MRSA's Evolving Epidemiology

Fred Gordin, MD, of the Infectious Diseases Section of the Veterans Affairs Medical Center (VAMC) in Washington, D.C., reports on the evolving epidemiology of MRSA in a commentary in the September issue of Infection Control and Hospital Epidemiology.  Gordin says that at his facility, about 40,000 patients receive care annually, and his research team reviewed infection control data on clinical MRSA isolates recovered during the period from 2001 to 2007.

Gordin (2010) explains, all new clinical isolates of MRSA were evaluated and categorized as either hospital-acquired MRSA (HA MRSA) or CA MRSA. We defined an isolate as HA MRSA if a MRSA-positive culture result was obtained at least 48 hours after admission to the hospital for a person without obvious signs of infection at the time of admission; an isolate was also categorized as HA MRSA if the patient had been in the facility, including the outpatient clinics, within 30 days before the onset of disease with MRSA. Isolates that did not fall under this definition were categorized as CA MRSA.

Gordin says that patients who were known to have MRSA infection or colonization were placed in contact isolation (patients remained in isolation until infection at the primary site was no longer active). The patients' electronic medical records were flagged, and healthcare workers were required to wear gloves and gowns for all procedures involving patient contact. A facility wide hand hygiene program was in place throughout the study period, emphasizing the use of an alcohol-based hand disinfectant that had been proven to be effective at reducing the nosocomial acquisition of MRSA at the VAMC. During the study period no active surveillance for MRSA was performed.

Gordin (2010) reports that the number of new clinical MRSA isolates increased during the study period, from 129 new MRSA isolates in 2001 to 221 new MRSA isolates in 2007. The number of HAMRSA isolates decreased during this period, from 78 isolates in 2001 to 46 isolates in 2007. This decrease occurred despite a stable number of bed days of care, so that the actual rate of HAMRSA infection decreased by about 41 percent from 2001 to 2007. Gordin (2010) also reports an almost fourfold increase in the number of CAMRSA isolates, from 41 isolates in 2001 to a peak of 183 isolates in 2005 and a total of 175 isolates in 2007.

The researchers also says there was a shift in the location where the HAMRSA isolates were recovered, from the inpatient wards to the outpatient clinical areas. The proportion of new MRSA clinical isolates recovered from the outpatient setting that were HAMRSA increased from 22 percent in 2001 to 50 percent from 2005 to 2007. There was also a shift in where the CAMRSA clinical isolates were recovered; from 2001 to 2004, the majority of CAMRSA clinical isolates were recovered from patients admitted to the inpatient wards; however, starting in 2005, the greatest number of CAMRSA clinical isolates was obtained from patients in outpatient areas.

Gordin (2010) says that a better understanding of the risk factors contributing to the significant increase in the incidence of MRSA infection in the community is needed, and that approaches to prevention, screening and treatment must reflect the changing epidemiology of MRSA.

Reference: Gordin FM. InsideOut: The Changing Epidemiology of Methicillin Resistant Staphylococcus aureus. Infect Control Hosp Epidemiol. 2010;31:983-985.