The majority of IE in the IVDU group is caused by Staphylococcus aureus, which is responsible for 50-75% of cases [20,21]. The streptococci and enterococci are the next-most-common organisms (7-10%), with small percentages caused by CoNS, Gram-negatives, and Candida species. Polymicrobial IE is relatively common in the IVDU population, occurring in up to 5% of cases.
S. aureus most commonly causes right-sided (tricuspid) endocarditis in the IVDU setting. In the review of definite S. aureus native valve IE cases from the ICE merged database (19791999) published by Miro et al., 131/149 (88%) cases in patients with a history of injection drug use involved the tricuspid valve . Of 170 patients with right-sided S. aureus IE, 131 (77%) provided a history of IV drug use. In the same study, MRSA was observed infrequently in the IVDU population: 6/43 (14.0%) patients with MRSA IE used IV drugs compared to 136/248 (54.8%) of those with infection caused by susceptible strains. However, increasing rates of MRSA in IVDU have been observed and outbreaks have been documented.
Gram-negative IE in drug users can be caused by organisms that are encountered only rarely in non-IVDU patients. Pseudomonas aeruginosa endocarditis is uncommon and occurs nearly exclusively in IVDU. Pseudomonas aeruginosa IE is usually right sided, but can involve left-sided valves, in which case the clinical course is more complicated . A cluster of 36 cases of Serratia marcescens IE was seen among heroin users in San Francisco in the 1970s, with high associated mortality . Campylobacter fetus, Pasteurella spp., Brucella spp., Bordetella spp., Franciscella tularensis, Aeromonas hydrophila, and Yersinia enterocolitica are other Gram-negative bacilli that are occasionally encountered in the setting of IV drug use.
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