Deadly Superbug in Southeast Michigan Shows Recent Decline

A new study finds a decrease in an emergent strain of the superbug methicillin-resistant S. aureus (MRSA) that is resistant to last line defense antibiotics. Researchers examined the prevalence of vancomycin-resistant Staphylococcus aureus (VRSA) infections in southeastern Michigan, where the majority of these infections have occurred in the U.S. The study is published in the December issue of Infection Control and Hospital Epidemiology.

“Vancomycin is one of the few antimicrobial agents that can treat infections caused by drug-resistant strains of S. aureus, such as MRSA,” says Alice Guh, MD, MPH, a lead author of the study. “Because alternative treatments are limited, widespread emergence of VRSA would pose a significant clinical and public health threat.”

As of 2012, 13 cases of the rare, but dangerous VRSA have been reported in the U.S. All VRSA cases have resulted from the transfer of genetic material between Inc18-positive vancomycin-resistant enteroccus (VRE) most common in non-E. faecium species (nfVRE) and pSK41-positive MRSA – considered precursor organisms to VRSA. To better understand this potentially dangerous multidrug-resistant strain of MRSA, researchers tested samples of the VRSA precursor organisms from a large, diverse collection of clinical isolates from two southeastern Michigan healthcare institutions. Researchers tested isolates of nfVRE cultured during 2006-2013. Positive samples of the MRSA strain were cultured during 2009-2011. The last reported case of VRSA in this region was December, 2009.

In total, 826 cases of nfVRE and 752 MRSA cases were received from both institutions. The researchers found the overall prevalence of VRSA precursor organisms from the sample cases were low during the testing period. Only 1.5 percent of VRE were Inc18-positive and 2.5 percent of MRSA were pSK41-positive.

Examining the trends, the authors found a significant decrease in the prevalence of Inc18-positive VRE after 2009, coinciding with the last VRSA case reported in Michigan. Risk factors associated with pSK41-positive MRSA included doses of intravenous vancomcyin, but no significant risk factors were identified for Inc18-positive VRE.

“Reasons for such a decrease are unknown but could reflect changes in the ecology of VRE due to natural fluctuations or interventions designed to reduce healthcare-associated VRE. Further evaluation is needed to confirm our findings and to better describe the evolving epidemiology of VRSA,” says Guh.

Reference: Albrecht V, et al. Prevalence of and Risk Factors for Vancomycin-Resistant Staphylococcus aureus Precursor Organisms in Southeastern Michigan. Infection Control and Hospital Epidemiology. 35:12. December 2014.

Source: Society for Healthcare Epidemiology of America (SHEA)