Community-associated methicillin-resistant s taphylococcus aureus in northwest Ontario : A five-year report of incidence and antibiotic resistance

BACKGROUND
The incidence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is traditionally high in remote areas of Canada with large Aboriginal populations. Northwestern Ontario is home to 28,000 First Nations people in more than 30 remote communities; rates of CA-MRSA are unknown.


OBJECTIVE
To determine the CA-MRSA rates and antibiotic susceptibilities in this region.


METHODS
A five-year review of laboratory and patient CA-MRSA data and antibiotic susceptibility was undertaken.


RESULTS
In 2012, 56% of S aureus isolates were CA-MRSA strains, an increase from 31% in 2008 (P=0.06). Reinfection rates have been increasing faster than new cases and, currrently, 25% of infections are reinfections. CA-MRSA isolates continue to be susceptible to many common antibiotics (nearly 100%), particularly trimethoprim/sulfamethoxazole, clindamycin and tetracycline. Erythromycin susceptibility stands at 58%.


DISCUSSION
Rates of CA-MRSA, as a percentage of all S aureus isolates, were higher than those reported in other primary care series. The infection rate per 100,000 is one the highest reported in Canada. Antibiotic susceptibilities were unchanged during the study period; the 99% susceptibility rate to clindamycin differs from a 2010 Vancouver (British Columbia) study that reported only a 79% susceptibility to this antibiotic.


CONCLUSION
There are very high rates of CA-MRSA infections in northwestern Ontario. Disease surveillance and ongoing attention to antibiotic resistance is important in understanding the changing profile of MRSA infections. Social determinants of health, specifically improved housing and sanitation, remain important regional issues.


T he incidence of community-associated methicillin-resistant
Staphylococcus aureus (CA-MRSA) has traditionally been high in remote areas of Canada with large Aboriginal populations (1-11). S aureus infections are increasingly found to be CA-MRSA, rendering beta-lactam therapy ineffective (12)(13)(14).
In northern Saskatchewan, 51% of Staphylococcus infections are CA-MRSA (2); in southern Manitoba, the rate is 40% (5). These infections most often affect children <10 years of age (30.4% of all CA-MRSA infections in northern Saskatchewan) and typically result in soft tissue infections (2). Associated risk factors for such high rates in these populations include overcrowding, inadequate housing and poor sanitation (1).
The purpose of the present study was to document the incidence of CA-MRSA in northwestern Ontario (2008 to 2012), examine the pattern of antibiotic resistance and review the seasonality of these infections.

Participants
The region is home to 28,000 First Nations people (Ojibway and Cree), most of whom lived in 28 remote, fly-in communities with populations between 400 and 2400 in northwestern Ontario. Regional inpatient and outpatient bacteriology was processed through the Sioux Lookout Meno Ya Win Health Centre. Population age grouping was ascertained by combining Indian and Northern Affairs Canada and Ontario Local Health Integration Network information in 2011.

RESULTS
The laboratory recorded 2451 MRSA isolates (new infections) from January 2008 to July 2012. The majority of the isolates (82%) were from soft tissue infections. The number of tissue swabs remained constant over the time period of the study (6000 to 8000 annually).
In 2012, 56% of the S aureus isolates were CA-MRSA strains as determined by antibiograms, which was an increase from 31% in 2008 ( Figure 1).
In 2010, a six-month in-hospital program screened registered inpatients or patients visiting the emergency department and found that 5% of nasal swabs tested positive for CA-MRSA. Other than this screening, all samples were taken for clinical management of infections.
The number of MRSA infections increased over the past five years and the increasing trend over the four years of complete data was borderline significant (P=0.06). Serious cases of sepsis and necrotizing pneumonia were rare but were also rising; one fatality from necrotizing pneumonia was recorded. Over the five-year period, skin and soft tissue infections increased by a factor of 2.4, while bacteremias, typically seen only once a year, were occuring almost monthly. The rate of CA-MRSA infection in the last year of complete data (2011) was 2482 per 100,000 population. Reinfections accounted for approximately 25% of infections ( Figure 2).
The infections were most commonly found in soft tissue and were more common in younger patients, with 35% of infections occurring in patients <10 years of age; the highest number of infections appeared in those zero to four years of age (Figure 3). Between 2008 and 2012, there was a seasonal trend to CA-MRSA infections, with the peak number of infections occurring in July and August (Figure 4).

DISCUSSION
The rates of CA-MRSA as a percentage of all S aureus isolates in the present study was higher than those reported in other primary care series (2,5,6,9,10,13). This may be due to the absence of running water in many of our regional communities, as well as inadequate housing (15). Our increasing reinfection rate may be a reflection of this reality because reinfections would more likely occur in an unchanged, high-risk environment. Additionally, the infection rate per 100,000 is one the highest reported in Canada and comparable with the 2011 northern Saskatchewan study (2), which showed a community range from 1460 infections per 100,000 to 4820 infections per 100,000.
Most of the infections in the present study were diganosed in soft tissue and this is consistent with most CA-MRSA studies (2). The reason for the high rate of reinfection in our study is not apparent. It is not clear whether these represent failure of therapy, persistent carriage or reinfection. We also identified a recent increase in bacteremia but it is too early to know whether this trend will continue.
Regarding seasonality, the number of infections increased during summer months when insect bites and perspiration are most prevalent in our region. Insect bites have been identified as an associated risk (40%) in a study by Golding et al (1).
Our regional antibiotic susceptibilities were unchanged during the study period. Trimethoprim/sulfamethoxazole, tetracycline and clindamycin all remained effective antibiotic choices. This differs from a study conducted in Vancouver (British Columbia) hospitals in 2010 (16), which showed only a 79% susceptibility to clindamycin. We continue to prospectively track antibiotic resistance through our hospital laboratory system.
Limitations of our study include difficulties in manually accessing information records in different formats by different hospital departments and outside agencies, whose protocols changed during the study period. Some data were not present and are recorded as such. Other than site tissue swab, gathering clinical information was beyond the scope of the study.

CONCLUSION
There are high rates of CA-MRSA infections in northwestern Ontario. Ongoing disease surveillance is important, as is continued attention to antibiotic resistance and stewardship to understanding the changing profile of MRSA infections. Social determinants of health; specifically, improved housing and sanitation, remain issues in many communities.