What is the most likely diagnosis and causative organism in an intravenous drug user (IVDU) with fever, new holosystolic murmur, and septic embolism on chest X-ray?

Infective Endocarditis in IVDU: Tricuspid Valve with Staphylococcus aureus

This clinical presentation is classic for right-sided infective endocarditis (IE) affecting the tricuspid valve, with Staphylococcus aureus as the causative organism. 1

Most Likely Valve Involved

The tricuspid valve is overwhelmingly the most affected valve in intravenous drug users with IE:

• Tricuspid valve involvement occurs in 73% of IE cases in IVDUs (either alone or in combination with other valves), making it the predominant site of infection 1
• Left-sided valves (aortic alone 7%, mitral alone 6%) are far less common in this population 1
• The presence of septic pulmonary emboli on chest X-ray strongly supports tricuspid valve involvement, as these emboli originate from right-sided vegetations traveling through the pulmonary circulation 1
• Roentgenographic evidence of septic pulmonary emboli is present in 87% of tricuspid valve IE cases 1

Important clinical caveat: Only 35% of IVDUs with IE demonstrate heart murmurs on admission, and only one-third show signs of tricuspid insufficiency (systolic regurgitant murmur louder with inspiration) 1. The presence of a new holosystolic murmur in this case is therefore a significant finding that should heighten suspicion.

Most Likely Causative Organism

Staphylococcus aureus is definitively the most common pathogen:

• S. aureus accounts for 60-90% of right-sided IE cases in IVDUs 1, 2
• In the most recent large series, S. aureus caused 60.8% of IE cases in IVDUs 1
• S. aureus is specifically identified as the most frequent pathogen in 80% of tricuspid valve infections 1
• Methicillin-resistant S. aureus (MRSA) is becoming increasingly prevalent, particularly in patients with previous hospitalizations, long-term addiction, and prior antibiotic use 1

Alternative Organisms to Consider (Less Likely)

While S. aureus predominates, IVDUs are at increased risk for unusual pathogens 1:

• Pseudomonas aeruginosa (13.5% in recent series) - particularly in pentazocine addicts, where antipseudomonal coverage should be added 1, 3
• Streptococci (16.2%) - more common in left-sided IE 1
• Candida species - especially in those using brown heroin dissolved in lemon juice, requiring antifungal therapy 1, 3
• Polymicrobial infections (8.1%) 1

Clinical Presentation Supporting This Diagnosis

The triad of fever, new holosystolic murmur, and septic pulmonary emboli on chest X-ray is pathognomonic for tricuspid valve endocarditis 1:

• Pulmonary findings dominate the clinical picture in tricuspid valve infection, with 30% experiencing pleuritic chest pain 1
• Chest roentgenogram abnormalities (infiltrates, effusion) occur in 75-85% of cases 1
• The most reliable predictors of IE in febrile IVDUs are visualization of vegetations by echocardiography and the presence of embolic phenomena 1

Diagnostic Approach

Immediate diagnostic steps should include:

• Obtain 3 sets of blood cultures from separate venipunctures before initiating antibiotics 1
• Transthoracic echocardiography (TTE) as first-line imaging - TTE is particularly valuable in right-sided IE due to the anterior location of the tricuspid valve and typically large vegetations 1
• Transesophageal echocardiography (TEE) if TTE is nondiagnostic or to assess for left-sided involvement 1
• Apply modified Duke criteria for definitive diagnosis 1

Empiric Antibiotic Coverage

Start empiric therapy immediately after blood cultures targeting S. aureus:

• Use vancomycin or daptomycin if local MRSA prevalence is high 1
• Add gentamicin initially (though may be discontinued after 3-5 days for uncomplicated cases) 1, 2
• If pentazocine use is suspected, add antipseudomonal coverage 3
• If brown heroin dissolved in lemon juice is used, add antifungal therapy for Candida species (not C. albicans) 1, 3

Once S. aureus is confirmed and susceptibilities known, uncomplicated right-sided IE can be treated with a 2-week course of nafcillin or cloxacillin plus aminoglycoside 1, 2