As today's Cavaliers all descend from only six dogs, any inheritable disease present in at least one of the original founding dogs can be passed on to a significant proportion of future generations. This is known as the founder effect and is the likely cause of the prevalence of MVD in the breed. The health problems shared with this breed include mitral valve disease, luxating patella, and hereditary eye issues such as cataracts and retinal dysplasia. The grades 3–4 are most severe where surgery will most likely be needed to correct the problem to avoid the development of arthritis and lameness in the limb. As many as half of all Cavalier King Charles Spaniels may have a congenital blood disorder called idiopathic asymptomatic thrombocytopenia, an abnormally low number of platelets in the blood, according to recent studies in Denmark and the United States. The expected average lifespan of a Cavalier King Charles Spaniel is between nine and fourteen years.The Cavalier King Charles changed dramatically in the late 17th century, when it was interbred with flat-nosed breeds.