# Infective Endocarditis - Diagnosis and Management

## Introduction

### Epidemiology

Infective endocarditis has an incidence of 3 to 15 per 100,000 person-years, and this incidence is increasing due to the aging population, the growing prevalence of prosthetic valves and cardiac devices, the expanding hemodialysis population, and the ongoing epidemic of injection drug use. In-hospital mortality remains devastating at 15 to 30 percent, with one-year mortality reaching 30 to 40 percent despite advances in diagnostic imaging and surgical techniques.

The microbiology of endocarditis has shifted substantially over recent decades. Staphylococcus aureus has surpassed viridans streptococci as the most common cause of infective endocarditis in developed countries, accounting for 30 to 40 percent of cases. Injection drug use-related endocarditis is predominantly right-sided, involving the tricuspid valve, with S. aureus responsible for more than 60 percent of cases. This epidemic parallels and is directly driven by the ongoing opioid crisis.

### Microbiology by Clinical Context

The differential microbiology of endocarditis varies predictably with the clinical context. Native valve community-acquired endocarditis is most commonly caused by S. aureus, viridans streptococci, and Enterococcus species. In injection drug users, S. aureus predominates at over 60 percent, followed by streptococci, Pseudomonas, and Candida. Early prosthetic valve endocarditis, occurring within 60 days of surgery, is most commonly caused by S. aureus, coagulase-negative staphylococci, gram-negatives, and Candida, while late prosthetic valve endocarditis mirrors the microbiology of native valve disease. Culture-negative endocarditis, which accounts for 5 to 10 percent of cases, may be caused by HACEK organisms, Coxiella burnetii, Bartonella, Brucella, Tropheryma whipplei, or fungi, though prior antibiotic administration remains the most common explanation for negative cultures.

| Clinical Context | Most Common Organisms | Key Features |
|---|---|---|
| Native valve, community-acquired | S. aureus (30-40%), viridans streptococci, Enterococcus | S. aureus now #1 in developed countries |
| Injection drug use | S. aureus (>60%), streptococci, Pseudomonas, Candida | Predominantly right-sided (tricuspid) |
| Early PVE (<60 days post-op) | S. aureus, CoNS, gram-negatives, Candida | Healthcare-associated flora |
| Late PVE (>60 days post-op) | Same as native valve disease | Mirrors community-acquired microbiology |
| Culture-negative (5-10%) | HACEK, Coxiella, Bartonella, Brucella, T. whipplei, fungi | Prior antibiotics most common cause of negative cultures |

## Diagnosis

### Modified Duke Criteria (2023 ESC Updated Criteria)

The diagnostic framework for infective endocarditis relies on the Modified Duke Criteria, which were updated in the 2023 ESC guidelines with the incorporation of advanced imaging modalities. Definite endocarditis requires 2 major criteria, or 1 major plus 3 minor criteria, or 5 minor criteria. Possible endocarditis is diagnosed with 1 major plus 1 minor criterion, or 3 minor criteria.

### Major Criteria

The major criteria encompass blood culture and imaging evidence. Blood culture criteria are met by the isolation of a typical organism from two separate blood cultures, including S. aureus, viridans streptococci, S. gallolyticus, HACEK organisms, or E. faecalis without a primary focus. Alternatively, persistently positive cultures defined as two cultures drawn more than 12 hours apart, or three of three cultures positive, or the majority of four or more cultures, satisfy this criterion.

Imaging evidence of endocardial involvement has been expanded beyond echocardiography. TTE or TEE findings of vegetations, abscess, pseudoaneurysm, perforation, or new dehiscence of a prosthetic valve constitute a major criterion. Cardiac CT demonstrating paravalvular lesions is now included. Additionally, 18F-FDG PET/CT showing abnormal metabolic activity around a prosthetic valve implanted more than 3 months earlier is recognized as a major criterion, reflecting the growing role of nuclear imaging in this disease.

### Minor Criteria

Minor criteria include a predisposing condition such as prosthetic valve, injection drug use, or structural heart disease; fever of 38.0 degrees Celsius or higher; vascular phenomena including arterial emboli, septic pulmonary infarcts, mycotic aneurysm, Janeway lesions, and conjunctival hemorrhage; immunologic phenomena including Osler nodes, Roth spots, glomerulonephritis, and positive rheumatoid factor; and microbiologic evidence that does not meet major criteria.

### Imaging Approach

The initial imaging study should be transthoracic echocardiography, which has a sensitivity of 50 to 60 percent for native valve vegetations and 30 to 40 percent for prosthetic valve disease, with specificity of approximately 95 percent. TEE is recommended when TTE is negative but clinical suspicion persists, in all prosthetic valve cases, and in complicated IE, achieving sensitivity exceeding 95 percent for native valve and 85 to 95 percent for prosthetic valve disease.

Cardiac CT has an emerging role in detecting paravalvular complications and demonstrates performance comparable to TEE for abscess detection. 18F-FDG PET/CT offers its greatest utility in prosthetic valve and cardiac implantable electronic device-related endocarditis, with the additional advantage of detecting metastatic infections and embolic events throughout the body. Cerebral MRI demonstrates subclinical cerebral lesions in 60 to 80 percent of patients with left-sided IE and should be considered in all such cases, as the detection of cerebral emboli influences surgical timing decisions.

<image>A diagnostic imaging comparison panel showing four imaging modalities for endocarditis. Panel 1: "TTE" showing a parasternal long-axis echocardiographic view with a vegetation on the mitral valve, labeled with sensitivity and specificity values. Panel 2: "TEE" showing a clearer view of a vegetation with an adjacent abscess cavity. Panel 3: "Cardiac CT" showing a prosthetic valve with paravalvular abscess in axial cross-section. Panel 4: "18F-FDG PET/CT" showing focal uptake around a prosthetic aortic valve (fused PET/CT image). Each panel should include sensitivity/specificity data and key indications. Use a medical imaging style with appropriate grayscale/color conventions for each modality.</image>

## Antimicrobial Therapy

### Native Valve - MSSA

Treatment of native valve MSSA endocarditis consists of nafcillin or cefazolin 2 grams intravenously every 8 hours for 6 weeks. Cefazolin is increasingly preferred based on FIRST trial data demonstrating non-inferiority with superior tolerability. Gentamicin adjunct therapy has been removed from current guidelines because it causes nephrotoxicity without providing a mortality benefit. Rifampin has no role in native valve endocarditis, as conclusively demonstrated by the ARREST trial.

### Native Valve - MRSA

MRSA native valve endocarditis is treated with vancomycin dosed to an AUC/MIC target of 400 to 600 for 6 weeks, or with daptomycin at 8 to 10 mg/kg intravenously daily for 6 weeks. Higher daptomycin doses are preferred for endocarditis given the need for maximal bactericidal activity. For persistent bacteremia, the daptomycin plus ceftaroline combination offers synergistic killing.

### Native Valve - Viridans Streptococci

Penicillin-susceptible viridans streptococci (MIC 0.12 or below) can be treated with penicillin G or ceftriaxone 2 grams intravenously every 24 hours for 4 weeks, or with a shorter 2-week course of ceftriaxone plus gentamicin. Relatively resistant strains (MIC 0.12 to 0.5) require ceftriaxone for 4 weeks plus gentamicin for the initial 2 weeks. S. gallolyticus endocarditis warrants mandatory colonoscopy, as 25 to 50 percent of these patients harbor colonic neoplasia.

### Native Valve - Enterococcus

The preferred regimen for ampicillin-susceptible enterococcal endocarditis is ampicillin 2 grams intravenously every 4 hours plus ceftriaxone 2 grams intravenously every 12 hours for 6 weeks. This double beta-lactam synergy regimen, established by the GAME cohort study, has supplanted the traditional ampicillin plus gentamicin combination by demonstrating equivalent efficacy with dramatically less nephrotoxicity. The synergistic mechanism involves ceftriaxone's saturation of PBP3, which enhances ampicillin's binding to PBP4 and PBP5 in enterococci. For VRE endocarditis, treatment options include daptomycin at 8 to 10 mg/kg with or without ampicillin (if ampicillin-susceptible), or linezolid, though linezolid data are limited and it is bacteriostatic.

### Prosthetic Valve Endocarditis (PVE)

Staphylococcal PVE requires a triple-drug regimen: vancomycin or nafcillin (based on susceptibility) plus rifampin 300 mg orally every 8 hours, added after 3 to 5 days of documented bacteremia clearance, plus gentamicin 1 mg/kg every 8 hours for the initial 2 weeks, with a total treatment duration of at least 6 weeks. Rifampin is specifically indicated in PVE because of its ability to penetrate the biofilm that forms on prosthetic material. Surgical consultation is mandatory for all cases of prosthetic valve endocarditis.

| Organism/Scenario | Regimen | Duration | Key Notes |
|---|---|---|---|
| NVE - MSSA | Cefazolin 2g IV q8h (or nafcillin 2g IV q4h) | 6 weeks | No gentamicin; no rifampin for native valve |
| NVE - MRSA | Vancomycin (AUC/MIC 400-600) or daptomycin 8-10 mg/kg IV daily | 6 weeks | Dapto + ceftaroline for persistent bacteremia |
| NVE - Viridans strep (MIC ≤0.12) | Ceftriaxone 2g IV q24h | 4 weeks (or 2 weeks with gentamicin) | S. gallolyticus: mandatory colonoscopy |
| NVE - Viridans strep (MIC 0.12-0.5) | Ceftriaxone 4 weeks + gentamicin 2 weeks | 4 weeks | Relatively resistant strains |
| NVE - Enterococcus (amp-susceptible) | Ampicillin 2g IV q4h + ceftriaxone 2g IV q12h | 6 weeks | GAME cohort: less nephrotoxicity than amp + gent |
| VRE endocarditis | Daptomycin 8-10 mg/kg ± ampicillin (if susceptible), or linezolid | 6 weeks | Limited data for linezolid; bacteriostatic |
| PVE - Staphylococcal | Vancomycin or nafcillin + rifampin (after clearance) + gentamicin (first 2 weeks) | ≥6 weeks | Rifampin added after 3-5 days of culture clearance |

### POET Trial (2019) and Oral Step-Down

The Partial Oral Treatment of Endocarditis (POET) trial was a landmark randomized trial demonstrating that oral step-down therapy after at least 10 days of intravenous treatment was non-inferior to completion of intravenous therapy for left-sided endocarditis. The trial applied strict eligibility criteria: patients had to be clinically stable with no abscess, a known susceptible organism, and an adequate oral regimen. Oral regimens used included amoxicillin or linezolid for staphylococci, amoxicillin for streptococci and enterococci, combined with a second agent such as rifampin or moxifloxacin. The POET findings are not applicable to prosthetic valve endocarditis within the first year, S. aureus endocarditis with high-risk features, or patients who cannot be reliably followed.

<image>A comprehensive management algorithm for infective endocarditis. Start with "Suspected IE: Blood cultures x3, TTE." Branch based on TTE results: "Vegetation seen" leads to "Definite IE - begin empiric therapy based on clinical context." "TTE negative but high suspicion" leads to "TEE" then either "Positive" (confirm IE) or "Negative" (reconsider diagnosis or repeat in 5-7 days). After confirmed IE, show parallel pathways: "Organism Identification" (with branches for MSSA, MRSA, Streptococcus, Enterococcus, HACEK, Culture-negative), "Surgical Evaluation" (with Class I indications listed), and "Metastatic Workup" (brain MRI, CT chest/abdomen/pelvis, spine MRI if back pain). Include a timeline bar at the bottom showing IV therapy duration (6 weeks) with an option for "POET-eligible patients: oral step-down after ≥10 days IV." Use a multi-level clinical algorithm format with color coding by organism.</image>

## Surgical Indications

### AHA/ACC Class I Indications for Surgery

The most common indication for surgery in endocarditis is heart failure resulting from valvular dysfunction, including severe regurgitation, obstruction, or fistula formation. Paravalvular abscess, fistula, or heart block from abscess erosion into the conduction system constitutes a clear surgical indication. Persistent bacteremia beyond 5 to 7 days despite appropriate antibiotic therapy and adequate source control mandates surgical evaluation. Fungal or highly resistant organism endocarditis, including MRSA PVE with a poor treatment response and VRE endocarditis, are additional Class I indications. Prosthetic valve dehiscence or obstruction also requires surgery.

### Timing of Surgery

Urgent surgery within days is indicated for heart failure, uncontrolled infection, and abscess formation. The question of surgery for embolic risk based on vegetation size remains controversial; large vegetations exceeding 10 mm with associated embolic events generally warrant surgical intervention, though surgery based on vegetation size alone is debated. When a cerebral embolism has occurred, surgical timing depends on the nature of the stroke: hemorrhagic stroke should prompt a delay of at least 4 weeks when possible, while ischemic stroke without hemorrhagic conversion may permit surgery within 1 to 2 weeks.

## Cardiac Device-Related Infections (CIED-IE)

Cardiac implantable electronic device infections span a spectrum from generator pocket infection (local erythema, warmth, drainage, or erosion) to lead-associated endocarditis with vegetations on leads or valve leaflets and systemic signs of infection. PET/CT is highly useful in differentiating pocket infection from lead endocarditis and in detecting septic emboli.

The management of CIED-IE requires complete device extraction via percutaneous lead extraction, which is mandatory for all cases. Antibiotics are administered for a minimum of 4 to 6 weeks for lead-associated endocarditis (2 weeks if bacteremia occurs without valvular vegetation). Reimplantation timing dictates placement of the new device on the contralateral side after at least 72 hours of documented negative blood cultures, extending to 14 days for S. aureus infections.

## Key Clinical Pearls

- S. aureus bacteremia mandates echocardiography -- TEE is preferred; TTE alone misses ~40% of vegetations
- Ampicillin + ceftriaxone is the preferred regimen for enterococcal endocarditis over ampicillin + gentamicin (GAME cohort data) -- dramatically less nephrotoxicity with equivalent outcomes
- Gentamicin has been removed as adjunct therapy for native valve S. aureus endocarditis -- causes nephrotoxicity without survival benefit
- The POET trial supports oral step-down for select patients with left-sided IE, but strict eligibility criteria must be met
- S. gallolyticus (bovis) endocarditis: always order a colonoscopy -- 25-50% have colonic neoplasia
- PET/CT is transformative for prosthetic valve and CIED-related endocarditis diagnosis
- Complete device extraction is mandatory for CIED-IE -- antibiotics alone without extraction have unacceptable failure rates

## References
1. Baddour LM, Wilson WR, Bayer AS, et al. Infective endocarditis in adults: diagnosis, antimicrobial therapy, and management of complications (AHA Scientific Statement). *Circulation*. 2015;132(15):1435-1486.
2. Iversen K, Ihlemann N, Gill SU, et al. Partial oral versus intravenous antibiotic treatment of endocarditis (POET). *N Engl J Med*. 2019;380(5):415-424.
3. Fernandez-Hidalgo N, Almirante B, Gavalda J, et al. Ampicillin plus ceftriaxone is as effective as ampicillin plus gentamicin for treating Enterococcus faecalis infective endocarditis. *Clin Infect Dis*. 2013;56(9):1261-1268.
4. Habib G, Lancellotti P, Antunes MJ, et al. 2023 ESC Guidelines for the management of endocarditis. *Eur Heart J*. 2023;44(39):3948-4042.
5. Fowler VG, Durack DT, Selton-Suty C, et al. The 2023 Duke-ISCVID criteria for infective endocarditis. *Clin Infect Dis*. 2023;77(4):518-526.