Equine Valvular Disease Study Guide

Overview and Clinical Importance

Valvular heart disease is one of the most commonly encountered cardiac abnormalities in horses. Unlike small animals where stenotic lesions are common, equine valvular disease is almost exclusively regurgitant in nature. The majority of valvular disease in horses is acquired rather than congenital, with degenerative changes being the most frequent etiology. Understanding the pathophysiology, clinical presentation, diagnostic approach, and management of valvular disease is essential for NAVLE success and clinical practice.

Approximately one-third of all horses develop a heart murmur during their lifetime. The most commonly affected valves are the mitral valve and aortic valve. While many horses with valvular regurgitation remain asymptomatic and can continue athletic careers, severe disease can lead to congestive heart failure, arrhythmias, and sudden death.

Cardiac Valve Anatomy Review

The equine heart contains four valves that ensure unidirectional blood flow. The atrioventricular (AV) valves (mitral and tricuspid) separate the atria from the ventricles, while the semilunar valves (aortic and pulmonic) guard the ventricular outflow tracts.

Cardiac Valve Characteristics

Heart Murmur Evaluation

Cardiac auscultation is the primary screening tool for valvular disease. Murmurs are characterized by timing, intensity (grade), quality, location (point of maximal intensity), and radiation.

Heart Murmur Grading Scale (1-6)

Mitral Valve Regurgitation (MR)

Mitral regurgitation is the most common clinically significant valvular lesion in horses, occurring in approximately 3% of the general horse population but up to 23% in performance Thoroughbreds. It results from incomplete closure of the mitral valve during systole, causing backflow of blood from the left ventricle into the left atrium.

Etiology

• Degenerative valve disease: Most common cause; age-related thickening, fibrosis, nodule formation
• Ruptured chordae tendineae: May cause acute, severe MR with sudden onset heart failure
• Bacterial endocarditis: Vegetative lesions causing valve destruction
• Mitral valve prolapse: Bulging of leaflet into LA during systole
• Secondary to LV dilation: Annular stretching from cardiomyopathy or severe AR
• Intensive training: Particularly in racehorses

Pathophysiology

During systole, blood regurgitates from the LV into the LA, causing volume overload of both chambers. Initially, the LV compensates with eccentric hypertrophy and may show increased fractional shortening. Progressive disease leads to LA dilation (increasing risk for atrial fibrillation), pulmonary venous congestion, pulmonary hypertension, and eventually right-sided heart failure.

Clinical Signs

• Many horses with mild-moderate MR are asymptomatic
• Exercise intolerance and decreased performance
• Tachypnea, cough, respiratory distress (left-sided CHF)
• Irregular rhythm if atrial fibrillation develops
• Ventral edema, jugular distension (right-sided CHF)

Auscultation Findings

Band-shaped (holosystolic) or crescendo systolic murmur heard best over the left 5th intercostal space at the level of the point of the elbow (mitral valve area/apex). The murmur may radiate dorsally and caudally. Mild MR may produce a mid-to-late systolic crescendo murmur.

Echocardiographic Findings

• 2D findings: Valve thickening, nodules, prolapse, flail leaflet, ruptured chordae
• Chamber changes: LA dilation (greater than 13.5 cm in 450-500 kg horse), LV dilation, rounding of LV apex
• Color-flow Doppler: Regurgitant jet into LA during systole; jet size correlates with severity
• M-mode: Increased fractional shortening (early compensation) or normal/decreased (decompensation)
• PA dilation: PA diameter greater than aortic diameter indicates severe left heart failure

Aortic Valve Regurgitation (AR)

Aortic regurgitation is the most common cause of diastolic murmurs in horses. The prevalence is estimated at 2-8%, with a median age of affected horses around 14 years. Most horses develop AR in middle age (10+ years) due to degenerative changes.

Etiology

• Degenerative valve disease: Most common; nodules, fenestrations, fibrosis of cusps
• Aortic valve prolapse: Common finding; one or more cusps prolapse into LV during diastole
• Bacterial endocarditis: Vegetative lesions
• Aortic root rupture/aneurysm: Congenital weakness of sinus of Valsalva - can be catastrophic
• Ventricular septal defect: Associated with prolapse of right coronary cusp

Pathophysiology

During diastole, blood regurgitates from the aorta back into the LV, causing LV volume overload and progressive eccentric hypertrophy. Diastolic runoff decreases aortic diastolic pressure, creating a widened pulse pressure and characteristic "bounding" or "water-hammer" pulse. LV dilation can stretch the mitral annulus, leading to secondary mitral regurgitation, which significantly worsens prognosis.

Clinical Signs

• Often asymptomatic despite loud murmur
• Hyperkinetic/bounding ("water-hammer") pulse: Due to widened pulse pressure
• Exercise intolerance (more common than with MR alone)
• Signs of left-sided CHF in severe cases
• At risk for ventricular arrhythmias

Auscultation Findings

Holodiastolic decrescendo murmur often described as "musical" or "honking" in character. Best heard over the left heart base (4th intercostal space), radiating dorsally and caudally. May also be heard on right side. The severity of regurgitation correlates better with pulse pressure than murmur intensity.

Echocardiographic Findings

• 2D findings: Valve thickening, nodules on cusp edges, valve prolapse, aortic root dilation
• Chamber changes: Progressive LV dilation, LA dilation if secondary MR develops
• Color-flow Doppler: Regurgitant jet into LV during diastole; jet width at vena contracta indicates severity
• M-mode: High-frequency diastolic flutter of anterior mitral valve leaflet (from regurgitant jet impact)
• Severe AR indicator: Premature mitral valve closure indicates markedly elevated LV end-diastolic pressure

Tricuspid Valve Regurgitation (TR)

Tricuspid regurgitation is commonly detected as an incidental finding during echocardiography. Mild, physiological TR is present in many clinically normal horses. Clinically significant TR is less common but can occur with several conditions.

Etiology

• Physiological/functional: Common in Standardbred racehorses due to exercise-induced annular dilation
• Secondary to pulmonary hypertension: Most often from chronic left heart failure
• Bacterial endocarditis: Often associated with jugular thrombophlebitis
• Ruptured chordae tendineae
• Cardiomyopathy/myocarditis

Clinical Signs

• Often asymptomatic with mild-moderate TR
• Jugular pulse (pulsation): Classic sign of significant TR
• Ventral/dependent edema (brisket, ventral abdomen, limbs)
• Ascites, hepatomegaly
• Exercise intolerance

Auscultation Findings

Band-shaped (holosystolic), coarse or blowing systolic murmur heard best over the right 4th intercostal space (tricuspid valve area), radiating dorsally. May be pansystolic with a coarse quality.

Pulmonic Valve Regurgitation (PR)

Clinically significant pulmonic valve disease is extremely rare in horses. Mild, physiological PR is commonly detected with color-flow Doppler in asymptomatic horses and is considered clinically insignificant.

Etiology

• Secondary to pulmonary hypertension: Most common cause of significant PR (from severe left heart failure)
• Bacterial endocarditis: Exceedingly rare at this valve
• Pulmonic valve rupture: Very rare

Summary: Valvular Lesions Comparison

Bacterial Endocarditis

Bacterial endocarditis is an uncommon but life-threatening infection of the heart valves characterized by vegetative lesions. In horses, the aortic and mitral valves are most commonly affected. Most affected horses are less than 3 years old. The prognosis is generally poor.

Common Bacterial Isolates

• Actinobacillus equuli: Most common
• Streptococcus zooepidemicus: Very common
• Staphylococcus spp.
• Other: Escherichia coli, Pseudomonas spp., Corynebacterium spp.

Risk Factors

• Jugular thrombophlebitis: Most commonly reported predisposing factor for tricuspid valve endocarditis
• Previous umbilical infection (foals)
• Pre-existing valve damage
• Immunocompromise

Clinical Signs

• Fever (intermittent or persistent): Key presenting sign
• New heart murmur or changing murmur character
• Shifting limb lameness: Due to septic emboli to joints
• Tachycardia
• Weight loss, depression, lethargy
• Signs of metastatic infection (nephritis, myocardial abscess, septic arthritis)

Diagnosis

• CBC: Leukocytosis, hyperfibrinogenemia, hyperglobulinemia
• Blood culture: Gold standard; negative culture does not rule out endocarditis
• Echocardiography: Oscillating vegetations on valve, valve thickening, regurgitation; most specific diagnostic tool

Treatment

• Long-term antibiotics: 4-6 weeks minimum; ideally based on culture and sensitivity
• Empirical choice: Penicillin + aminoglycoside initially pending culture results
• Supportive care for cardiac dysfunction (diuretics, ACE inhibitors if in CHF)

Prognosis

Generally poor. Even if bacteremia is cleared, permanent valvular damage often leads to progressive heart failure. Aortic valve involvement carries the worst prognosis. Mitral and tricuspid involvement may have slightly better outcomes if caught early.

Exam Focus: For NAVLE - When you see a young horse (less than 3 years) with FEVER + NEW HEART MURMUR + SHIFTING LAMENESS, think bacterial endocarditis immediately. Blood culture and echocardiography are the key diagnostics. Actinobacillus equuli and Streptococcus spp. are the most common isolates.

Treatment of Valvular Disease

There is no curative treatment for degenerative valvular disease in horses. Management focuses on monitoring disease progression and treating congestive heart failure when it develops.

Pharmacologic Management of Equine CHF

Prognosis and Safety for Athletic Use

Poor Prognostic Indicators

General Recommendations

• Mild valvular disease: Generally safe for athletic use; annual monitoring recommended
• Moderate valvular disease: Low-level exercise may be appropriate; avoid intense exercise; monitor every 6 months
• Severe valvular disease: Not safe to ride; risk of sudden death; horses with severe AR should not carry riders