The Cavalier King Charles changed dramatically in the late 17th century, when it was interbred with flat-nosed breeds. Until the 1920s, the Cavalier shared the same history as the smaller King Charles Spaniel. Breeders attempted to recreate what they considered to be the original configuration of the breed, a dog resembling Charles II’s King Charles Spaniel of the Restoration. Various health issues affect this particular breed.
Cavaliers can notably be prone to mitral valve disease, which leads to heart failure. This appears in many Cavaliers at some point in their lives and is the most common cause of death. Some serious genetic health problems, including early-onset mitral valve disease (MVD), the potentially severely painful syringomyelia (SM), hip dysplasia, luxating patellas, and certain vision and hearing disorders are health problems for this breed. 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. Cavaliers are also affected by ear problems, a common health problem among spaniels of various types, and they can have such other general conditions as hip dysplasia, which are common across many types of dog breeds.
During the early part of the 18th century, John Churchill, 1st Duke of Marlborough, kept red and white King Charles type spaniels for hunting. The duke recorded that they were able to keep up with a trotting horse. His estate was named Blenheim in honor of his victory at the Battle of Blenheim. Because of this influence, the red and white variety of the King Charles Spaniel and thus the Cavalier King Charles Spaniel became known as the Blenheim.
Attempts were made to recreate the original Cavalier King Charles Spaniel as early as the turn of the 20th century, using the now extinct Toy Trawler Spaniels. These attempts were documented by Judith Blunt-Lytton, 16th Baroness Wentworth, in the book “Toy Dogs and Their Ancestors Including the History And Management of Toy Spaniels, Pekingese, Japanese and Pomeranians” published under the name of the “Hon. Mrs Neville Lytton” in 1911.