The Cavalier King Charles Spaniel has been a breed for over 1000 years. However, during that time there were “bottlenecks” in the breeding of these dogs where only a small number of dogs were bred.
Researchers studied eight different breeds and found that the Cav had the greatest number of disease-causing genes than any of the other breeds studied. They are especially prone to heart disease.
Note: The study only sampled a limited number of dogs of each breed studied. While this is a good indicator, I would have liked to see a larger number of dogs tested from a wider geographical area. However, breeders who sincerely love their breed, what ever it is, can improve the breed by selective breeding for the right reasons. Too many people breed indiscriminately and do not test their dogs for genetic defects. As a canine behavior consultant, I have seen the results of this for my entire career.