Chronic valvular heart disease is the most important heart disease in dogs. Many breeds are affected, especially smaller breeds. Most aged dogs have some degree of valvular degeneration (called endocardiosis), however, only those with substantial valve disease develop heart symptoms. This condition is a degenerative condition, probably predisposed by genetic factors. It is not caused by an infection or related to bad teeth (though this is a common misconception).

The essential valvular abnormalities are either increased "floppiness" of the mitral valve in the heart, or more often, shortening and thickening of this valve. The tricuspid heart valve is also affected in some dogs. The degeneration causes the valves to close improperly. Leaking of the valve causes blood to move backwards creating a heart murmur and limiting the amount of blood that can be pumped to the body. Severe leaking can occur when one or more of the fine strands that support the valve (the chordae tendineae) rupture.

A percentage of dogs with chronic mitral disease develop a condition called pulmonary hypertension, which is high blood pressure in the arteries of the lung. These dogs often develop fluid accumulation in the abdominal cavity and are prone to bouts of weakness or fainting.

The consequence of moderate to severe valvular heart disease is typically congestive heart failure. The symptoms of heart failure include exercise intolerance, breathing difficulties or coughing and obvious fluid accumulation in the chest cavity or the abdomen.

Mild cases of chronic valvular disease do not limit the dog, but severe cases cause heart failure and can be lethal.

Other medical problems can lead to symptoms similar to those encountered in valvular heart disease. It is important to exclude these conditions before establishing a definite diagnosis:

Congenital heart disease (birth defects of the heart)

Cardiomyopathy (heart muscle disease, common in dogs)

Pericardial disease (fluid accumulation around the heart)

Bronchitis (inflammation of the bronchial tree, similar to chronic "smoker's cough")

Lung diseases (a variety of diseases of the lung, including pneumonia and lung cancer)

Heartworm disease

Pulmonary fibrosis (scarring of the lung)