Methicillin-resistant Staphylococcus pseudintermedius in Atlantic Canada: epidemiology and culture methods

The main aim of this thesis was to obtain information on the distribution and\nprevalence of methicillin-resistant Staphylococcus pseudintermedius (MRSP) in Atlantic\nCanada at both a regional Veterinary Teaching Hospital (VTH) and a regional diagnostic\nlaboratory, as well as exploring culture methods for the detection of this pathogen.\nThe strain type distribution of MRSP using the direct repeat unit (dru) typing\napproach in isolates collected at a regional diagnostic laboratory in Atlantic Canada is\ndescribed in Chapter 2. Ninety-four isolates were successfully typed and demonstrated to\nbelong to 18 different dru types. The majority of isolates belonged to dt11a (30.9%),\ndt10h (24.5%), dt9a (18.1%), and dt11af (10.6%). The remaining 15.9% isolates were\ndistributed between 13 different dru types, nine of which have not been previously\nidentified: dt5k, dt6t, dt8ag, dt9ba, dt9bd, dt10bz, dt10cc, dt10cj, and dt11ca. The\npredominant dru types identified in this study were similar to those found in Ontario,\nCanada; however, cluster 9a appears to be less common in Atlantic Canada. A\nsignificant difference in the distribution of clusters between provinces was detected (p =\n0.01). Notably, differences in the dru types were observed between Newfoundland and\nthe Maritime provinces (New Brunswick, Nova Scotia, and Prince Edward Island).\nCluster 10h was not present in the isolates from Newfoundland, while it was the\npredominant cluster in samples from PEI. Cluster 11a was the predominant cluster in\nboth New Brunswick and Nova Scotia. Resistance to ≥2 antimicrobials (in addition to β-\nlactams) was observed in 71.4% of the isolates, with isolates belonging to cluster 9a\nbeing significantly more resistant to all antimicrobials tested except fusidic acid, chloramphenicol, and doxycycline. This study confirms that dru type distributions can\nvary significantly across the same country, and previously unreported dru types were\nidentified in Atlantic Canada.\nChapter 3 describes the presence of MRSP in a regional VTH environment after\ncleaning and disinfection and its potential to cause hospital-acquired infections. MRSP\nwas isolated from 16/137 (12% [95% CI 6.8%–18.3%]) of environmental samples after\nthe first cleaning and disinfection cycle, and 1/14 (7% [95% CI 0.2%–36.0%]) of\nenvironmental samples after the second cleaning and disinfection cycle. Culture results\nand data retrieved from medical records implicated a hospital-acquired MRSP infection\ninvolving two canine patients. Pulsed-field gel electrophoresis (PFGE) revealed that the\nMRSP isolates from these patients were clonal, further supporting a hospital-acquired\ninfection. Since the same hospital personnel handled the MRSP positive patients in this\ncase, transmission between patients was most likely via hospital personnel. PFGE of\nstrains cultured from the environment and patient revealed that environmental strains\nwere clonal to those from the patient associated with that area; however, non-clonal\nstrains were also recovered from the same environmental area. The presence of MRSP in\nthe hospital environment after cleaning and disinfection and a suspected hospitalacquired\ninfection highlights the importance of a robust infection control program in\nthese facilities.\nIn Chapter 4, the prevalence of MRSP in clinical samples collected from dogs\ndeemed high-risk for MRSP was estimated. This chapter also compared four culture methods for MRSP: traditional culture, mannitol salt agar with 2μg/mL oxacillin\n(MSAox), mannitol salt enrichment broth (EB) and MSAox, and EB and traditional\nculture. A total of 741 samples were analyzed from 556 individual dogs between\nFebruary 2013 and April 2014. The prevalence of MRSP in samples detected by any\nmethod was estimated at 13.4% (95% CI 11.1%–16.0%). When prevalences were\ncompared according to culture methods, EB and MSAox had the highest prevalence\n(11.2% [9.1%–13.7%]), followed by EB and traditional (10.8% [8.8%–13.2%]), then\ntraditional (10.1% [8.1%–12.5%]), with MSAox having the lowest prevalence (8.9%\n[7.1%–11.2%]). MRSP prevalence using the traditional culture did not differ significantly\nfrom any of the three selective culture methods. Culture with MSAox detected\nsignificantly fewer MRSP than either of the enrichment broth methods. The addition of\nenrichment broth to current methodology is recommended, particularly in patients with a\nprevious history of MRSP positive specimens. These results highlight how differences in\nculture methodology can influence prevalence estimates; therefore, caution should be\nexercised when comparing studies investigating prevalence. The MSAox had a low\nsensitivity for the detection of MRSP at 24 and 48 hours, and is not recommended as a\nrapid culture-based screening tool. The estimated MRSP prevalence in this study is\nsimilar to findings of previous studies in Canada and other parts of the world.

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