Outbreak in Alberta of community-acquired (USA300) methicillin-resistant Staphylococcus aureus in people with a history of drug use, homelessness or incarceration

Abstract

Background: The USA300 strain of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) can cause severe infection and is increasingly recognized as a cause of community outbreaks. In 2004, an outbreak was identified in the Calgary Health Region (CHR).

Methods: MRSA isolates were identified with standard methods at a central regional laboratory and typed via pulsed-field gel electrophoresis (PFGE). Isolates were tested by PCR for mecA, Panton-Valentine leukocidin (PVL), SCCmec, and spa genes. Cases were defined as such if a clinical isolate of the USA300 strain was noted between January 1 and September 30, 2004, and the patient had lived or traveled in CHR within 2 years before symptom onset. Demographic, clinical and risk data on all such cases were collected from several sources for statistical analysis. A case was defined as high-risk if the patient had a history of drug use, homelessness or incarceration.

Results: Of 40 isolates with the USA300 PFGE pattern, all tested positive for PVL, SCCmec type IVa and spa type 008. Almost all infections (39/40, 98%) involved skin and soft tissues, except for 1 death from necrotizing hemorrhagic pneumonia; a notable proportion (38%) required hospital admission or intravenous antimicrobial therapy. The outbreak centred on the high-risk population in CHR (70%; risk ratio 169.4, 95% confidence interval 86.1-333.0).

Interpretation: People with histories of illicit drug use, homelessness or recent incarceration were at highest risk for infection with CA-MRSA. The emergence and spread of this virulent strain has important implications for treatment and public health in Canada.

Fig. 1: Typing results for outbreak-related clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) in the Calgary Health Region (CHR). Left panel: Staphylococcal cassette chromosome mec (SCCmec) typing results. Lanes 1–6, representative outbreak-associated clinical isolates; lane M, molecular size markers, 1 Kb Plus DNA Ladder (Invitrogen). Control strains used: types I (NCTC10442), II (N315), III (85/ 2082), IVa (CA05), IVb (8/6–3P), IVc (MR108), IVd (JCSC 4469) and V (WIS [WBG8318]-JCSC3624), corresponding to distinct amplicon sizes of 613, 398, 280, 776, 493, 200, 881 and 325 bp, respectively. In addition, a 147-bp mecA gene (methicillin-resistant determinant) amplicon was present in all strains. Note: bp = base pairs. Right panel: Pulsed-field gel electrophoresis patterns of USA 300 strains of MRSA circulating in the CHR between Jan. 1 and Sept. 30, 2004. Lane 1, pattern A (31 of 40 cases, 78%); lane 2, USA300 reference strain pattern; lane 3, pattern B (7 cases, 18%); lane 4, pattern C (2 cases, 5%).

Fig. 2: Number of cases that furnished the USA300 strain of methicillin-resistant Staphylococcus aureus (MRSA) in the Calgary Health Region by month of specimen collection, from Jan. 1, 2004 through Sept. 30, 2004 (n = 40; 7 historical isolates were included for comparison). A further 310 infections were identified through ongoing surveillance up to Dec. 31, 2005 (8–34 cases per month).