Cavalier King Charles Spaniel vs Maltese

Heart disease isn't a risk for Cavaliers so much as a near-certainty — about half develop a heart murmur from mitral valve disease by age 5, and veterinary sources note that essentially every Cavalier that lives long enough will eventually develop it, making it the breed's leading cause of death.

That famous pure-white coat sits on a genuinely fragile toy dog prone to a congenital heart defect, dental disease serious enough to affect other organs, and a skeleton delicate enough that a bad fall off a sofa is a real injury risk, not just a grooming project.