Infective Endocarditis

Etiology

• Infective endocarditis (IE) is presumed to result from injury to the valvular endothelium or endocardium, exposing subendothelial collagen to which platelets, fibrin, and eventually bacteria adhere.
• Native valve infective endocarditis (NVIE) is usually caused by gram-positive cocci. S. aureus is the most common pathogen followed by viridans group streptococci, enterococci, and coagulase-negative staphylococci. Increasing rates of S. aureus bacteremia have contributed to a rising incidence of acute bacterial endocarditis (ABE) and health care–associated endocarditis (related to IV catheters and invasive procedures).
• Enterococcus species cause 5%–10% of cases of subacute bacterial endocarditis (SBE).
• Bacteremia from distant foci of infection or dental procedures are frequent seeding events.
• Streptococcus gallolyticus (former S. bovis) bacteremia and endocarditis are associated with lower gastrointestinal tract disease, including neoplasms. Groups B and G streptococcal endocarditis may also be associated with large intestinal pathology.
• Gram-negative and fungal IEs occur infrequently and are usually associated with injection drug use or prosthetic heart valves.
• Early prosthetic valve endocarditis (PVE) (within the first year of surgery) commonly occurs in the first 2 months and is typically caused by S. aureus, coagulase-negative staphylococci, gram-negative bacilli, and Candida spp. Late-onset PVE is caused by the same organisms seen in NVIE.
• Coagulase-negative staphylococci (e.g., Staphylococcus epidermidis) primarily occur in patients with prosthetic heart valves, although NVIE is increasing, particularly in healthcare settings. Staphylococcus lugdunensis is associated with a high rate of perivalvular extension and metastatic spread, resembling S. aureus clinically.
• HACEK is an acronym for a group of fastidious, slow-growing, gram-negative bacteria (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella species) that account for 5%–10% of community-acquired cases of IE.

Risk Factors

Acquired structural heart disease (e.g., degenerative valve disease, rheumatic heart disease), congenital disease (e.g., bicuspid aortic valve, ventricular septal defect), injection drug use, prosthetic heart valves, intravascular devices, chronic hemodialysis, and a prior history of endocarditis are predisposing factors for endocarditis.