Meticillin-resistant Staphylococcus aureus (MRSA) acquisition risk in an endemic neonatal intensive care unit with an active surveillance culture and decolonization programme

Abstract

Background: Meticillin-resistant Staphylococcus aureus (MRSA) is a leading cause of healthcare-associated infection in the neonatal intensive care unit (NICU). Decolonization may eliminate bacterial reservoirs that drive MRSA transmission.

Aim: To measure the association between colonization pressure from decolonized and non-decolonized neonates and MRSA acquisition to inform use of this strategy for control of endemic MRSA.

Methods: An eight-year retrospective cohort study was conducted in a level-4 NICU that used active surveillance cultures and decolonization for MRSA control. Weekly colonization pressure exposures were defined as the number of patient-days of concurrent admission with treated (decolonized) and untreated (non-decolonized) MRSA carriers in the preceding seven days. Poisson regression was used to estimate risk of incident MRSA colonization associated with colonization pressure exposures. The population-attributable fraction was calculated to assess the proportion of overall unit MRSA incidence attributable to treated or untreated patients in this setting.

Findings: Every person-day increase in exposure to an untreated MRSA carrier was associated with a 6% increase in MRSA acquisition risk [relative risk (RR): 1.06; 95% confidence interval (CI): 1.01-1.11]. Risk of acquisition was not influenced by exposure to treated, isolated MRSA carriers (RR: 1.01; 95% CI: 0.98-1.04). In the context of this MRSA control programme, 22% (95% CI: 4.0-37) of MRSA acquisition could be attributed to exposures to untreated MRSA carriers.

Conclusion: Untreated MRSA carriers were an important reservoir for transmission. Decolonized patients on contact isolation posed no detectable transmission threat, supporting the hypothesis that decolonization may reduce patient-to-patient transmission. Non-patient reservoirs may contribute to unit MRSA acquisition and require further investigation.

Keywords: Decolonization; Intensive care unit; Meticillin-resistant Staphylococcus aureus; Staphylococcal infections; Transmission.

Figure 1

Two-week Snapshot of MRSA Colonization Pressure in the NICU. Three hypothetical patients are shown. Some neonates that acquire MRSA colonization are decolonized (treated). Above we depict how neonates contribute to and are exposed to treated and untreated colonization pressure. In week 1, patients 1 and 3 are at risk for MRSA colonization and are exposed to seven days of untreated person-time from an untreated MRSA carrier (patient 2). In week 2, only patient 1 remains at-risk for incident MRSA colonization and is exposed to two days of untreated person-time (patient 3) as well as seven days of treated person-time (patient 2).

Figure 2

Study Flowchart. Flowchart detailing the population used to calculate colonization pressure exposures (left) and the at-risk population that was followed for outcome of incident MRSA colonization (right). Of the 3,783 at-risk neonates who were surveilled at least once after the first two days of admission (prevalent period), 91 neonates acquired MRSA colonization during NICU stay. Four colonization cases identified after MRSA infection, as indicated by clinical culture obtained during routine care, were not included as incident cases as colonization was assumed to be acquired through an endogenous process.

Figure 3

Quarterly Incidence Rate of MRSA Colonization Acquisition During Study Period. Rates are reported per 1,000 patient-days. Figure depicts low-level, endemic transmission throughout study period. Poisson regression line shown in blue. There was not a significant downward trend in rate of incident MRSA colonization (p=0.62).