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Brief Report: Vancomycin-Resistant Staphylococcus aureus --- New York, 2004

Staphylococcus aureus is a common cause of hospital- and community-acquired infections (1,2). The development of vancomycin-resistant enterococci in 1988 led the way to the emergence of vancomycin-resistant S. aureus (VRSA) (minimum inhibitory concentration [MIC] >32 µg/mL [3]), first recognized in 2002 (4--7). This report describes the third documented clinical isolate of VRSA from a patient in the United States and provides evidence of failure to detect this VRSA by commonly used automated antimicrobial susceptibility testing.

On March 17, a urine culture obtained from a resident of a long-term--care facility yielded S. aureus. The isolate was tested for antimicrobial susceptibility by using Microscan® overnight panels (Dade Behring, Deerfield, Illinois); vancomycin MIC was 4 µg/mL. Further testing by Etest® (AB Biodisk North America, Inc., Piscataway, New Jersey) indicated that the isolate was resistant to vancomycin (MIC >256 µg/mL). After notification and subsequent analysis by the New York State Department of Health (NYSDOH), the isolate was forwarded to CDC, where it was confirmed to be VRSA (vancomycin MIC = 64 µg/mL, using the National Committee for Clinical Laboratory Standards broth microdilution reference method). The isolate contained both the mecA and vanA genes mediating oxacillin and vancomycin resistance, respectively. The isolate was susceptible to chloramphenicol, linezolid, minocycline, quinupristin-dalfopristin, rifampin, and trimethoprim-sulfamethoxazole.

The patient remains in a long-term--care facility, and NYSDOH is investigating the case. The goals of the investigation include assessment of infection-control practices and whether transmission to other patients, health-care providers, family, and other contacts has occurred. Previous investigations of VRSA demonstrated no transmission among contacts (5,6).

This VRSA isolate appears to be unrelated epidemiologically to the VRSA isolate identified previously in Michigan and Pennsylvania (5,6). Although the New York isolate contained the vanA resistance gene, the vancomycin MIC of the isolate appeared low when tested initially by an automated method. Additional testing at CDC indicated that Microscan® and Vitek® (bioMerieux, Hazelwood, Missouri) testing panels and cards available in the United States did not detect vancomycin resistance in this VRSA isolate. Consequently, additional VRSA infections might have occurred but were undetected by laboratories using automated methods. Potential VRSA isolates should be saved for confirmatory testing, and clinical microbiology laboratories must ensure that they are using susceptibility testing methods that will detect VRSA. The most accurate form of vancomycin susceptibility testing for staphylococci is a nonautomated MIC method (e.g., broth microdilution, agar dilution, or agar-gradient diffusion) in which the organisms are incubated for a full 24 hours before reading results. Therefore, when performing automated susceptibility testing of S. aureus strains, particularly methicillin-resistant S. aureus, laboratories should include a vancomycin-agar screening plate containing 6 µg/mL of vancomycin and examine the plate for growth after 24-hour incubation.

The public health response to identification of this VRSA infection is ongoing. Use of proper infection-control practices and appropriate antimicrobial agent management can help limit the emergence and spread of antimicrobial-resistant microorganisms, including VRSA. CDC recommends contact precautions when caring for patients with these infections, including 1) placing the patient in a private room; 2) wearing gloves and a gown during patient contact; 3) washing hands after contact with the patient, infectious body tissues, or fluids; and 4) limiting the use of patient-care items to individual patients. In addition, the number of persons caring for a patient with VRSA or vancomycin-intermediate S. aureus should be minimized (e.g., by assigning dedicated staff to care for the patient)*. Isolation of S. aureus with confirmed or presumptive vancomycin resistance should be reported immediately through state and local health departments to the Division of Healthcare Quality Promotion, National Center for Infectious Diseases, CDC, telephone 800-893-0485.

Reported by: M Kacica, MD, New York State Dept of Health. LC McDonald, MD, Div of Healthcare Quality Promotion, National Center for Infectious Diseases, CDC.

Acknowledgments

This report is based in part on contributions by C Scott, DJ Bopp, MS, NB Dumas, G Johnson, DJ Kohlerschmidt, P Kurpiel, RJ Limberger, PhD, KA Musser, PhD, B Wallace, MD, P Smith, MD, New York State Dept of Health.

References

  1. CDC. National Nosocomial Infections Surveillance (NNIS) report, data summary from January 1992--June 2001. Am J Infect Control 2001;29:404--21.
  2. Lowy F. Staphylococcus aureus infections. N Engl J Med 1998;339:520--32.
  3. National Committee for Clinical Laboratory Standards. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically, 6th ed. Approved standard, M7-A6. Wayne, Pennsylvania: National Committee for Laboratory Standards, 2003.
  4. CDC. Staphylococcus aureus resistant to vancomycin---United States, 2002. MMWR 2002;51:565--7.
  5. CDC. Vancomycin-resistant Staphylococcus aureus---Pennsylvania, 2002. MMWR 2002;51:902.
  6. Chang S, Sievert DM, Hageman JC, et al. Infection with vancomycin-resistant Staphylococcus aureus containing the vanA resistance gene. N Engl J Med 2003;348:1342--7.
  7. Whitener CJ, Park SY, Browne FA, et al. Vancomycin-resistant Staphylococcus aureus in the absence of vancomycin exposure. Clin Infect Dis 2004;38:1049--55.

* Additional CDC guidelines for preventing spread of VRSA are available at http://www.cdc.gov/ncidod/hip/vanco/vanco.htm.

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