Limited information exists in the literature regarding bone and joint infections in patients with IDA [2
]. Some authors reported P aeruginosa as the most common pathogen [7
], but others identified S aureus as the most common [2
]. The recent increase of ORSA infections in patients with IDA highlights the potential for progressive changes in the microbiology of bone and joint infections associated with IDA [3
]. Knowledge of the microbiology of these infections would help optimize empiric antimicrobial therapy in patients with IDA; therefore, we determined the most common pathogen in bone and joint infections in patients with IDA, the rate of oxacillin resistance in these infections caused by S aureus, and compared the microbiology of osteomyelitis versus septic arthritis in these patients.
We acknowledge several limitations. First, it is a retrospective record review and some patients with IDA and with osteomyelitis or septic arthritis may have not been identified. Second, culture results were missing in 17 patients, who were excluded from the study. Third, the identification of eligible patients relied on self-admission of IDA by the patient and some patients may have denied use of illicit drugs for various reasons. Moreover, the exact time frame of IDA was not evident and we cannot differentiate between patients who were actively injecting drugs at the time of admission versus the ones who were not injecting at the time their symptoms developed. Fourth, all testing was performed in a clinical laboratory and no further testing for molecular typing was obtained to differentiate between community-acquired and hospital-acquired ORSA. Finally, when extracting data from records we did not record details of the origin and number of the available intraoperative cultures; therefore, we cannot rule out that the S. epidermidis identified in some of our patients without hardware in place is not a contaminant.
Our data suggest bone and joint infections associated with IDA are predominantly caused by Gram-positive bacteria, with a progressive increase of ORSA. A considerable proportion of infections are caused by Gram-negative and anaerobic microorganisms. Older studies identified P aeruginosa as the most common pathogen (Table ). Kido et al. reported that P aeruginosa was the pathogen in 27 (84%) of 32 patients with osteomyelitis or septic arthritis [11
]. Roca and Yoshikawa [18
] identified P aeruginosa in 14 (64%) of 22 skeletal infections and stated the microbiology of these infections was far less diverse than their anatomic location. Miskew et al. identified P aeruginosa as the pathogen in all 35 bone and joint infections in their series [14
]. Similarly, in series of patients with IDA and acute endocarditis, Gram-negative bacteria were the major pathogens [12
]. The high rate of P aeruginosa infections in injecting drug abusers has been attributed to contaminated water used for preparation of injection [20
]. In contrast, Chandrasekar and Narula [6
] reported S aureus was the most common pathogen, identified in 56% (25 of 45) of their patients, whereas P aeruginosa was present in 11% (five of 45). Other studies from the 1980 s and later found S aureus present in 55 to 77% of musculoskeletal infections in patients with IDA [2
]. We found S aureus was the infecting microorganism in the majority of cases (52%, 111 of 215) with P aeruginosa in only 7% (15 of 215) of infections. The decline in the importance of P aeruginosa in musculoskeletal infections associated with IDA after 1980 has been attributed to the fact that in the 1970 s pentazocine, which is soluble in cold water, was a common drug of abuse, whereas in the 1980s heroin became common but required heating of the water, which may have reduced contamination of the water with Pseudomonas [10
Literature on musculoskeletal infections in patients with IDA
After evaluating sensitivities among the S aureus isolates in our series, an alarming increase in oxacillin resistance was documented. From 1998 to 2005, the incidence of oxacillin resistance among S aureus isolates in our series rose substantially from 21% to 73%. Only three of the existing studies documented ORSA bone and joint infections in patients with IDA [6
], with considerable variability. Two studies from the USA by Ang-Fonte et al. [2
] in 1985 and by Chandrasekar and Narula [6
] in 1986 found that ORSA comprised 7 of 18 S aureus infections and 16 of 25 S aureus infections, respectively, but no such data have been reported recently. In contrast, a study from Spain from 1991 reported ORSA in only 1 of 27 S aureus infections [5
]. Progressive increase of ORSA in patients with IDA has been demonstrated in soft tissue abscesses [1
] and all types of infection overall [3
]. A recent study evaluating all types of infections reported, from 2006 to 2008, 49% (18 of 37) of clinical isolates from patients with IDA were ORSA, compared with 7% (four of 59) in 2000 to 2003 and 0% (zero of 48) in 1999 to 2000.
We found that in patients with osteomyelitis there was a markedly higher prevalence of polymicrobial infections (46% versus 15%), infections due to Gram-negative organisms (24% versus 9%), and anaerobic infections (19% versus 6%) compared to patients with septic arthritis. Chandrasekar and Narula [6
] evaluated 7 patients with osteomyelitis and 33 patients with septic arthritis and identified S. aureus in 4 of 7 patients with osteomyelitis versus 16 of 33 patients with septic arthritis, and P aeruginosa in none of 7 osteomyelitis cases versus 5 of 33 septic arthritis cases.
Based on our findings, vancomycin should be considered in the empiric antibiotic therapy of bone and joint infections in these patients while waiting for definitive culture results. However, the extensive use of vancomycin has been associated with increasing minimum inhibitory concentrations for ORSA. Empiric coverage for Gram-negative organisms and anaerobes should also be considered, especially in cases of osteomyelitis.
Bone and joint infections in patients with IDA are predominately caused by Gram-positive bacteria. S aureus is the most common pathogen and oxacillin resistance has greatly increased. A considerable proportion of infections, especially osteomyelitis, are caused by Gram-negative and anaerobic microorganisms. This information, in combination with local resistance patterns, will help the treating physician select antibiotics for empiric administration until cultures results are available.