**1. Introduction**

Chronic mitral valvular insufficiency (CMVI) is the most common cause of congestive heart failure (CHF) in small breed dogs [1, 2] and is characterized by progressive myxomatous degeneration of the atrioventricular valves [3]. Mitral regurgitation (MR) is the most common sequel to CMVI, which causes volume overload at the left atrial (LA) and left ventricle (LV) and progresses to CHF [4]. Underlying causes of CMVI have yet been identified, although aging and genetic causes were suggested in a certain breed of dogs [5–8] and humans [4]. CMVI is more often seen in old and small breeds of dog. Higher prevalence of CMVI was noticed in Cavalier King Charles Spaniels (CKCS) and Dachshund [9, 10]. CMVI has been noticed in ~50% of 6‐ to 7‐year‐old CKCS dogs and ~50% of 10‐year‐old Dachshund dogs. Those two

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

studies strongly suggested genetic etiology for this disease, although other study found the aging is the major cause for this disease [11]. In these dog breeds, the CMVI is occurred at younger age and progressed more rapidly. List of dog breeds having high prevalence rate for CMVI are Chihuahuas, Malteses, Yorkshire Terriers, Poodles, Papillons, Pekingeses, Miniature Pinschers, Bologneses, Dachshunds, Shih Tzus, Cairn Terriers, Miniature Schnauzers, Bichon Frises, Carvalier King Charles Spaniels, Pugs, West Highland White Terriers, Fox Terriers, Boston Terriers, Welsh Terriers, Whippets, American Cocker Spaniels, Beagles, German Shepherds, and Great Danes [12], although almost one‐quarter of dogs over the age of 10 have degenerative changes on mitral valve in any breeds of dog. Higher prevalence rate in Maltese and Shih Tzu has been found in some Asian countries including Korea, Japan, and Taiwan [13].

CMVI is an animal model of human mitral valve prolapse (MVP), which is suggested of poly‐ genic inheritance [14]. Several canine studies also suggested polygenic inheritance for CMVI [6, 8, 15]. Male dogs have higher rate of prevalence almost 1.5 times than female dogs [3]. One study found that the mitral valve was affected in ~60% case of CMVI, and tricuspid valve only was affected in ~10% of CMVI, while both atrioventricular valves (mitral valve and tricuspid valve) were affected in ~30% of CMVI [16].
