Prevalence and Characterization of Community Acquired MRSA in HighRisk Individuals in Toronto

1
Prevalence and Characterization of Community
Acquired MRSA in High-Risk Individuals in Toronto

• B. Borgundvaag, B. M. Willey, P. Gnanasuntharam,
  A. Rostas, N. Kreiswirth, V. Porter, L. Louie, H.
  Wong, P. Gnanasuntharam, M. Loftus, B. Le, E.
  Boyd, A. Gelosia, T. Svoboda, W. McIsaac, A.
  McGeer

2
Background

• MRSA first described in 1961, long been
  recognized as nosocomial pathogen
• In 2003, HA-MRSA represented up to 40 of US
  Staph isolates, 10.4 in Canada
• Recently, new strains of MRSA have emerged,
  called CA-MRSA, and become dominant in some
  communities

3
Background

• Epidemiological genetic vs definition
• CA-MRSA clones are genetically distinct from
  HA-MRSA, not just spill-over into community
• Well adapted to spread within the community,
  potential to cause severe disease
• CA-MRSA may now be acquired in hospitals/instituti
  ons spilling into hospital from community
  settings

4
CA-MRSA vs HA-MRSA
-2
Common
Barton et al, Can J Infect Dis Med Micro Vol 17
Suppl C Sept/Oct 2006
5
CA-MRSA Virulence

• CA-MRSA (esp PVL) infection associated with
  higher hospitalization rates, treatment failure
  rates, and relapse rates
• PVL is an extra-cellular product of S aureus that
  is cytolytic (pore-forming activity) to
  neutrophils, monocytes and macrophages
• Tends to cause infection in children and young
  adults, socially disadvantaged, IV drug users,
  inmates

Davis et al, J. Clin. Micro. June 07
6
Importance of MRSA Colonization

• Study of 812 Military Recruits found 3 colonized
  with CA-MRSA, 28 with MSSA
• SSTI infection rates for CA-MRSA colonized
  individuals was 38, vs 3 for MSSA colonized
  individuals

Ellis et al. Clinical Infections Disease,
200439 pp. 971-979
7
MRSA Colonization Rates?

• The prevalence of CA-MRSA amongst community Staph
  isolates (wounds) in USA up to 75
• Presently, little is known regarding MRSA
  colonization rates in Canada
• Murray et al (2005) -1.4 (FPU)
• Borgundvaag et al (2009) -1 (FPU)
• Last year we demonstrated 4.1 colonization rate
  homeless shelter population

8
Purpose

• To determine if MRSA colonization rates are
  changing in a high-risk population of homeless
  men
• Characterize the isolated strains of MRSA and
  determine associated antibiotic resistance
  patterns

9
Methods

• 319 consenting residents of mens homeless
  shelter, between June 6 and Aug 15, 2008
• Separate swabs of nares, axilla, and visibly open
  sores
• Questionnaire regarding risk factors for MRSA
  colonization
• Swabs processed as per standard laboratory
  procedures
• Students t-test, Fishers exact and Chi-squared
  test for association as appropriate

10
Results 2007-08
11
Results 2007-08
p0.001
12
MRSA Homeless 2008
13
MRSA Homeless 2008
14
MRSA Homeless 2008
15
Summary

• MRSA colonization rates in this population of
  homeless showed significant (3X) increase
  year-year
• Most important factors associated with MRSA
  colonization were ethnic origin, and chronic skin
  condition
• 36/38 isolates were CA-MRSA strains, and overall
  28 of MRSA isolates were clinda resistant, with
  multiple resistant strains emerging.

16
Limitations

• Homeless men population may not the most reliable
  historians
• Relatively small study
• Point prevalence only
• Single location
• Single gender

17
Conclusions

• CA-MRSA colonization much higher in this group
  than in a general FPU population (1)
• CA-MRSA colonization rates appear to be on the
  increase
• The implications of this increase in CA-MRSA
  colonization rate - yet to be determined,
  particularly WRT transmission, clinical illness
  etc

18
(No Transcript)
19
Supplemental Slides
20
PVL
Barton et al, Can J Infect Dis Med Micro Vol 17
Suppl C Sept/Oct 2006
21
Micro Methods

• Swabs were incubated in brain heart infusion
  broth at 37oC overnight and planted plated the
  next day to Mannitol Salt Agar (MSA) for
  detection of all S. aureus (including susceptible
  strains i.e. MSSA), and to Oxoids Denim Blue
  agar (DBA) for detection of MRSA. DBA were
  incubated for a full 24h at 37oC, while MSA were
  incubated for up to 36h.
• Suspicious colonies from both media were tested
  using the Pastorex Staph Plus (BioRad) and tube
  coagulase (Remel) tests to identify S. aureus.
  Positive colonies were tested using the CLSI
  Oxacillin Screen Agar and PBP-2a MRSA screen
  (Denka Seiken, Japan) to determine susceptibility
  to methicillin. Antimicrobial susceptibility
  testing was performed using by CLSI broth
  microdilution MIC panels prepared in house.
  Induction of clindamycin resistance in
  erythromycin-resistant but apparently
  clindamycin-susceptible isolates was performed
  using the Double Disc Test in accordance with
  CLSI.
• SmaI pulsed field gel electrophoresis (PFGE) was
  performed to determine the clonal lineage of the
  MRSA strains and PCR was performed to determine
  their SSCmec type.

22
Background