New insights concerning methicillin-resistant Staphylococcus aureus disease

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) has been a common nosocomial pathogen since the 1960s, and has been a major problem in hospitals worldwide. In 2006, the European Antimicrobial Surveillance System (EARSS), a free network that connects more than 600 laboratories in 28 European countries, recorded one incidence of MRSA per 100 000 patient-days, ranging from 0.2 in Denmark to 26.9 in Portugal (http://www.rivm.nl/earss/). In 2005, data from The Surveillance Network-USA (TSN), an electronic surveillance network that collects microbiology data from 300 clinical microbiology laboratories across the USA, reported that MRSA rates were 59%, 55% and 48% for strains from non-intensive-care unit (ICU) inpatients, ICU patients, and outpatients, respectively [1]. Two meta-analyses showed that bloodstream infections (BSIs) due to MRSA are associated with almost two-fold higher mortality rates than those due to methicillin-susceptible S. aureus [2,3]. Costs were significantly higher for MRSA BSIs than for methicillin-susceptible S. aureus BSIs [4]. Risk factors include: degree of compliance with hand disinfection procedures, use of antimicrobials, underlying diseases, prior hospitalization, surgery, duration of hospitalization, central venous catheterization and endotracheal intubation, enteral feeding, admission to the ICU, and nursing staff workload [5–8]. A 1-year study, carried out at an ICU, showed that urgent admission, values of APACHE II score at 24 h, bronchoscopy and days of staff deficit were all independent risk factors for acquisition of nosocomial MRSA. When a simple stochastic model was fitted, staff deficit was the only factor that was significantly associated with cross-transmission. It was predicted that a 12% improvement in adherence to hand hygiene might have compensated for staff shortage and prevented transmission during periods of overcrowding, shared care, and high workload [6]. British and US guidelines recommend that patients should be screened routinely before ICU admission in a hospital where MRSA is endemic [9,10]. A systematic review of isolation policies in the hospital management of MRSA demonstrated that intensive concerted interventions, which include isolation policies, can substantially reduce MRSA infections, even in settings with a high level of endemic MRSA [11]. Recently, MRSA infections have been diagnosed with increasing frequency upon hospital admission. In a cohort study of 127 patients with MRSA bacteraemia, diagnosed upon hospital admission, independent risk factors included a history of MRSA colonization or infection within 90 days, presence of a central venous catheter, and skin ulcers or cellulitis [12,13]. A meta-analysis of MRSA infections identified within 24–72 h of hospitalization documented a prevalence of community-acquired MRSA infections, defined as infections in patients without any known risk factors for MRSA, of £0.24% [14,15]. These ‘communityacquired’ MRSA strains arise from two different patient populations: those with true community-acquired infections due to MRSA strains that have emerged de novo from community-based S. aureus strains, and those with infections due to healthcare-associated MRSA strains that have been acquired in hospital or during a previous exposure to a healthcare setting or intervention. Increased adherence to hand-washing guidelines and controlled use of antibiotics may be two of the few modifiable factors offering a potential for primary prevention of MRSA infection in the hospital setting. The efficacy of a multimodal, centrally coordinated, multisite hand hygiene culture-change programme for reducing rates of MRSA was assessed in Victorian hospitals. Increased compliance with hand hygiene recommendations was associated with a significant reduction of MRSA BSIs [16,17]. The importance of a dose–effect association, supporting a causal relationship between MRSA and antimicrobial drug