Surveillance of Staphylococcus aureus Bacteraemias by Specialty in Wales: Trends over Time

Background: Surveillance of S. aureus bacteraemias was made mandatory by the Welsh Assembly Government in April 2001.  To aid interpretation of data collected and identify target areas for control measures, medical specialty was added to the data set in October 2001.Eight years of continuous surveillance data for both meticillin sensitive (MSSA) and meticillin resistant (MRSA) S. aureus bacteraemias are available for analysis.

Objective: To examine the changes over time in specialties where S. aureus bacteraemias are identified.

Methods: S. aureus bacteraemia data are entered quarterly into a web- reporting system by infection control personnel. Blood culture sets taken, positive blood cultures and total MRSA and MSSA bacteraemias are entered for each acute hospital. For each S. aureus bacteraemia, the specimen date and specialty of patient when the blood culture was taken are added. Reports are deduplicated on a 14 day basis. 84 specialties are provided as options, which were collapsed into 15 broad specialty groups for analysis.

Results: Routine surveillance data to April 2009 showed a 27% decrease in total MRSA bacteraemias in Wales between year 1 and 8 of surveillance, whereas total MSSA bacteraemias increased by 5%. The decreasing trend for MRSA and the stable state of MSSA remained when number of blood cultures taken was taken into account.

8304 S. aureus bacteraemias had a specialty recorded in the 8 years of specialty surveillance (98% of reports). Bacteraemias were reported from 11/15 possible specialties, but the top 4 specialties (medicine, surgery, emergency, Intensive Treatment Unit (ITU)) included 94% (7793/8304) of samples. Noticeable differences in trends over time were noted for the top 4 specialties (see figure). MRSA bacteraemias from patients in medicine, surgery and ITU followed the Wales trend, with substantial decreases between years 1 and 8 of surveillance. MSSA bacteraemias remained stable, although some decrease in medicine was noted. Both MRSA and MSSA bacteraemias from patients in emergency departments, however, increased substantially over time (300% increase in MRSA bacteraemias, 225% increase in MSSA between year 1 and 8 of surveillance).

Conclusions: Interventions by infection control teams appear to have had a significant affect on numbers of MRSA bacteraemias reported in Wales, but little affect on MSSA bacteraemia numbers, suggesting perhaps that MSSA is less linked to control of cross-infection in hospital settings. Increases were observed in both MRSA and MSSA bacteraemias in patients in emergency departments over the time period, which may indicate a higher burden of hospital-linked S. aureus in the community or community acquisition. Further investigations examining trends in specialty throughputs, blood culture taking practices and lengths of stay over time period, may clarify the trends observed.