The most common degenerative valve disease of the dog: myxomatous thickening of the mitral leaflets causing progressive mitral regurgitation, left atrial dilatation and congestive heart failure in advanced stages. In the Cavalier King Charles Spaniel and the Dachshund there is a strong genetic predisposition, but it is NOT a monogenic disease: the genetic test estimates risk, it does not determine the disease.

Cavalier King Charles Spaniel and Dachshund, among other small breeds. In the CKCS the majority of dogs develop a murmur from 7-9 years of age according to clinical series; no genetic frequencies per locus have been published systematically (limited data).

- Left apical systolic (mitral) murmur, increasing in intensity with age\n- Cough, exercise intolerance and dyspnoea\n- Syncope in some cases\n- Signs of congestive heart failure (pulmonary oedema, ascites)\n- Many dogs remain asymptomatic for years

The disease has been recognised for decades as the most prevalent acquired heart disease of the dog, with a marked predisposition of the Cavalier King Charles Spaniel. Genome-wide association studies have identified risk loci: Madsen and colleagues (2011) described regions on CFA13 and CFA14 in the CKCS; Axelsson and colleagues (2021) associated variants in NEBL with the disease in the Dachshund; and Mead and colleagues (2022) linked variants of the Nebulette locus (NEBL) with severity in the CKCS. The polygenic basis and the influence of age explain why selection relies on cardiological assessment rather than on a single genetic test.

- The genetic test estimates risk, it does not certify disease: combine it with cardiological assessment\n- Select breeding animals with lower genetic risk and, above all, with absence of murmur/normal valve on echocardiography before breeding\n- Do not breed with animals with a significant murmur or cardiomegaly\n- Given the very high prevalence in CKCS, it is neither ethical nor viable to exclude animals solely on the basis of a genetic risk profile: prioritise gradual selection over several generations\n- Repeat auscultation/echocardiography annually in breeding animals

Differential diagnosis with infective endocarditis, cardiomyopathy and shunts. ACVIM staging (B1, B2, C, D). Echocardiography (VHS ratio, left atrial size, fractional shortening) guides follow-up and the initiation of pimobendan in advanced stage B2. Annual monitoring recommended in predisposed breeds.

1. Madsen MB et al. 2011, Identification of 2 loci associated with development of myxomatous mitral valve disease in Cavalier King Charles Spaniels. J Hered 102(Suppl 1):S62-7. PMID: 21846748. 2. Axelsson E et al. 2021, The genetic consequences of dog breed formation - Accumulation of deleterious genetic variation and fixation of mutations associated with myxomatous mitral valve disease in cavalier King Charles spaniels. PLoS Genet 17(9):e1009726. PMID: 34473707. 3. Mead SE et al. 2022, Genetic Variants at the Nebulette Locus Are Associated with Myxomatous Mitral Valve Disease Severity in Cavalier King Charles Spaniels. Genes (Basel) 13(12):2292. PMID: 36553559. 4. Olsen LH et al. 2026, A Combination of Alleles in LMOD2 and a lncRNA is Strongly Associated With Myxomatous Mitral Valve Disease in Cavalier King Charles Spaniels. Anim Genet. PMID: 42708524. OMIA:000654-9615.