Methicillin-resistant Staphylococcus aureus (MRSA) is an old enemy.

However, the findings of a study published on August 11 in Clinical Infectious Diseases (CID) suggests infectious disease and public health specialists may need to think about the troubling gram-positive bacteria in a new way. That’s because the team of researchers working with the US Department of Veterans Affairs (VA) and the Veterans Health Administration have found that a “substantial proportion” of hospital-associated MRSA infections actually occur following patient discharge.

“The first step is recognizing that assessments of whether infection control practices have been successful should not be confined to what happens to patients between admission and discharge,” Richard E. Nelson, PhD, research health scientist, IDEAS Center, George E. Wahlen Department of Veterans Affairs Medical Center, and research associate professor, division of epidemiology, University of Utah School of Medicine told Contagion®. “Symptoms from infections that were picked up during a hospital stay may not manifest until days or even weeks later. For this reason, contact precautions, improved hand hygiene efforts, and other infection control strategies designed to prevent transmission in the inpatient setting should have an impact on post-discharge infections as well.”

Even though recent research has reported a decline in the incidence of hospital-associated infections caused by MRSA, mortality rates associated with them are up to 3 times higher than those of many newly emerging pathogens, according to a paper published in July 2017 in the journal Infection Control & Hospital Epidemiology. Historically, the vast majority of research designed to assess the true burden of hospital-associated infections caused by MRSA have focused on cases identified prior to discharge. Thusly, control measures have been geared toward infection prevention pre-discharge as well.

However, recently, multiple studies have estimated that more than 60% of MRSA infections actually occur after discharge. In fact, all VA hospital admissions have been screened for MRSA carriage and all invasive infections have been documented (with time of admission and discharge) since 2007.

To better understand the patient risk for post-discharge MRSA infections, Dr. Nelson and his colleagues built a dataset of 985,626 first admissions (94% male; mean age: 64.8 years) into VA acute care hospitals from January 2008 through December 2015 in whom surveillance tests for MRSA carriage were performed. They categorized patients as not colonized, importers, or acquirers based on the presence of positive MRSA surveillance tests during their inpatient stay. Patients with pre-discharge MRSA infections were excluded.

Using multivariable Cox proportional hazards and logistic regression models, they calculated the proportion of MRSA infections occurring prior to discharge and at 30, 90, 180, and 365 days post-discharge for each of the 3 colonization groups to assess the relationship between MRSA colonization status and infection. In all, 983,916 of the patient admissions included in the analysis did not have an MRSA infection during hospitalization and they were followed for infection post-discharge.

Overall, Dr. Nelson et al found that 903,190 (91.6%) of the patients were not colonized with MRSA during their hospitalizations, while 72,388 (7.3%) were importers and 10,048 (1.0%) were acquirers. The MRSA infection rate across the pre-discharge and 180-day post-discharge time period was 5.5% in importers and 7.0% in acquirers not directly admitted to the intensive care unit (ICU) and 11.4% in importers and 11.7% in acquirers directly admitted to the ICU. Notably, the pre-discharge hazard ratio for MRSA infection was 29.6 for importers and 28.8 for acquirers, compared to those not colonized; however, 63.9% of all MRSA pre- and post-discharge infections among importers and 61.2% among acquirers occurred within 180 days post-discharge.

The authors of the CID paper describe their work as “an important first step” in analyzing the true impact of interventions such as contact precautions in preventing hospital-associated infections caused by MRSA post-discharge. Dr. Nelson noted that future research evaluating the “costs and consequences of contact precautions” for patients admitted to the ICU or the general ward “would guide decision makers as to when and where to implement” these approaches.

“Infection or carriage surveillance is a crucial step in identifying where transmissions are occurring within the hospital so as to direct resources to the areas where they are needed most and to evaluate how well the infection control program is working,” Dr. Nelson said. “Our analysis would not have been possible without the VA’s nationwide surveillance program which allowed us to differentiate between patients who were colonized on admission from those who acquired colonization during their stay.

“[Ideally], infections whose symptoms do manifest in the post-discharge period would be identified or treated during follow-up visits in an outpatient setting,” he added. “With proper follow-up care, costly readmissions could be avoided.”