Canada Communicable Disease Report

May 2008

Volume 34

Number 05

Monthly Report

Investigation of community-associated methicillin-resistant Staphylococcus aureus in a remote northern community, Nunavut, Canada

A Dalloo, MSc (1, 2), I Sobol, MD, CCFP, MHSc (3), C Palacios (3), M Mulvey, PhD (4), D Gravel, BScN, MSc (5), L Panaro, MD (1)

  1. Canadian Field Epidemiology Program, Public Health Agency of Canada, Ottawa, Ontario, Canada
  2. Centre for Immunization and Respiratory Infectious Diseases, Public Health Agency of Canada, Ottawa, Ontario, Canada
  3. Nunavut Department of Health and Social Services, Iqaluit, Nunavut, Canada
  4. National Microbiology Laboratory, Winnipeg, Manitoba, Canada
  5. Centre for Communicable Diseases and Infection Control, Public Health Agency of Canada, Ottawa, Ontario, Canada

Introduction

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) is an increasingly frequent cause of skin infections that are resistant to commonly prescribed antibiotics(1-3). CA-MRSA refers to a MRSA infection with onset in the community in an individual without any established MRSA risk factors such as recent hospitalization, surgery, residence in a long-term care facility, or the presence of invasive medical devices(1-3) . In Canada, CA-MRSA was first reported among an Aboriginal community in Alberta in the late 1980's(4) . As MRSA is not a nationally reportable disease in Canada, the prevalence of CA-MRSA in the general population is unknown but it is considered low.

The Nunavut Department of Health and Social Services (NDHSS) identified an increase in the number of laboratory-confirmed cases of MRSA since December 2006 in a remote Inuit community of over 2,000 people. This community accounted for 80% of the MRSA cases identified in Nunavut in 2007. This report summarizes the findings of the initial outbreak investigation.

Methods

A case of CA-MRSA was defined as a community-onset infection in an individual between 10 August, 2006 and 16 August, 2007, diagnosed in the outpatient setting or within 48 hours after hospital admission, whose MRSA infection was laboratory-confirmed. The patient had no history of dialysis, a percutaneous device or indwelling catheter, surgery, hospitalization, being a resident in a long-term care facility or previous positive MRSA culture in the year before the onset of illness.

Diagnosis of CA-MRSA was defined by culture of MRSA from a normally sterile site. Swabs from symptomatic patients were cultured at the Baffin Regional Hospital. Batches of S. aureus isolates were then sent to Kasper Dynacare Laboratories in Edmonton for confirmatory testing and to the National Microbiology Laboratory in Winnipeg for molecular characterization. Molecular typing was conducted using pulsed-field gel electrophoresis (PFGE) as previously described(5) . Detection of mecA and the Panton-Valentine leukocidin toxin genes was conducted using PCR(6).

A standardized questionnaire (adapted from an existing Canadian study examining a current outbreak of CA-MRSA in First Nations communities in Saskatchewan(7,8) ) was completed on all persons with a positive MRSA culture between August 2006 and August 2007. The questionnaire captured demographic, clinical, laboratory and risk factor information.

Data analysis was performed on cases meeting the case definition using EpiData Analysis version 1.1 and Microsoft Office Excel 2003.

Results

Of the total 52 investigation reports sent by NDHSS, 43 (83%) individuals met the case definition. The epidemic curve for these CA-MRSA cases is shown in Figure 1.

Figure 1: Epidemic curve of laboratory-confirmed CA-MRSA cases in a remote northern community, Nunavut by week of specimen collection date, 10 August, 2006 to 16 August, 2007 (n = 43)

Figure 1: Epidemic curve of laboratory-confirmed CA-MRSA cases in a remote northern community, Nunavut byweek of specimen collection date, 10 August, 2006 to 16 August, 2007 (n = 43)

Males accounted for 58% of cases, and the median age was 18 years of age (range: 2 to 60 years). Ninety-three percent of cases were < 40 years of age, and those aged 5 to 9 years and 20 to 29 years had the highest age-specific cumulative incidence of infection at 26 cases per 1,000 population. Those aged 40 to 59 years had the lowest age-specific cumulative incidence at two cases per 1,000 population. No hospitalizations or deaths were reported.

Antibiotics were prescribed for 36 cases (84%), and skin and soft tissue infections (SSTI) accounted for the majority (98%) of cases. Among SSTI, 55% were characterized as abscesses found mostly in the abdomen and buttocks.

The most common risk factors were exposure to health care workers in the past year (83%), antibiotic prescriptions in the past year (65%), eczema, psoriasis and chronic skin conditions (37%), travel out of the province (26%), household contact of person with MRSA (24%) and exposure to person with a skin condition/infection (21%).

Twenty isolates were available for further study. Molecular typing revealed 95% (n = 19) were CMRSA7 (USA400) while the other strain was a CMRSA2. All of the CMRSA7 isolates contained the PVL toxin whereas the CMRSA2 isolate did not.

Discussion

The increased number of CA-MRSA cases reported in this remote Canadian community differs from American community outbreaks. Most studies report outbreaks among illicit drug users, sports teams, or inmates in prisons or correctional centers(2). CA-MRSA outbreaks in Canada to date have featured men who have sex with men, marginalized incarcerated populations, or remote First Nations communities on the Prairies(9-12).

MRSA is a reportable disease in a number of jurisdictions across Canada; however, it is not a nationally reportable disease. The prevalence of this organism has been defined among a selected number of tertiary care institutions through the Canadian Nosocomial Infection Surveillance Program (CNISP), which reports the annual incidence MRSA in patients admitted to sentinel hospitals participating in the CNISP network. Sentinel hospitals report a cumulative incidence of 7.4 cases per 1,000 patient admissions(13). The incidence in this remote community is about 60% higher than the nosocomial CNISP incidence, but comparable to what has been reported in northern communities of Saskatchewan and Manitoba(7,14,15). This figure probably underestimates the true prevalence of this organism, as laboratory-based surveillance systems do not capture symptomatic patients who are not swabbed or those who are asymptomatically colonized.

Comparison of this Inuit community's CA-MRSA cases and those associated with the Prairies' First Nations' outbreak revealed a similar epidemiologic profile (e.g., younger age groups and risk factors such as exposure to health care workers in the past year, antibiotic prescriptions in the past year, eczema, psoriasis and chronic skin conditions, being a household contact of a person with MRSA)(15). The questionnaire facilitated comparison between these two outbreaks, and no unusual risk factor or disease presentation was identified.

Control measures implemented to date (e.g., basic practices for good personal hygiene, education; emphasizing appropriate infection control measures; promoting appropriate antibiotic use) follow the established Canadian guidelines for the prevention and control of CA-MRSA(2). However, health care workers adopted optimal treatment for abscesses (incision and drainage rather than antibiotic use) in June 2007, almost a year after the increase was noted. Without an adequate infrastructure (e.g., overcrowded housing and limited potable water) in this isolated northern community, effective control continues to be challenging.

The most significant limitation of this investigation was its reliance on previously collected data (many questionnaires had illegible, incomplete or missing data which made interpretation difficult). Further work is needed to better define the extent of the outbreak (e.g., clinical as well as laboratory-confirmed cases) and examine the risk factors for transmission within the community.

CMRSA7 first caused community outbreaks in southern Manitoba in the late 1990's and subsequently spread to northern Manitoba in the early 2000's(16). High rates continue to be reported in some northern communities in Manitoba(15). This strain has also been reported as causing community outbreaks in northern Saskatchewan(14). It would appear as if CMRSA7 is continuing to spread northward possibly through travel from residents between these communities.

Conclusion

The descriptive epidemiology of the CA-MRSA cases in this remote Inuit community was similar to other remote First Nations' community outbreaks in Canada. This report provides the basis for a more in depth investigation. Further work is underway to improve control measures.

Acknowledgements

The authors thank the following for their assistance: Dr. G. Osborne, Nunavut Department of Health and Social Services, Iqaluit, Nunavut; D. Boyd and G. Golding, National Microbiology Laboratory, Winnipeg, Manitoba; M. Ofner-Agostini, F. Bergeron and C. Weir, Blood Safety Surveillance and Health Care Acquired Infections Division, Public Health Agency of Canada, Ottawa, Ontario.

References

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