Resistant Staphylococcus aureus Infections in the United States: A New Classification, a New Resistance and the Implications for Surveillance, Prevention, and Control.

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) infections are often defined as healthcare-associated (HA) or community-associated (CA) based on three different classification schemes: healthcare risk factor, infection type, or susceptibility pattern. This dissertation analyzed the sensitivity, specificity, and utility of these classifications using MRSA case data from Michigan.

MRSA infections were voluntarily reported to the Michigan Department of Community Health (MDCH) from October 2004 through December 2005. Data on patient demographics, risk factors, and infection information were recorded on the MDCH MRSA Report Form and submitted with laboratory susceptibility test results. A total of 2,151 non-duplicate MRSA infections were reported. Pulsed-field gel electrophoresis (PFGE) tests were conducted on 244 randomly selected isolates from reported cases.

The first project classified MRSA infections as HA or CA using each of the three classification schemes, then examined results for inconsistency across methods. Comparison of HA and CA results using the common classification schemes revealed a large proportion of inconsistent results. The second project used PFGE test result as a gold standard to consider the three classification schemes and other important contributing variables aimed at producing an improved multivariable classification model. This new model using infection type, susceptibility pattern, age and hospitalized as variables better predicted PFGE classification of HA or CA than any other single classification method. The third project evaluated accuracy of the new classification model and used it to define the epidemiology of Michigan MRSA infections. This analysis revealed that MRSA is prevalent across Michigan and CA-MRSA, particularly among males, blacks, people within correctional facilities, and people presenting to emergency departments. A final project produced a comprehensive review of the first seven cases of emergent vancomycin-resistant Staphylococcus aureus (VRSA) in the US. All VRSA cases had a history of prior MRSA and enterococcal infection or colonization; all had several underlying conditions and most had received vancomycin prior to their VRSA infection.

In conclusion, the improved method to categorize MRSA infections as HA or CA, and characterization of the VRSA cases, provides new knowledge that will help to accurately target control efforts and prevention methods and messages to better combat this adept and evolving bacterium.