Reduced-susceptibility staph on the increase
CDC has documented 15 cases in last 18 months
The Centers for Disease Control and Prevention is stepping up efforts to identify and analyze rapidly emerging staph strains with reduced susceptibility to vancomycin. The strains have a minimum inhibitory concentration (MIC) of four for vancomycin, about half the level of resistance to the antibiotic as vancomycin-intermediate Staphylococcus aureus (VISA). Infection control professionals are urged to assist in the surveillance, which is being done in part to assess the need and efficacy of recommended precautions.
There have been six confirmed cases of VISA in the United States since 1997, but the CDC has confirmed 15 cases of the reduced-susceptibility strains in the last 18 months alone, Scott Fridkin, MD, a medical epidemiologist in the CDC’s hospital infections program, said recently in Toronto at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC).
"We actually have one report where there was spread to a family member of this organism," he emphasized. "I believe it went from the son, who was [previously] hospitalized, to the mother, who was somewhat ill and subsequently was hospitalized with a Staph aureus infection with this same MIC of four. So there has been spread documented."
As previously reported in Hospital Infection Control, that case involved a patient who came in to a hospital colonized with an MRSA strain that barely registered for any reduced susceptibility to vancomycin, with lab tests showing it had less than one MIC. However, the level of resistance increased to four MIC after the patient was administered vancomycin for 45 days during hospitalization.1 (See HIC, August 2000.)
While the pathogens are of concern in their own right, the presence of the reduced-susceptibility strains may precede the emergence of VISA in a patient population, Fridkin said. "In attempting to get a sense of how frequent or how common this organism is in the U.S., we’ve [initiated] enhanced case finding to identify these patients — these infections — as soon as possible," he said. "We’ve established partnerships with industry and academia. [CDC is] basically working with some of the other surveillance systems, the private surveillance systems that are out there, [asking] if they identify any of these to please let us know. We’ve offered expedited confirmatory testing. The reason we’re trying to identify these patients is to help assess what prevention strategies the infection control teams are using and also to try and help coordinate the public health response."
In that regard, the CDC is asking that any S. aureus isolate for which the vancomycin MIC is greater than or equal to four to be sent to the agency for confirmatory testing. "We definitely have some concerns that these patients are very similar to patients with actual VISA," he said. "There are infection control and treatment
implications."
Currently, the CDC is recommending that patients with the reduced-susceptibility strains be placed under contact isolation similar to that recommended for methicillin-resistant S. aureus. Infection control measures for VISA are considerably more stringent, including wearing masks, culturing contacts, and monitoring compliance with precautions.
CDC guidelines also call for infection control professionals to minimize the number of health care contacts for VISA patients, and that is commonly being interpreted as one-to-one nursing, Fridkin said. Evaluating possible spread to health care workers, other patients, and contacts also is recommended for VISA, he said as a reminder, although he conceded that such efforts could become expensive if interpreted broadly. No secondary transmission of VISA has been identified.
"Monitoring personnel for acquisition of the isolate is commonly an oversight, and you want to highlight that," he said. "It’s important. We need to assess the efficacy of these precautions."
The common denominator in many of the VISA cases reported so far has been prolonged administration of vancomycin, Mary Rogeman, MD, hospital epidemiologist at the VA Medical Center in Baltimore and professor of Medicine at the University of Maryland, said in another ICAAC session.
"I think it is very important for us to remember this, particularly in our clinical practices, as we note patients who are receiving prolonged courses of vancomycin," she said. "Being on a clinical service this month, I have one patient in our medical intensive care unit who is going on her ninth week of vancomycin, and I am quite frightened that we’re going to be isolating this as well."
Reference
1. Greene L, Chodoff A, Hollick G, et al. Evidence for community transmission of methicillin-resistant Staphylococcus aureus with a propensity for reduced vancomycin susceptibility. Presented at the Association for Professionals in Infection Control and Epidemiology. Minneapolis; June 18-22, 2000.
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