Bit & Bridle

Equine Infectious Endocarditis (IEE)

Other Names

  • Infective Endocarditis (IE)
  • Bacterial endocarditis
  • Valvular endocarditis

What is Equine Infectious Endocarditis?

Equine Infectious Endocarditis (IEE) is an infection of the endocardium — the thin smooth membrane lining the interior of the heart chambers and covering the cardiac valves. It is characterised by the development of vegetative lesions on the valve leaflets or, less commonly, on the mural endocardium (the walls of the heart chambers). These vegetations are masses of fibrin, platelets, inflammatory cells, and bacteria that attach to the valve surface and progressively destroy its architecture.

The condition is uncommon in horses compared with dogs and humans, but when it does occur it is a serious and often life-threatening diagnosis. The mitral valve and the aortic valve are most frequently affected, though any valve can be involved. Tricuspid or pulmonic valve endocarditis does occur and may be associated with contamination following intravenous catheterisation or injection.

Because the infected vegetations shed bacteria and emboli into the bloodstream, infectious endocarditis frequently causes systemic signs of infection alongside cardiac dysfunction. The combination of cardiac and systemic disease makes it one of the more complex conditions in equine internal medicine.

Causes

Infectious endocarditis arises when bacteria establish themselves on the endocardial surface, particularly on valve leaflets. Predisposing factors include:

  • Bacteraemia: Transient episodes of bacteria entering the bloodstream — from distant infections (dental disease, respiratory infection, skin infections, gastrointestinal sepsis) or from procedural contamination (intravenous catheter infection, perivascular injection) — are the primary route by which bacteria reach the heart
  • Pre-existing valve disease: Damaged or structurally abnormal valves provide a surface that bacteria can more easily colonise
  • Intravenous catheterisation: Prolonged IV catheter use is a well-recognised risk factor, particularly for right-sided endocarditis involving the tricuspid valve
  • Common bacterial species: Streptococcus equi, Staphylococcus aureus, Actinobacillus equuli, and various gram-negative organisms are the most frequently isolated pathogens

Symptoms and Signs

Clinical signs are variable and often non-specific, which contributes to delayed diagnosis:

  • Fever: Often intermittent and may be the most consistent finding — a horse with unexplained recurring fever should prompt cardiac evaluation
  • Weight loss and progressive poor condition
  • Depression and lethargy
  • Exercise intolerance
  • New or changing heart murmur: The development of a new murmur — particularly if regurgitant in character — is a key diagnostic clue in a febrile horse
  • Signs of congestive heart failure: In advanced cases — ventral oedema, elevated jugular venous pressure, pleural effusion, respiratory difficulty
  • Embolic events: Bacterial emboli shed from the vegetations can lodge in the kidneys, lungs, joints, and brain, causing lameness (septic arthritis), respiratory signs, neurological signs, or renal dysfunction
  • Sudden death: May occur from rupture of a vegetation, sudden valve failure, or cardiac arrhythmia

Diagnosis

Diagnosis requires a high index of suspicion in any febrile horse with a murmur or evidence of embolic disease:

  • Echocardiography (cardiac ultrasound): The most important diagnostic tool. Two-dimensional and Doppler echocardiography allows direct visualisation of vegetative lesions on valve leaflets, assessment of valve function, and evaluation of chamber dimensions and cardiac function. Serial examinations track progression.
  • Haematology and biochemistry: Neutrophilia, elevated fibrinogen, elevated serum amyloid A, anaemia of chronic disease, and elevated cardiac troponin are common findings
  • Blood culture: Multiple blood cultures taken during febrile episodes improve the chance of identifying the causative organism. Essential for guiding antibiotic selection. Results may be negative in up to 30–40% of cases despite genuine infection.
  • Auscultation: Careful cardiac auscultation at multiple sites. A new regurgitant murmur in a febrile horse should be considered infectious endocarditis until proven otherwise.
  • Urinalysis: Haematuria or proteinuria may suggest renal embolic disease

Treatment

Treatment requires prolonged antibiotic therapy based on culture and sensitivity results where available:

  • Long-term intravenous antibiotics: The thick fibrin matrix of the vegetation provides a physical barrier to antibiotic penetration. Treatment typically requires 4–8 weeks of intravenous antibiotic therapy, usually with combinations of penicillin and an aminoglycoside (such as gentamicin), guided by culture results.
  • Monitoring: Serial echocardiography to assess vegetation size and valve function. Serial blood work to track inflammatory markers. Renal function monitoring is essential when aminoglycosides are used.
  • Treatment of concurrent infection: Any identified primary source of bacteraemia (dental abscess, septic joint, respiratory infection) must be addressed
  • Heart failure management: Where congestive heart failure is present, diuretics (frusemide) and ACE inhibitors may be used to manage fluid overload
  • Prognosis monitoring: Horses that develop significant valve insufficiency, cardiac arrhythmias, or embolic complications may not be salvageable for athletic function

Surgical treatment (valve replacement) is not feasible in horses with current technology. Medical management is the only option.

Prevention

  • Use IV catheters aseptically and replace them according to recommended protocols — do not leave indwelling catheters in place longer than necessary
  • Treat all sources of bacteraemia promptly — dental disease, skin infections, respiratory infections, and urinary tract infections should not be left untreated
  • Perform jugular vein injections carefully to avoid perivascular contamination and subsequent thrombophlebitis
  • Maintain rigorous hygiene around any procedures involving venous access

Prognosis

The prognosis for equine infectious endocarditis is guarded to poor. A significant proportion of horses diagnosed with the condition die or are euthanased within weeks to months, either from progressive heart failure, embolic complications, or arrhythmia. Horses that survive the acute phase and respond to antibiotic therapy may stabilise, but are likely to have permanent valve damage that limits athletic function. Long-term prognosis for return to any level of performance work is guarded, though some horses manage light use with appropriate monitoring.

When to Call the Vet

  • Any horse with unexplained recurring fever — particularly if associated with weight loss or lethargy
  • Detection of a new heart murmur, especially in a horse that has recently been ill or had an IV catheter
  • A horse showing signs of congestive heart failure — ventral oedema, laboured breathing, jugular distension
  • Sudden onset lameness in one or multiple joints in a febrile horse — this may represent septic arthritis from endocarditic emboli

Frequently Asked Questions

Is infectious endocarditis common in horses?

No — it is relatively uncommon compared with many other cardiac and systemic conditions in horses. However, it is important to recognise because it carries a serious prognosis and can cause sudden deterioration or death if not identified and treated. It should be considered in any horse with unexplained fever and a murmur.

Can a horse survive infectious endocarditis?

Some do, particularly with early aggressive antibiotic treatment. However, long-term prognosis for full athletic recovery is poor in most cases due to the valve damage left behind. Horses treated successfully may lead a comfortable life at light work or as companions, but returning to competition is unlikely in most cases.

How long does antibiotic treatment last for endocarditis?

Treatment is prolonged — typically 4–8 weeks of intravenous antibiotic therapy, which is logistically challenging and expensive in horses. Once clinical improvement is established and inflammatory markers have normalised, some cases can be transitioned to oral antibiotics for the remainder of the course, though this is not always possible depending on the organism involved. See BEVA clinical resources for current treatment guidance.

Can endocarditis spread from horses to humans?

The risk of zoonotic transmission of bacterial endocarditis from horses to humans is extremely low under normal conditions. Standard hygiene precautions when handling a sick horse are appropriate. The bacteria involved are typically those that colonise the horse’s own flora and are not typically spread via casual contact. However, immunocompromised individuals should exercise additional caution around sick animals.