Problems associated with the heart valves constitute the most common form of heart disease in dogs and include chronic valve disease, congestive heart failure and bacterial endocarditis. The risk of valvular heart disease is related to the type of dog. While heart valve diseases are known to affect around 50% of Calvalier King Charles spaniels, it is relatively uncommon in medium, large, and giant breeds. The causes of the difference in susceptibility between one breed and another are genetically based, though not, as yet, fully understood. Breeds with a high incidence of valvular heart disease include the Miniature Schanauzer, Calvalier King Charles spaniel, Chihuahua, Lhasa Apso, Miniature Poodle, and Yorkshire Terrier.
What is unexpected in the case of mitral valve disease is that the majority of affected breeds display an average adult weight of less than 9 kg. Why is it so much more prevalent in small breeds as opposed to large? What do all small dog breeds share that might generate the optimal conditions for mitral valve disease (MVD) to develop? It appears that many of the at-risk breeds share common ancestry from an early small dog that has transmitted susceptibility genes to its descendants. A final trait that is shared between small dogs that might contribute to the development of MVD is extended lifespan. It is often noted that small dogs live longer, on average, than large dogs. MVD is reported to be a disease of the aging heart.1
Chronic Valve Disease
Chronic valve disease is an important cause of heart failure, a condition in which the heart cannot keep up with the workload. For unknown reasons, the flaps of the valves between the atria and ventricles can become thickened and distorted until their edges no longer meet when the valve shuts. This defect allows some blood to be forced back into one or both of the atria when the ventricles contract to expel blood from the heart. The seepage increases as the valve defect worsens. Sometimes some of the cordae tendinae rupture. If this happens, that part of the valve becomes useless.
During the initial stages of the valvular heart disease, there may be no obvious signs that anything is wrong. This condition is usually discovered during a routine examination, when a veterinarian hears a heart murmur. When valve disease is present, the mitral valve, which lies between the atrium and the ventricle on the left side of the heart, is usually affected. The equivalent valve on the right of the heart, the tricuspid valve, is affected in only about one third of cases. Treatment of valvular heart disease will not prolong a dog's life expectancy.
Congestive Heart Failure
A heart affected by a disease that reduces its pumping efficiency (most often chronic valve disease) can usually "compensate" for months or years. It usually does so by increasing in size. Eventually, the underlying disease leads to congestive heart failure, in which blood becomes backed up in the veins. This buildup, in turn, forces fluid out of the circulation into the body tissues, in the lungs and elsewhere.
Congestive heart failure can occur gradually or suddenly. The early signs include reduced exercise tolerance and lethargy. Because congestive heart failure occurs in older dogs, these signs are often mistaken as natural age-related changes. Soon, a dry, non-productive cough develops, initially after exercise and at night. As heart failure persists, more changes are noticed. Some dogs lose their appetite, lose weight, and breathe more rapidly. In some cases, fluid that has been forced out of the veins may cause ascites and swelling of the limbs. Eventually, fluid back up in the lungs may cause a frothy cough or production of a bubbly pink fluid. In the late stages of heart failure, a dog braces itself on its elbows and extends its head to breathe. The dog's gums and tongue become blue. The pulse becomes rapid and irregular. Fainting usually occurs with the slightest exertion. A veterinarian diagnoses congestive heart failure when the typical signs are present and are consistent with known heart disease. If there is no known heart disease, chest and abdominal X-rays, ECG, and echocardiography are performed. If the heart failure primarily affects the right side of the heart, a blood test may be carried out for heartworm infestation.
Miniature poodles are susceptible to heart valve disease
Dogs with heart disease but no signs of heart failure are treated normally and given a full exercise routine. When the first signs of heart failure develop, a dog benefits from treatment from a combination of ACE (angiotensin-converting enzyme) inhibitors and diuretics such as frusemide (furosemide). The ACE inhibitors reduce the workload on the heart by causing blood vessels to widen. Dietary treatment for heart failure remains controversial. Excess salt in the diet should be avoided, but the benefit of low-salt diets is, as yet, not proven. Antioxidants may be of value for some dogs with congestive heart failure such as selenium and vitamin E. Routine, daily light exercise is beneficial as long as it does not cause the dog to cough, tire easily, or breathe rapidly.
Bacterial endocarditis is an infection of the heart valves and the lining of the heart muscle. Bacteria enter the bloodstream from wounds or infections elsewhere in the body. Gum disease is one possible source of bacteria. The bacteria invade the heart valves, where they produce cauliflower-like masses called vegetations. Some of the vegetations break off and spread via the blood stream to infect other parts of the body. Dogs being treated with immunosuppressant drugs, including corticosteroids, are at increased risk of bacterial endocarditis. The fact that infection can and does affect other parts of the body means that dogs often have nonspecific signs suggestive of a number of diseases. These signs include lethargy, loss of appetite, fever, shaking, lameness, and personality changes. An affected dog usually has a heart murmur, often changing in intensity from day to day. Chest X-rays, ECG, and echocardiography are used to confirm the diagnosis. A blood culture identifies the bacteria and the antibiotic drugs that are likely to be effective against them. An appropriate antibiotic, selected according to bacterial culture, is given intravenously for the first week. Oral antibiotic therapy will continue for the long term. Affected dogs are monitored closely. Repeat X-rays and echocardiography will show the vegetations shrinking in size. Even so, the outlook is not good; congestive heart failure may occur at any time.
Mitral valve disease is a prominent component of most types of mucopolysaccharidosis, a lysosomal storage diseases. In this disorder the accumulation of the mucopolysaccharides heparan, dermatan, and chondroitin sulfates results in abnormal collagen (main structural protein in connective tissues) structure. The abnormal collagen leads to weakening of mitral valve, mitral regurgitation, and the need for valve replacement, a major surgical procedure.2
- Myxomatous mitral valve disease in dogs: Does size matter?
- Pathogenesis of Mitral Valve Disease in Mucopolysaccharidosis VII Dogs (2015)