Methicillin-resistant Staphylococcus aureus in horses : aspects of detection, clinical infection and the potential for zoonotic transmission
This thesis is an investigation of several aspects of methicillin-resistant ' Staphylococcus aureus' (MRSA) in horses. Evaluation of a real-time polymerase chain reaction (PCR) assay for rapid identification of MRSA directly from nasal swabs in horses was performed by comparing the test to enrichment culture. Despite previous validation for use with human nasal swabs, this test performed poorly. Initially only 176/293 samples yielded valid PCR results due to uncharacterized factors associated with equine nasal specimens. Two of 176 and 167/176 samples were positive and negative, respectively, by PCR and culture. Seven of 176 samples were PCR positive/culture negative, whereas 0/176 samples were PCR negative/culture positive. The 'kappa' statistic was 0.35, which represented poor agreement between the tests. A retrospective case study of 115 horses infected with MRSA was performed to provide an overview of the characteristics of clinical MRSA infections in horses. A broad range of clinical infections were reported, but opportunistic infections were the most common. Community-associated (CA) infections (56/114, 49.1%) were also common. Overall 93/111 (83.8%) cases survived to discharge. Significant associations were found between CA-MRSA infection and previous hospitalization, previous gentamicin therapy and incisional infections. Mortality was significantly associated with intravenous catheterization, CA-MRSA infection, and dissemination of the MRSA infection. A study of MRSA colonization in veterinary personnel attending an equine veterinary conference found a sample prevalence of 26/257 (10.1%). Colonization with MRSA was associated with having been diagnosed with or having treated a horse diagnosed with MRSA in the last year (odds ratio [OR] 8.41, ' P' = 0.015; OR 3.27, 'P' = 0.039, respectively). Hand washing between infectious cases (OR 0.27, 'P' = 0.009) and between farms (OR 0.35, 'P' = 0.047) were protective factors. These data support suggestions that equine veterinary personnel may be at increased risk of colonization with MRSA. This was the first study to demonstrate a statistically significant association between hand hygiene practices and a measurable clinical outcome in veterinary medicine. Further research is required to better understand the role of MRSA in equine medicine and people who work with horses. This research will be useful for directing future studies in this area.