 Nearly all Cavaliers eventually suffer from disease of the mitral valve, with heart murmurs which may progressively worsen, leading to heart failure. This condition is polygenic (affected by multiple genes), and therefore all lines of Cavaliers worldwide are susceptible. It is the leading cause of death in the breed. A survey by The Kennel Club of the United Kingdom showed that 42.8 % of Cavalier deaths are cardiac related. The next most common causes are cancer (12.3 %) and old age (12.2 %). The condition can begin to emerge at an early age and statistically may be expected to be present in more than half of all Cavalier King Charles Spaniels by age 5. It is rare for a 10-year-old Cavalier not to have a heart murmur. While heart disease is common in dogs generally – one in 10 of all dogs will eventually have heart problems – mitral valve disease is generally (as in humans) a disease of old age. The "hinge" on the heart's mitral valve loosens and can gradually deteriorate, along with the valve's flaps, causing a heart murmur (as blood seeps through the valve between heartbeats) then congestive heart failure. The Cavalier is particularly susceptible to early-onset heart disease, which may be evident in dogs as young as one or two years of age.
