Infective Endocarditis
Infective endocarditis (IE) is an infection of the endocardium — the inner lining of the heart — that almost always settles on the heart valves. It is one of the few infections where a smouldering fever and a new heart murmur can, if missed, kill a patient within weeks through valve destruction, heart failure, or a shower of septic emboli to the brain. What makes it fascinating and dangerous is that the infection hides inside a fibrin-platelet clot where antibiotics and immune cells struggle to reach, which is why treatment is measured in weeks, not days.
For a student, endocarditis is the perfect stress-test of clinical reasoning: it links microbiology, cardiology, immunology, and pharmacology, and it rewards the doctor who thinks "could this fever be coming from a valve?" before the complications arrive. This page teaches the disease the way an infectious-disease physician actually reasons through it at the bedside.
Learning Objectives
- Explain how a sterile clot on a damaged valve becomes an infected vegetation.
- Identify the major causative organisms and the clinical settings that predict each one.
- Apply the modified Duke criteria to make or reject the diagnosis.
- Recognize the classic peripheral and embolic signs and their pathophysiology.
- Outline the principles of prolonged bactericidal therapy and the indications for surgery.
- Know who genuinely needs antibiotic prophylaxis and who does not.
Quick Answer
Infective endocarditis is infection of a heart valve, usually bacterial. It begins when turbulent blood flow or valve damage lets platelets and fibrin form a small sterile clot, which is then seeded by bacteria during a bout of bacteraemia (from teeth, skin, gut, or an intravenous line). The organism multiplies inside this vegetation, protected from immune clearance. The patient presents with fever, a new or changing murmur, and features of embolism or immune complex deposition. Diagnosis rests on blood cultures plus echocardiography, formalised in the modified Duke criteria. Treatment requires several weeks of intravenous bactericidal antibiotics, tailored to the organism, and surgery in about half of cases for heart failure, uncontrolled infection, or recurrent emboli. Prognosis depends heavily on how early it is caught.
Where It Came From
Physicians described the disease long before they understood it. In 1885 William Osler delivered the landmark Gulstonian Lectures on "malignant endocarditis," pulling together autopsy findings, the peripheral signs, and the relentless clinical course into the first coherent picture of the disease. Several eponymous signs — Osler's nodes among them — still carry his name. Before antibiotics, endocarditis was essentially uniformly fatal; the vegetation was a sanctuary no drug could enter.
The real need that drove the field was this sanctuary problem. Even after penicillin arrived in the 1940s, doctors found that ordinary courses failed and the infection relapsed. The insight was that bacteria buried deep in an avascular, matrix-rich vegetation reach enormous densities and enter a slow-growing, metabolically quiet state where bacteriostatic drugs are useless. This is why the specialty settled on prolonged, high-dose, bactericidal therapy — a principle worked out through hard clinical experience of relapses. In 1994 Durack and colleagues at Duke University published diagnostic criteria combining microbiological and echocardiographic evidence, later modified in 2000, because clinicians badly needed a reproducible way to diagnose a disease with such protean presentations. More recently the epidemiology has shifted from young patients with rheumatic valves to older patients with prosthetic valves, degenerative disease, intracardiac devices, and injection drug use.
How a Vegetation Forms: The Pathogenesis
The central lesion is the vegetation — a mass of platelets, fibrin, microorganisms, and inflammatory cells stuck to a valve. Its formation is a two-step story.
First, the endothelial surface must be disrupted. Turbulent flow across an abnormal valve, a jet lesion, an indwelling catheter, or degenerative calcification exposes underlying collagen and tissue factor. Platelets and fibrin deposit on this spot, forming a small sterile thrombus called non-bacterial thrombotic endocarditis (NBTE).
Second, bacteria must arrive and stick. Transient bacteraemia is surprisingly common — vigorous tooth-brushing, chewing, or a dental extraction releases oral streptococci into the blood many times a week. If circulating organisms possess adhesins (such as the surface proteins of Streptococcus and Staphylococcus), they bind to the fibrin-platelet nidus. Once attached, they trigger more platelet aggregation and wrap themselves in a growing fibrin coat. Inside this shell they reach densities of 10^9 to 10^11 organisms per gram, protected from neutrophils and antibody, and shielded by poor blood supply from antibiotic penetration.
This explains everything downstream. Valve tissue is progressively destroyed, causing regurgitation and heart failure. Fragments break off as septic emboli, lodging in the brain, spleen, kidneys, or (in right-sided disease) the lungs. Circulating immune complexes deposit in small vessels, producing glomerulonephritis and some of the classic skin signs.
A worked clinical vignette
A 62-year-old man with a bioprosthetic aortic valve presents with three weeks of low-grade fever, night sweats, fatigue, and a poor appetite. On examination he has a new diastolic murmur, a tender pulp in one fingertip, and small painless red spots on his palms. Three sets of blood cultures grow Streptococcus gordonii (a viridans streptococcus). Transoesophageal echocardiography shows a 12 mm vegetation on the prosthetic aortic valve with early dehiscence.
Reasoning: subacute course plus viridans streptococci plus a vegetation on a prosthesis. He meets the Duke criteria (two major criteria: typical organism in persistently positive cultures, and echocardiographic evidence). He needs several weeks of intravenous therapy and early surgical review because prosthetic valve involvement with dehiscence carries high risk.
The Causative Organisms and What They Tell You
The organism is a clue to the source and the tempo of disease.
- Viridans group streptococci (for example S. sanguinis, S. mitis) come from the mouth and cause the classic subacute picture — indolent fever over weeks on a previously abnormal valve. Historically the most common cause of community-acquired native-valve IE.
- Staphylococcus aureus causes acute, aggressive disease that can destroy a normal valve in days. It is the leading cause overall in many modern series, strongly associated with injection drug use, intravascular catheters, and healthcare contact. In injection drug users it classically hits the tricuspid (right-sided) valve, causing septic pulmonary emboli.
- Enterococci (E. faecalis) arise from the gastrointestinal or genitourinary tract, often in older men after urological procedures.
- Coagulase-negative staphylococci (for example S. epidermidis) are the leading cause of early prosthetic valve endocarditis, seeded at surgery.
- Streptococcus gallolyticus (formerly S. bovis) is famously linked to colonic neoplasia — its presence should trigger a colonoscopy.
- The HACEK group (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) are fastidious Gram-negative oral organisms causing culture-negative or slow-growing IE.
- Culture-negative endocarditis (around 5 to 10 percent) often reflects prior antibiotics or fastidious organisms such as Coxiella burnetii (Q fever) and Bartonella, needing serology and molecular testing.
Making the Diagnosis: The Modified Duke Criteria
The Duke criteria combine two major and several minor items. A definite clinical diagnosis needs two major, or one major plus three minor, or five minor criteria.
Major criteria:
- Positive blood cultures — a typical organism from two separate cultures, or persistently positive cultures with a compatible organism, or a single positive for Coxiella burnetii.
- Evidence of endocardial involvement — echocardiographic vegetation, abscess, or new prosthetic valve dehiscence, or a new valvular regurgitation.
Minor criteria: predisposing heart condition or injection drug use; fever above 38 C; vascular phenomena (arterial emboli, septic pulmonary infarcts, Janeway lesions, conjunctival haemorrhage); immunologic phenomena (glomerulonephritis, Osler's nodes, Roth spots, rheumatoid factor); and microbiological evidence not meeting a major criterion.
The practical backbone of diagnosis is three sets of blood cultures drawn from separate sites before antibiotics, plus echocardiography. Transthoracic echo (TTE) is the first test; transoesophageal echo (TOE) is far more sensitive and is needed for prosthetic valves, suspected abscess, or a negative TTE with high suspicion.
The classic peripheral signs
| Sign | What it is | Mechanism |
|---|---|---|
| Osler's nodes | Painful nodules on finger/toe pulps | Immune complex vasculitis (Osler = Ouch) |
| Janeway lesions | Painless red macules on palms/soles | Septic microemboli |
| Roth spots | Retinal haemorrhages with pale centres | Emboli plus immune reaction |
| Splinter haemorrhages | Linear marks under nails | Microemboli to nailbed vessels |
| Splenomegaly, clubbing | Chronic signs | Long-standing subacute infection |
Real-World Applications
- Any patient with unexplained fever and a prosthetic valve or known valve disease should prompt blood cultures and echocardiography before reflexive antibiotics obscure the diagnosis.
- Staphylococcus aureus bacteraemia is treated as a red flag: guidelines recommend echocardiography in most cases because occult endocarditis changes the treatment duration entirely.
- Injection drug use with fever and pleuritic chest pain suggests right-sided IE with septic pulmonary emboli — a chest picture of multiple peripheral nodules.
- Dental and cardiac surgical teams apply prophylaxis rules to the small group of highest-risk patients.
- Growth of Streptococcus gallolyticus prompts investigation of the colon for cancer.
Common Mistakes
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Starting antibiotics before drawing blood cultures. Because the diagnosis and the entire treatment plan hinge on identifying the organism, even a single dose of antibiotic can render cultures negative and force weeks of guesswork. Unless the patient is septic and unstable, draw three culture sets first. Correction: culture before you treat, from separate venepunctures.
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Assuming a normal transthoracic echo excludes endocarditis. TTE misses many vegetations, especially on prosthetic valves and small lesions. A negative TTE with a compatible clinical picture does not rule out IE. Correction: proceed to transoesophageal echo when suspicion is real.
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Using bacteriostatic or short antibiotic courses. Because organisms sit protected and slow-growing inside the vegetation, only prolonged bactericidal therapy (typically 4 to 6 weeks intravenously) reliably sterilises it. Treating like a simple soft-tissue infection leads to relapse. Correction: give bactericidal drugs for the full guideline-directed duration.
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Over-prescribing prophylaxis. Modern guidelines dramatically narrowed prophylaxis to only the highest-risk patients undergoing specific procedures, because the everyday bacteraemia from chewing and brushing dwarfs the risk from a single dental visit. Correction: reserve prophylaxis for the defined high-risk groups.
Comparison and Connections
| Feature | Acute IE | Subacute IE |
|---|---|---|
| Typical organism | Staphylococcus aureus | Viridans streptococci, enterococci |
| Valve | Often normal | Usually abnormal/damaged |
| Tempo | Days | Weeks to months |
| Peripheral signs | Fewer (too fast) | Classic Osler's/Roth/Janeway |
| Toxicity | High, rapid failure | Indolent, constitutional |
It helps to separate IE from non-bacterial thrombotic endocarditis (sterile vegetations in cancer or lupus — the same clot without infection) and from acute rheumatic carditis (immune valve inflammation after streptococcal pharyngitis, not a valve infection). All three damage valves but by different mechanisms. IE also connects to sepsis (see Critical Care Medicine) and to the antimicrobial reasoning taught in Pharmacology and Microbiology.
Practice Questions
Recall
Q: Name the two major modified Duke criteria. A: (1) Positive blood cultures with a typical organism (or persistently positive cultures / a single positive for Coxiella burnetii), and (2) evidence of endocardial involvement on echocardiography (vegetation, abscess, or new prosthetic dehiscence) or new valvular regurgitation.
Understanding
Q: Why must endocarditis be treated with weeks of bactericidal antibiotics rather than a short course? A: Bacteria inside the vegetation reach very high densities in an avascular, fibrin-rich matrix with poor blood supply and are metabolically slow. Antibiotics penetrate poorly and immune cells cannot reach them, so only prolonged, high-dose bactericidal therapy sterilises the lesion and prevents relapse.
Application
Q: A young man who injects drugs has fever, pleuritic chest pain, and multiple peripheral nodular opacities on chest imaging. Which valve and organism do you suspect? A: Right-sided (tricuspid) endocarditis, most likely Staphylococcus aureus, with septic pulmonary emboli producing the lung lesions.
Analysis
Q: Blood cultures grow Streptococcus gallolyticus (bovis). Beyond treating the valve, what further step is essential and why? A: Colonoscopy. This organism is strongly associated with colorectal neoplasia, which acts as the portal of entry, so the underlying colonic lesion must be sought and managed.
FAQ
Is a heart murmur always present in endocarditis? No. A new or changing murmur is a classic clue, but early or right-sided disease may have no audible murmur, so its absence never excludes the diagnosis.
How many blood cultures do I really need? Three sets from separate venepuncture sites, ideally spaced out and before antibiotics. This maximises the chance of catching the continuous low-grade bacteraemia and helps distinguish true infection from contamination.
Why does staph endocarditis behave so differently from strep endocarditis? Staphylococcus aureus is highly virulent, produces tissue-destroying toxins, and can invade normal valves rapidly, causing acute disease. Viridans streptococci are less aggressive and typically seed already-damaged valves, giving the slow subacute picture.
When is surgery needed? The main indications are heart failure from valve destruction, uncontrolled infection (persistent bacteraemia or abscess despite antibiotics), and prevention of embolism from large or mobile vegetations. Roughly half of patients require surgery.
Who actually needs antibiotic prophylaxis before dental work? Only the highest-risk patients — those with prosthetic valves or prosthetic repair material, previous endocarditis, certain congenital heart defects, or valve disease after a heart transplant — and only for procedures involving manipulation of gingival or periapical tissue. Most people with murmurs do not need it.
Quick Revision
- IE is infection of a heart valve; a vegetation of fibrin, platelets, and bacteria is the core lesion.
- Two steps: valve damage forms a sterile clot (NBTE), then bacteraemia seeds it.
- Viridans strep = subacute on damaged valves; S. aureus = acute, can hit normal valves and the tricuspid in injection drug use.
- Diagnose with three blood culture sets plus echo, formalised by the modified Duke criteria; TOE beats TTE.
- Osler's nodes hurt (immune); Janeway lesions do not (emboli).
- Treat with 4 to 6 weeks of intravenous bactericidal antibiotics; surgery for heart failure, uncontrolled infection, or recurrent emboli.
- Prophylaxis is only for the highest-risk patients and specific procedures.