Methicillin-resistant Staphylococcus aureus colonization in schoolteachers in Ontario.

A prospective study of methicillin-resistant Staphylococcus aureus (MRSA) colonization was performed involving teachers at a science teachers' conference in Toronto, Ontario. Nasal swabs and questionnaire data were collected from consenting individuals. MRSA colonization was identified in seven of 220 (3.2%) participants. No colonized individuals reported recent contact with the health care system, antimicrobial therapy, residence with health care workers or previous MRSA infections. Methicillin-susceptible S aureus colonization was identified in 72 of 220 (33%) individuals. The prevalence of MRSA colonization was higher than expected for a purportedly low-risk population.


Methicillin-resistant Staphylococcus aureus colonization in schoolteachers in Ontario
A prospective study of methicillin-resistant Staphylococcus aureus (MRSA) colonization was performed involving teachers at a science teachers' conference in Toronto, Ontario. Nasal swabs and questionnaire data were collected from consenting individuals. MRSA colonization was identified in seven of 220 (3.2%) participants. No colonized individuals reported recent contact with the health care system, antimicrobial therapy, residence with health care workers or previous MRSA infections. Methicillin-susceptible S aureus colonization was identified in 72 of 220 (33%) individuals. The prevalence of MRSA colonization was higher than expected for a purportedly lowrisk population.
or contact with an individual diagnosed with MRSA, and animal contact.

MRSA identification, characterization and typing
Swabs were placed in 2 mL of enrichment broth consisting of 10 g/L tryptone T, 75 g/L sodium chloride, 10 g/L mannitol and 2.5 g/L yeast extract for 24 h at 35°C. An aliquot of broth was inoculated onto mannitol salt agar for isolation of methicillinsusceptible staphylococci and onto selective chromogenic agar (CHROMagar, BD Diagnostics, USA), and was incubated at 35°C. Plates were read at 24 h and 48 h, and presumptive staphylococcal isolates were subcultured and identified based on colony morphology, Gram stain, catalase reaction, tube coagulase assay and S aureus latex agglutination test (Pastorex Staph Plus, Bio-Rad Laboratories Ltd, Canada). Methicillin resistance was confirmed by demonstration of growth on Mueller Hinton agar, with 6 μg/mL oxacillin for 24 h at 35°C, and by demonstration of penicillin-binding protein 2a antigen using a latex agglutination antibody screening test (Denka Seiken Co Ltd, Tokyo, Japan). MRSA isolates were typed via SmaI pulsed-field gel electrophoresis, and categorized according to Canadian epidemic MRSA (CMRSA) clones (13). Isolates were tested for Panton-Valentine leukocidin (PVL) genes, lukS and lukF using real-time polymerase chain reaction (14).

Statistical analysis
Prevalence and 95% CIs were calculated for overall S aureus, MRSA and MSSA colonization. Categorical comparisons were performed using χ 2 analyses. Logistic regression was used to evaluate the association between age and colonization. P<0.05 was considered significant for all comparisons. Risk factors for S aureus colonization were evaluated via stepwise forward logistic regression. Variables achieving a liberal significance level of P≤0.15 in the univariate analyses were considered for inclusion in the multivariable model. Variables achieving a P<0.05 level in the final model were considered significant; ORs with 95% CIs were calculated.
The prevalence of methicillin resistance among S aureus isolates was 8.9% (seven of 79 isolates). No significant variables for MRSA colonization were identified using univariable analysis. Multivariable analysis was not performed because of the low prevalence. No individuals reported contact with a household member known to be infected or colonized with MRSA.

DISCUSSION
The present study is the first to evaluate MRSA colonization in teachers in Canada; the 3.2% colonization rate was unexpectedly high. This is particularly true when one considers that   (15,16); the potential impact of child contact on MRSA colonization should be considered. Similarly, contact with children was also identified as a significant risk factor for MRSA colonization in dogs (17). While one can hypothesize that child contact was a potential reason for the MRSA colonization rate identified here, it was not proven in the present study; future studies should evaluate the role of child contact on communitybased colonization rates. It is also plausible that the MRSA colonization rate identified simply reflects an increased baseline community colonization rate. The population prevalence of MRSA colonization in the United States has been estimated to be 0.94% (8); however, that estimate was based on data from 2001 to 2002, and it is unknown whether there has been an increase in colonization since that time. Similar Canadian studies have not been reported, with available studies focusing on high-risk groups, and reporting rates of 32% in hospitalized individuals (10), 7.4% in intravenous drug users (11), 13% in horse owners and veterinarians (18), and 20% in pig farmers (19), but the markedly different populations limit any reasonable comparisons. Further studies of people in the community without identifiable risk factors would be useful. The predominance of CMRSA-2 was not surprising because this is currently the most common MRSA strain causing infections in Canada (20). Identification of CMRSA-10, also known as USA300, was similarly unsurprising because this PVL-containing clone appears to be emerging in Canada (4,21) and is a tremendous cause of concern in the United States (22,23).
The prevalence of MSSA colonization in the present study (36%) was similar to various other studies. This result, therefore, suggests that the self-sampling and swab handling practices were appropriate and that the MRSA rate reported in the present study was not an underestimation of the true prevalence.
The association between MSSA colonization and the male sex has been reported elsewhere (3,6), but the association between pet ownership and MSSA colonization has not been previously reported. Animal contact risk factors were evaluated because of reports (24)(25)(26)(27) linking animals with MRSA infection and colonization in humans. The association of animal contact with MSSA colonization was interesting, and supports the hypothesis that animals can be associated with transmission of MRSA to humans, and that the role of pets in colonization and infection in humans should be further investigated. The protective effect of weekly contact with animals, not including the individuals' household pets (ie, farm animals or other peoples' pets), and MSSA colonization was unexpected, and an explanation for this is not readily apparent. Unfortunately, specific details regarding this type of animal contact were not collected.
Potential limitations of the study and biases of the study population need to be considered. Participation was voluntary, and it is unclear whether that affected the prevalence of MRSA. Random selection of participants would have been ideal, but was not possible for ethical reasons. As discussed above, in hindsight, it is possible that teachers may not reflect the true population prevalence because of frequent contact with children, but this hypothesis is unproven.
The present study has raised questions about whether the prevalence of MRSA colonization in individuals in the community is higher than has been previously considered, or whether schoolteachers may be at a higher risk for MRSA exposure. Further study of both of these questions is indicated to better understand the epidemiology of this concerning pathogen. Furthermore, the association between pet ownership and MSSA colonization, while not readily explained, provides additional support to the role of animals, especially household pets with which people often have close and prolonged contact, in the epidemiology of infectious diseases.
ACKNOWLEDGEMENT: The authors thank the Science Teachers' Association of Ontario/L'association des professeurs des sciences de l'Ontario for their assistance with this study.
Methicillin-resistant S aureus colonization in schoolteachers