Two more VRSA cases reported in Michigan

Novel plasmid or surveillance artifact?

Clinicians and public health officials in Michigan have identified both the fifth and sixth cases of vancomycin-resistant Staphylococcus aureus (VRSA), meaning four of the first six cases of the emerging pathogen have occurred in a single state.

The fifth U.S. case was confirmed last October in Michigan, and right on the heels of that another appeared there in late December. The latter has yet to be officially reported, but the former was described last week in Chicago at the annual meeting of the Society for Healthcare Epidemiology of America (SHEA). Dawn Sievert, MS, an epidemiologist at the state department of community health in Lansing, MI, updated the situation at SHEA.

"The sixth case just occurred at the very end of December — beginning of January," she told Hospital Infection Control. "So I am just wrapping that up. I am closing that investigation because the patient no longer has VRSA present. It hasn’t been reported at a national meeting. We intend to do that, and we do that each time we close a case."

As described at SHEA, the fifth VRSA case occurred in a 58-year-old female with a history of morbid obesity, hypertension, asthma, chronic bronchitis, and arthritis.1 She developed a post-surgical site infection in June 2005. While the patient was recovering in a rehabilitation facility from July through November, cultures taken from the abdominal wound in August and September each grew a variety of organisms, including methicillin-resistant Staphylococcus aureus (MRSA). The patient received vancomycin both in August and September for a total of four weeks. Subsequently, an abdominal wound culture taken on Oct. 11 grew Pseudomonas aeruginosa, vancomycin-resistant Enterococcus faecalis (VREF), and VRSA, Hiebert reported.

VRSA detected in lab

The VRSA was initially identified by the hospital laboratory using a vancomycin agar screen plate (6ug/mL) and a Microscan microdilution testing panel for which the vancomycin minimum inhibitory concentration (MIC) was reported as greater than or equal to 16 ug/mL. The isolate was susceptible to daptomycin, gentamicin, linezolid, rifampin, tetracycline, and trimethoprim-sulfamethoxazole. The patient did not receive antimicrobial therapy for the VRSA-positive wound culture, as there was no longer apparent infection. Subsequent cultures did not recover VRSA from the wound. However, VRSA was recovered from a surveillance culture of the groin in close proximity to the wound site. Five days of chlorhexidine showers were used to decolonize the patient and follow-up cultures were negative for VRSA. Surveillance nasal cultures collected from 26 patient contacts were negative for VRSA. Infection control measures — which apparently worked — included a private room, dedicated staff and equipment, contact precautions, and health care provider education.

The mechanism of resistance in all of the U.S. cases since the bug debuted in 2002 has been a genetic transfer from vancomycin-resistant enterococci (VRE) to MRSA. The vancomycin-resistance determinant vanA, typically found in VRE but never in clinical staph strains, was found in the VRSA isolates in all of the cases. The recipe has become a familiar one: prolonged administration of vancomycin in the presence of coinfection or colonization with MRSA and VRE. Four of the six documented cases have occurred in Michigan, but there appear to be no clonal similarities in the strains, she notes. "They are unrelated," she says.

Still, the Centers for Disease Control and Prevention is looking at the Michigan isolates to see if there is something strange afoot. "The CDC is taking isolates from our MRSA and our VRE to see if perhaps we have a plasmid that is more easily shared — a broad-spectrum plasmid," she says. "Is it [emerging] in Michigan vs. isolates you would find in other states? These are ongoing investigations."

More likely, the case count reflects a classic surveillance artifact: Michigan is finding more cases because it is looking harder.

"Because Michigan had the first case — and that was big because no one had seen it before — our [state] lab worked very hard to make sure that our clinical labs were very aware that this was a possibility," Sievert says. "[We told them] this is how you look for it and if you have any questions call us immediately. We basically are on heightened alert in Michigan for this. There may be many hospitals that may not be looking and are missing [cases]. States that haven’t had to deal with it don’t know what they are looking for yet."

If that’s true, it suggests cases are going undetected elsewhere. Though the emergence of VRSA is ominous, the early cases are not yet fulfilling fears of a deadly superbug. No secondary transmission has been detected in any of the cases.

Reference

  1. Sievert D, Dyke T L, Bies S. Fifth case of vancomycin-resistant Staphylococcus aureus in United States and third for Michigan. Abstract 162. Society for Healthcare Epidemiology of America. Chicago; March 18-21, 2006.