Heart mitral valve disease is the leading cause of death of cavalier King Charles spaniels throughout the world. MVD is a polygenetic disease which afflicts over half of all cavaliers by age 5 years and nearly all cavaliers by age 10 years. MVD, common to Cavalier King Charles Spaniels is a degeneration of the heart's mitral valve, one of four sets of valves in a dog's heart. As the mitral valve degenerates, the valve no longer fully closes after each pumping action, allowing some blood to flow backwards through them from the ventricle back into the atrium. As the condition worsens, more and more blood is able to backflow through the valve. In the final stages, the valve's struts sometimes break, causing the valve to collapse completely. MVD results in congestive heart failure in the CKCS. About 10% of all dogs suffer from some form of heart disease. Mitral valve disease is the most common heart disorder in older dogs of all breeds. However, in the cavalier King Charles spaniel, the prevalence of MVD is about 20 times that of other breeds. Also in cavaliers, the onset of the disease typically is much earlier in the life of the dog. It has been reported that, once diagnosed, mitral valve disease is much more rapid in cavaliers than in other breeds, possibly reaching a life-threatening stage within as little as 1 to 3 years, rather than the average 3 to 5 years. To a lesser extent, cavaliers also suffer from deterioration of their tricuspid valves.