Dog Myxomatous Mitral Valve Disease (MMVD)

Myxomatous mitral valve disease is the most common heart condition in dogs, particularly affecting small and toy breeds. Early detection and treatment can significantly extend a good quality of life.

Key Facts

MMVD affects the mitral valve between the left atrium and left ventricle, causing the valve leaflets to thicken and degenerate over time, leading to progressive leakage (regurgitation)
It is the most common cardiovascular disease in dogs, affecting an estimated 10% of all dogs and the majority of Cavalier King Charles Spaniels by age 10
Cavalier King Charles Spaniels have a genetic predisposition with earlier onset and faster progression; responsible breeding programmes use cardiac auscultation to screen breeding dogs
The disease progresses through stages: Stage A (at-risk breed), Stage B1 (murmur present, no cardiac enlargement), Stage B2 (murmur + cardiac enlargement, still asymptomatic), Stages C and D (clinical heart failure)
The landmark EPIC trial demonstrated that starting pimobendan treatment in Stage B2 dogs (before heart failure onset) significantly delays time to onset of heart failure
Signs of heart failure include exercise intolerance, coughing (especially at night), increased breathing rate, and eventually laboured breathing and collapse
Chest X-rays and echocardiography are used to assess cardiac size and function; resting respiratory rate monitoring at home is a valuable early warning system
With appropriate treatment (pimobendan, ACE inhibitors, furosemide), dogs can live comfortably with heart failure for months to years

Welfare Considerations

MMVD causes progressive deterioration in the dog's ability to exercise and breathe comfortably. Dogs in early stages may feel well but are at increasing risk of sudden decompensation into acute heart failure — a frightening and painful emergency. The welfare challenge is that earlier intervention (Stage B2 pimobendan) requires owners to invest in cardiac monitoring before their dog shows obvious illness, which requires proactive veterinary engagement. Once in heart failure, the primary welfare goal becomes maintaining quality of life through medication optimisation and minimising breathlessness. The final stages of untreated or refractory heart failure — severe breathlessness, coughing, and exhaustion — cause significant distress. Timely euthanasia when quality of life can no longer be maintained is an important welfare decision owners should feel supported in making.

What You Can Do

For Cavalier King Charles Spaniels and other predisposed breeds, start annual cardiac auscultation by age 1–2
Learn to monitor your dog's resting respiratory rate — above 30 breaths per minute at rest is a warning sign
If a murmur is detected, pursue echocardiography to stage the disease and guide treatment decisions
Ask your vet about the EPIC trial findings and whether pimobendan is appropriate for your dog's stage
Support the Cavalier Health breeding health scheme to reduce genetic burden of MMVD in future generations