There are many specific acquired heart diseases that have been identified in horses. The ones discussed below are the most common.
Degenerative Valve Disease
This acquired disease is characterized by thickening of the heart valves. In horses, degenerative valve disease most often affects the aortic, mitral, and tricuspid valves. The condition generally occurs slowly over time and is most common in middle-aged and older horses. In many horses, signs are uncommon because significant left ventricular volume overload and enlargement do not occur.
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.
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. Equine infectious anemia virus see Disorders Affecting Multiple Body Systems of Horses: Equine Infectious Anemia tends to cause myocarditis. Less common causes include severe deficiencies of vitamin E or selenium and ingestion of monensin (a feed additive found in some cattle feeds).
Signs of right-sided heart failure are common and include accumulation of fluid in the abdomen, congestion in veins, and large pulsations in the jugular vein. A murmur or arrhythmia may be noted on examination.
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 endocardium is the thin membrane that lines the heart cavity. Infection of the endocardium 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. 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.
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. see Heart and Blood Vessel Disorders of Dogs: Infective Endocarditis
The pericardium is the membrane that surrounds the heart. When fluid builds up within this membrane, 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.
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; these may include unwillingness or inability to exercise, lack of appetite, listlessness, and 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.
High Blood Pressure (Hypertension)
Systemic hypertension is an increase in the body's blood pressure. There are 2 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. Pulmonary hypertension is elevation of blood pressure in the blood vessels of the lungs. In horses, pulmonary hypertension may occur as a result of left-sided congestive heart failure. (For more information on high blood pressure, seesee Heart and Blood Vessel Disorders of Dogs: High Blood Pressure (Hypertension).)
Last full review/revision July 2011 by Davin Borde, DVM, DACVIM; Benjamin J. Darien, DVM, MS, DACVIM; Ase Risberg, VMD