Although intravenous drug abuse is a recognized risk factor for infectious endocarditis, this complication is not a frequent complication among intravenous drug users. The incidence of
Uncommon Pathogens in Endocarditis of Intravenous Drug Abusers11
Group B Streptococcus (Streptococcus agalactiae) 12 Staphylococcus epidermidis
Gram negative bacteria (Pseudomonas, Serreatia, etc.)
Anaerobic bacteria (Bacteroides, Veillonella, etc.) Fungi (Candida)
infective endocarditis in intravenous drug abusers is estimated at 1.5-2.0 cases per 1000 intravenous drug abusers admitted to the hospital.1 Intravenous drug abusers with infective endocarditis are more likely to be young men (ave. age = 29 years, M:F = 3:1) compared with non-addicts with endocarditis (ave. age = 50, M:F = 2:1).2 The frequency of underlying heart disease in intravenous drug abusers with endocarditis is 26% compared with 60% of non-addicts with endocarditis. In a cohort of 85 intravenous drug abusers, echocardiography failed to detect any valvular vegetation consistent with endocarditis.3 Eight intravenous drug abusers had thickened or redundant leaflets (with or without prolapse) of the mitral, aortic, or tricuspid valve. Focally thickened leaflets of the mitral and tricuspid valves have been reported in other series of asymptomatic intravenous drug abusers who were examined by echocardiography.4 These subtle morphologic abnormalities may be the stratum upon which endocarditis builds. Most researchers agree that endothelial injury or damage initiates fibrin, platelet, and bacterial deposition that produce endocarditis.
In Dressler and Robert's series of 80 autopsied intravenous drug abusers with infective endocarditis, the tricuspid valve was involved in half of the victims compared with 15% of victims dying of acute endocarditis that did not use intravenous drugs.5 However, IV drug abusers can and often have left-sided valve involvement. The aortic and mitral valves are involved in 35% and 30% of intravenous drug abusers with infective endocarditis. The majority (82%) of acute endocarditis in intravenous drug abusers is caused by Staphycoccus aureus compared with streptococcal species that commonly cause endocarditis in victims not injecting intravenous drugs.6 A minority (18%) of S. aureus isolates are methacillin resistant. Other bacteria co-infect 9% of intravenous drug abusers with S. aureus endocarditis. Streptococcus viridans causes right-sided endocarditis in 11% of intravenous drug abusers. Candida endocarditis is usually superimposed on a previous episode of bacterial endocarditis and has a more indolent clinical course. Unusual pathogens causing endocarditis in intravenous drug abusers are summarized in Table 1.
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