Methicillin-resistant Staphylococcus aureus (MRSA) is a primary cause of nosocomial infections in human hospitals and is a predominant cause of skin and soft tissue infections in adults. MRSA is problematic because its resistance to multiple antimicrobials makes it difficult to treat. Additionally, MRSA can easily spread among people and the environment. MRSA strains isolated from companion animals are often identical to the prevailing clones found among people within the same region, and these strains share similar features with those strains from humans. Also troubling for veterinary patients, is the presence of methicillin resistance in the S. intermedius group (SIG) of bacterial organisms, which includes S. intermedius , S. pseudintermedius, and S. delphini. Healthy dogs frequently have S. pseudintermedius on their skin, and it is a primary cause of canine pyoderma. While a common inhabitant of canine skin, S. pseudintermedius is also an opportunistic pathogen in animals, and it has been shown to cause infection in humans due to zoonotic transmission of the agent and to acquisition from unknown sources. The mecA gene transforms these isolates into methicillin-resistant Staphylococcus pseudintermedius (MRSP), which are often multidrug-resistant.
We first wanted to determine the occurrence of both S. aureus and the SIG in canine, feline, and equine patients visiting The Ohio State University Veterinary Medical Center (OSU-VMC) and the antimicrobial resistance patterns associated with these pathogens. We found that S. aureus accounted for less than 3% of all bacterial isolates from these patients, but isolates within the SIG accounted for approximately 18% of all bacterial isolates from patient samples during this same time period.
Our second objective was to determine the occurrence of MRSA among patients and the environment in a veterinary teaching hospital and to compare the predominant clonal features of patient isolates with MRSA strains of known public health significance. Samples were obtained as described above. Polymerase chain reaction (PCR) was used to detect the mecA gene in S. aureus isolates and to determine which SCCmec types were present among these strains. Pulsed-field gel electrophoresis (PFGE) fingerprinting was used to assess the genetic similarity of the patient strains with each other and with environmental strains collected over a similar time period. Although S. aureus was isolated infrequently, all available isolates were tested for both phenotypic oxacillin resistance and genotypic oxacillin resistance conferred by the mecA gene. Approximately 50% of canine and equine isolates and approximately 20% of feline isolates were MRSA as determined by phenotypic oxacillin resistance or the presence of the mecA gene; in rare cases, the oxacillin phenotype and genotype did not agree. Canine and feline isolates were most commonly SCCmec type II, characteristic of healthcare-associated MRSA (HA-MRSA) isolated from people, and isolates from equine patients were most often SCCmec type IV (84.6%), characteristic of community-associated MRSA (CA-MRSA). Analysis of PFGE data showed our small animal MRSA isolates were most similar to USA100, a genotype often seen with HA-MRSA, and this same finding was true for the majority of environmental samples collected from small animal areas of the hospital. MRSA strains from equine patients were most similar to USA500 (by PFGE), a genotype also associated with HA-MRSA.
Our third objective was to determine the occurrence of methicillin-resistance among the SIG and characterize the features of these isolates in our patient population. Overall, SIG isolates accounted for 18.2% of all bacterial organisms cultured from dogs, cats, and horses. The proportion of methicillin-resistant SIG isolates ranged from 37.6-48.9% in dogs, 30-58.8% in cats, and 0-20% in horses. Those isolates categorized as methicillin-resistant based only on phenotypic oxacillin resistance represented a smaller percentage of isolates than those strains which were categorized as methicillin-resistant based on genotypic oxacillin resistance; thus, many SIG isolates carried the mecA gene but did not necessarily exhibit phenotypic resistance to oxacillin. Canine and feline mecA-positive SIG isolates were most commonly SCCmec type V(VII), while around one-third these isolates from dogs were non-typeable with the PCR methods described in this study.
The majority of S. aureus and SIG in canine and feline patients and S. aureus in equine patients were often multidrug resistant. Additionally, a high proportion of isolates appeared to be methicillin-resistant. Small animal MRSA strains commonly exhibited features similar to HA-MRSA. These findings are pertinent to public health because humans and small animals may transmit MRSA to each other through occupational exposure or companionship. A similar scenario may also occur with horses and people. Methicillin resistance among the SIG isolates, MRSP in particular, poses a risk to veterinary patients and may be inadvertently spread between animals by people.
|Commitee:||Burkhard, Mary Jo, Daniels, Joshua B., Hillier, Andrew, Rajala-Schultz, Paivi|
|School:||The Ohio State University|
|Department:||Veterinary Preventive Medicine|
|School Location:||United States -- Ohio|
|Source:||DAI-B 78/11(E), Dissertation Abstracts International|
|Subjects:||Public health, Epidemiology|
|Keywords:||Antimicrobial resistance, MRSA, MRSP, Methicillin, Veterinary teaching hospital|
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