Methicillin‐Resistant Staphylococcus aureus Infection and Colonization Among Hospitalized Prisoners
We assessed methicillin‐resistant Staphylococcus aureus (MRSA) infection and colonization in hospitalized prisoners. Of 434 admission surveillance cultures, 58 (13%) were positive for MRSA. The sensitivity of admission surveillance cultures of samples from the anterior nares was 72% and increased to 84% when the calculation included cultures of wound samples. Hospitalized prisoners are at high risk for MRSA infection and colonization, and surveillance should include cultures of nares and wound samples.
Received July 26, 2006; accepted November 28, 2006; electronically published May 25, 2007.
Community‐associated methicillin‐resistant Staphylococcus aureus (MRSA) infection has been increasingly recognized among prisoners.1‐5 Data on clinical infection with MRSA underestimate the true proportion of patients colonized with MRSA. Despite increased reports of MRSA infection among incarcerated populations, to our knowledge there has been no attempt to describe the overall burden of colonization and infection in this specific population, a potential high‐risk group for targeted active surveillance programs. We assessed the prevalence of MRSA colonization and infection among incarcerated patients upon transfer to an acute care facility.
Methods
This study was approved by the institutional review board at the University of Maryland Medical Center (UMMC; Baltimore, Maryland). This was a prospective cohort study of incarcerated patients at the UMMC, a 656‐bed, tertiary care medical center that operates a 13‐bed correctional health unit (CHU). The CHU is the only facility dedicated to providing care to prisoners in the state of Maryland. During 2002‐2003, there was a 3‐fold increase in the number of CHU patients who had MRSA identified from clinical cultures. In light of this evidence, we implemented an active surveillance program in October 2003 to identify incarcerated persons with MRSA colonization on admission to the UMMC.
From October 2003 through December 2004, we obtained anterior nares samples for surveillance cultures from all prisoners within 48 hours after transfer to the CHU. Cultures were processed by use of standard methods.6 Patients who were colonized or infected with MRSA were placed under contact isolation (ie, gloves and gowns were required for entry into their rooms) for the duration of their hospitalization.
We used pulsed‐field gel electrophoresis (PFGE) to determine the similarity of all available MRSA isolates, using SmaI‐digested chromosomal DNA according to protocol.7 The study isolates and standard MRSA types from the Centers for Disease Control and Prevention (ie, USA100‐USA800) were compared by use of the Dice coefficient of band‐based similarity with the unweighted pair group method, in accordance with the criteria used by Tenover and colleagues.7,8
Colonization was defined as an admission surveillance culture of an anterior nares specimen that was positive for MRSA, whereas infection was defined as any clinical culture that was positive for MRSA. An admission surveillance culture was defined as any culture of a sample obtained within 48 hours after transfer from a correctional facility to the index hospital. Patients for whom samples for surveillance culture were not obtained at admission were excluded. Patients could have had multiple admissions during the study period before their first surveillance or clinical culture was positive for MRSA; however, after being identified as infected or colonized with MRSA, their subsequent admissions were excluded from analysis. We calculated relative risks to assess the association between the detection of MRSA colonization or infection on admission (ie, a positive admission surveillance culture or a positive clinical culture collected within 48 hours of admission) and demographic characteristics, previous healthcare exposures, and the correctional facility of origin.
Results
From October 2003 through December 2004, a total of 388 patients were transferred to the CHU, accounting for 514 admissions (1.3 admissions per patient). Samples for admission surveillance cultures were obtained at 434 (84%) of the admissions. Of the 434 eligible admissions, 374 (86%) were of men and 60 (14%) were of women. Forty‐two (10%) of the admission surveillance cultures were positive for MRSA. Of the 434 admissions, 50 (11.5%) had surveillance and/or clinical cultures positive for MRSA within 48 hours after the patient was transferred from a correctional facility. An additional 8 (1.8%) admissions had clinical cultures positive for MRSA more than 2 days (range, 2.5‐12.3 days) after admission to the unit. Data for these 8 admissions were excluded from measurements of admission risk, though the isolates were submitted for PFGE.
During the study period, 58 patients had cultures positive for MRSA, with 30 (52%) of the patients identified by surveillance culture alone (Table). An additional 16 (28%) of the patients were identified as infected with MRSA on the basis of clinical culture results, despite having surveillance cultures negative for MRSA. Nine (56%) of these positive clinical cultures were of wound samples; other, less common culture‐positive samples included sputum, blood, and abscess fluids. Seven of the 9 wound cultures were of samples obtained less than 48 hours after admission. Surveillance cultures of anterior nares specimens identified MRSA in 42 of 58 patients, yielding a sensitivity of 72%, whereas a combination of surveillance cultures and admission cultures of wound specimens identified MRSA in 49 of 58 patients (Table), yielding a sensitivity of 84%. The “gold standard” for this analysis was a positive result of any surveillance or clinical culture (eg, blood culture) during the patient's stay at the index hospital.
Women were at higher risk for MRSA carriage on admission, compared with men (relative risk [RR], 2.46 [95% confidence interval {CI}, 1.42‐4.27]), whereas prior admission to the index hospital within the past year was not significantly associated with MRSA carriage (RR, 1.41 [95% CI, 0.82‐2.42]). Persons transferred from Baltimore city jails were more likely to have MRSA recovered from culture on admission, compared with patients transferred from other correctional facilities outside the city jail system (RR, 2.63 [95% CI, 1.46‐4.74]). Using stratified analysis, we observed that transfer from city jails was a significant risk factor for MRSA carriage among patients without prior admissions to our facility (RR, 3.31 [95% CI, 1.54‐7.11]), compared with those who had prior admissions (RR, 1.71; 95% CI, 0.68‐4.3]).
Fifty‐six (97%) of the 58 confirmed MRSA isolates from unique patients were available for typing by PFGE. PFGE results suggested that the isolates had 10 distinct PFGE types, according to published criteria.7 Three predominant PFGE groups were observed among these isolates, with the largest group (21 isolates [36%]) being similar to USA300. However, clonal isolates were not specific to a particular correctional facility.
Discussion
Current guidelines recommend active surveillance and isolation of patients who are at high risk for MRSA infection and colonization, such as those with prior healthcare exposures and those admitted to intensive care units.9,10 In this study, admission prevalence of MRSA colonization or infection among prisoners transferred to a tertiary care facility was 11.5%. In comparison, during the same period, 11% of nonprisoners admitted to our medical and surgical intensive care units were colonized with MRSA on admission (data not shown).
Female sex was a significant risk factor for MRSA colonization or infection, and this result was similar to previous findings regarding clinical cultures performed for prisoners in Texas. Baillargeon et al.1 hypothesized that this may be partly attributed to a sex‐biased increased use of healthcare services. Among patients without previous admissions to our facility, persons transferred from Baltimore city jail facilities were 3 times as likely to have MRSA recovered from culture, compared with their counterparts from jail systems outside the city. Persons awaiting trial or serving relatively short sentences (6 months or less) are housed in Baltimore city facilities, as opposed to those serving longer sentences, who are housed in detention facilities outside Baltimore. Despite being incarcerated for shorter periods, the prevalence of MRSA positivity among persons transferred from Baltimore city jail facilities was 17%. The relatively short‐term residency period of those in the city jail system suggests that duration of exposure to incarceration facilities may not be the sole contributing factor to the prevalence of MRSA infection and colonization among these patients.
There are several potential limitations of our study. Although previous admissions to our facility were used as a proxy for prior healthcare use and/or exposure, we were unable to fully assess previous healthcare encounters that occurred beyond the scope of our facility’s medical records. Furthermore, this study did not account for length of incarceration prior to admission or potential comorbidities, including immunosuppression, which could have impacted the results. Finally, compliance with obtaining surveillance culture samples was not 100%, and as a result we were unable to determine the prevalence of MRSA positivity among the 16% of patients for whom cultures were not performed.
Although numerous studies have described clinical manifestations of MRSA infection and colonization among the incarcerated, ignoring the potential impact of colonization in other populations has already been demonstrated to have negative consequences.9 This study suggests that incarcerated persons transferred to acute care facilities are a high‐risk population for MRSA colonization and infection and should be considered for targeted active surveillance. Supplementing cultures of anterior nares samples with cultures of wound samples may improve the sensitivity of culture for detection of MRSA.
Acknowledgments
Financial support. Financial support was provided by Veterans Affairs Health Services Research and Development Service Research Career Development Award (RCD‐02026‐1 to E.N.P.) and National Institutes of Health Career Development Award (K25: AI‐58956 to D.M.H.).
Potential conflicts of interest. All authors report no conflicts of interest relevant to this article.
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Presented in part: 44th Interscience Conference on Antimicrobial Agents and Chemotherapy; Washington DC; November 2004 (Abstract K‐1856).