This study was approved by the Institutional Review Board of Marshfield Clinic and Marshfield Clinic Research Foundation. Fifteen MRSA isolates were recovered from 15 patients in a correctional facility over a 13-month period (). These isolates were recovered from SSTI wound samples submitted to Marshfield Laboratories. The patients were housed in 7 of 10 units with a common recreation yard at a 1,200-inmate facility in Wisconsin from May 2002 to May 2003. Infections with MRSA were rare in this facility; the last reported case of MRSA was 16 months ago. Because of increased number of SSTIs during this period, the Wisconsin Division of Public Health initiated an investigation to determine whether these strains were epidemiologically related.
Figure 1 Timeline of incarceration and isolation of methicillin-resistant Staphylococcus aureus isolates from different patients. Top panel: baseline shows months in which a particular isolate was recovered and patient was identified as infected; y-axis shows (more ...)
All strains were typed by pulsed-field gel electrophoresis (PFGE) and staphylococcal cassette chromosome mec (SCCmec) and tested for virulence genes. Only the first isolate of each PFGE-based clonal group and 1 additional isolate from the same clone were analyzed with spa and multilocus sequence typing.
All 15 patients were men (average age 39 years) and had SSTIs at various body sites (). All patients, except for inmate Y08, who entered the facility in the seventh month of the outbreak (), were incarcerated for 3 to 56 months before the outbreak. Dates of incarceration for 2 inmates could not be determined. All 15 isolates were resistant to β-lactams but sensitive to ciprofloxacin, gentamicin, rifampicin, tetracycline, trimethoprim-sulfamethoxazole, and vancomycin.
Figure 2 Pulsed-field gel electrophoresis (PFGE)–based dendrogram of methicillin-resistant Staphylococcus aureus strains isolated during the outbreak. A genetic similarity index scale is shown above the dendrogram. Strain numbers, clone identification, (more ...)
PFGE analysis grouped these isolates into 2 clonal groups, USA400 (n = 7, 47%) and USA300 (n = 8, 53%) (). The first isolate and a randomly selected second isolate of USA300 and USA400 clonal groups were determined to be sequence type (ST) 8 and ST1, respectively. The representative USA300 strains were spa type t008 (YHGFMBQBLO), and USA400 strains were spa type t128 (UJJFKBPE). All 15 strains in both clonal groups were type IVa SCCmec and positive for virulence factor Panton-Valentine leukocidin (PVL) genes (lukSF-PV) and staphylococcal enterotoxin gene sek. Isolates of USA400 were also positive for sea, sec, seh, sel, and fnbA. USA300 strains were positive for fnbA and fnbB.
Both PFGE profiles () of isolates of USA400 clone were previously observed in Native American communities in Wisconsin throughout the 1990s (4
). However, ethnicity of the patients in the current study was not determined. PFGE profiles of USA300 strains were indistinguishable from USA300–0114 type strain (9
). Like USA400 strains, USA300 strains were sensitive to many classes of antimicrobial drugs. However, unlike type 0014 strain, USA300 strains in this study were sensitive to tetracycline. All 8 strains in the USA300 clonal group were resistant to erythromycin but lacked inducible clindamycin resistance by the D-test (data not shown), whereas only 3 (43%) USA400 isolates were resistant to erythromycin and showed inducible clindamycin resistance.
After the outbreak investigation, the facility instituted specific infection control measures for inmates and staff members. Measures included promoting frequent hand washing and improving sanitation of laundry, linens, showers, bathrooms, and equipment in the recreation yard. Inmates were educated about personal hygiene and consequences of sharing needles and other sharp objects. Subsequently, the number of MRSA cases in this facility decreased substantially from 1.25 cases per month during the study period to 0.67 cases per month over the next 6 months.