valve or immune dysfunction can increase the likelihood that infective endocarditis will occur. In horses, the aortic and mitral valves are most commonly affected. The tricuspid valve is rarely affected, and pulmonic valve infective endocarditis is exceedingly rare.

Infected blood clots form on affected valves. Pieces of blood clots can dislodge and travel to other parts of the body, including the nervous system, digestive tract, urinary system, reproductive tract, and joints. Signs vary depending on where the infected blood clots lodge, but affected animals usually have a murmur, fever, weight loss, and fatigue. The condition can also lead to heart failure, which can result in trouble breathing. Veterinarians use blood tests, echocardiography (ultrasonography), and electrocardiography to diagnose the condition.

Treatment is directed at controlling signs of congestive heart failure, resolving any significant arrhythmias, killing the bacteria that started the infection, and eliminating the spread of infection. Horses may also be treated with medications to prevent the formation of blood clots on the affected valve. If the aortic valve is severely affected, the heart failure may not respond to treatment and the horse is unlikely to survive. The outlook is better when the infection is mild and affects the mitral and tricuspid valves.

The pericardium is the membrane that surrounds the heart. When fluid builds up within this membrane (called pericardial effusion), the heart is compressed. The pressure on the heart reduces its ability to pump blood. This condition is called cardiac tamponade. The compression significantly affects blood circulation and causes swollen jugular veins and accumulation of fluid in the abdomen. In addition, too little oxygen reaches the body’s tissues. Pericardial effusion rarely occurs in horses.

Pericarditis is inflammation of the pericardium, which can lead to pericardial effusion. In horses, septic pericarditis (resulting from infection) and pericarditis of an unknown cause are the most commonly reported types of pericardial disease. Signs vary based on the amount of fluid that has accumulated.

Common signs of pericarditis include:

• a history of a recent respiratory tract infection

• fever

• lack of appetite

• depression

Less common signs of pericarditis include:

• weakness

• distension of the jugular veins

• muffled heart sounds

• unwillingness or inability to exercise

• listlessness

• abdominal swelling due to fluid accumulation

Blood tests show signs of an infection in horses with suspected septic pericarditis.

Horses with cardiac tamponade require urgent treatment. Pericardiocentesis (inserting a needle into the sac to withdraw the fluid) is the most common initial treatment. Pericardiocentesis is relatively easy to perform and serious complications are rare. Broad-spectrum antibiotics, intravenous fluids, and corticosteroids may also be given. If the cause of the pericarditis is not known, it is important to watch for signs of a recurrence.

Systemic hypertension is an increase in the body’s blood pressure. There are two major types of systemic hypertension. Essential (primary) hypertension, which is of unknown cause, is rare in most animals, but common in humans. Secondary hypertension results from a specific underlying disease, such as equine metabolic syndrome. If untreated, high blood pressure can lead to damage in other organs throughout the body, such as the eyes and the kidneys.

Pulmonary hypertension is elevation of blood pressure in the blood vessels of the lungs. In horses and other animals, it is usually caused by another disease that increases blood flow to the lungs (caused by, for example, a ventricular septal defect or patent ductus arteriosus) or that increases the resistance to blood flow within the lungs (caused by, for example, blood clots within the lungs). Pulmonary hypertension may occur in horses as a result of left-side congestive heart failure.

Signs of pulmonary hypertension are usually those seen with right-side heart failure, such as:

• swelling of the limbs and underside of the abdomen and chest

• jugular distension and exaggerated pulses

• accumulation of fluid in the abdomen

• tiring easily (especially after exercise)

The diagnosis is usually made using echocardiography (ultrasonography). Treatment is typically ineffective, and the outlook is often poor. The best chance for a successful longterm outcome is when the underlying disease can be identified and treated, but this is rare.