What are the atypical causes of infective endocarditis?

Atypical Causes of Infective Endocarditis

Atypical causes of infective endocarditis include fastidious organisms (HACEK group, Bartonella, Coxiella burnetii, Brucella, Legionella, Mycoplasma, Tropheryma whipplei), fungi (Candida and Aspergillus), nutritionally variant streptococci (Abiotrophia/Granulicatella), non-HACEK Gram-negative bacteria, and rare pathogens like Corynebacterium, Neisseria elongata, and various intracellular bacteria.

Fastidious and Culture-Negative Organisms

The most clinically significant atypical causes are those responsible for blood culture-negative endocarditis (BCNIE). Coxiella burnetii (Q fever) and Bartonella species are the two most prevalent causes of culture-negative IE, with serological testing positive in 47.7% of BCNIE cases, primarily for these two organisms 1. These require specialized diagnostic approaches including serology and PCR rather than routine blood cultures 2.

Key Fastidious Pathogens:

• HACEK group organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella): Gram-negative bacilli requiring prolonged incubation and specialized laboratory notification 2

• Bartonella species: Requires doxycycline plus gentamicin for 2 weeks, with treatment extending 4-6 weeks total 2

• Coxiella burnetii: Demands prolonged therapy with doxycycline plus hydroxychloroquine for >18 months, with treatment success defined as anti-phase I IgG titre <1:200 2

• Brucella species: Treated with triple therapy (doxycycline, cotrimoxazole, rifampin) for ≥3-6 months 2

• Legionella, Mycoplasma, and Tropheryma whipplei (Whipple's disease agent): All require extended doxycycline-based regimens 2, 3

Nutritionally Variant Streptococci

Abiotrophia defectiva represents an underrecognized cause of IE because it fails to grow on routine media like blood agar and MacConkey agar 4. This organism causes more fulminant disease with higher mortality due to frequent embolic episodes and valvular destruction, requiring ceftriaxone or vancomycin combined with gentamicin plus surgical valve replacement 4.

Fungi

Fungal IE occurs predominantly in prosthetic valve endocarditis (PVE), IV drug users, and immunocompromised patients, with Candida and Aspergillus species predominating 2. Aspergillus typically results in culture-negative IE. Mortality exceeds 50%, and treatment mandates combined antifungal therapy plus surgical valve replacement 2.

Non-HACEK Gram-Negative Bacteria

These organisms account for 1.8% of IE cases and include various aerobic Gram-negative bacilli 2. Management requires early surgery plus prolonged therapy (≥6 weeks) with bactericidal combinations of beta-lactams and aminoglycosides, sometimes supplemented with quinolones or cotrimoxazole 2.

Rare and Emerging Pathogens

Recent literature documents increasingly diverse causative agents 5:

• Corynebacterium species: Exceptionally rare, even in patients without conventional risk factors 6
• Neisseria elongata: Part of normal oropharyngeal flora but can cause endocarditis originating from dental sources, predominantly affecting left-sided valves with high embolic risk 7
• Other rare causes: Lactobacillus, Klebsiella, Salmonella, Gemella, Campylobacter, Aeromonas, Yersinia, Nocardia, Pasteurella, Listeria, Erysipelothrix 3

Critical Diagnostic Approach

When evaluating suspected atypical IE:

1. Notify the laboratory immediately if fastidious organisms are suspected, as specialized investigations and prolonged incubation are required 2

2. Obtain three blood culture sets at 30-minute intervals before antibiotics 2

3. Pursue advanced diagnostics: Serology for Coxiella and Bartonella, PCR on valve tissue (identifies pathogens in two-thirds of cases), and specialized staining (Warthin-Starry, Gimenez, PAS) on excised valves 1, 3

4. Evaluate epidemiological factors systematically: Prior infections, antimicrobial exposure, clinical course severity, and extracardiac infection sites 1

5. Consult infectious disease specialists from the Endocarditis Team for all culture-negative cases 1, 2

Common Pitfalls

• Failing to consider atypical pathogens in culture-negative IE leads to delayed diagnosis and increased mortality
• Relying solely on routine blood cultures misses fastidious organisms requiring specialized media
• Not obtaining valve tissue for pathologic examination and PCR when available
• Inadequate treatment duration for intracellular organisms (many require months of therapy)
• Missing the dental source in cases caused by oral flora like Abiotrophia or Neisseria elongata