Correlation between animal nasal carriage and environmental methicilin-resistant Staphylococcus aureus isolates at U.S. horse and cattle farms

Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, 615 N. Wolfe Street, Baltimore, MD 21205, United States.
Veterinary Microbiology (Impact Factor: 2.51). 06/2012; 160(3-4). DOI: 10.1016/j.vetmic.2012.06.032
Source: PubMed


Animals on farms may be a potential reservoir and environmental source of methicillin-resistant Staphylococcus aureus (MRSA). Expanded surveillance methods for animal-associated MRSA are needed. To develop an environmental sampling method and to determine the correlation between animal and environmental MRSA positivity in the farm setting, we sampled horses, cattle, and their local environments at several farms in the mid-Atlantic United States. We obtained nasal swabs from 13 racehorses at first visit, and 11 racehorses at the same farm eight weeks later. We also sampled 26 pleasure horses and 26 beef cattle from two additional farm sites. Sterilized electrostatic cloths were used to collect dry dust samples from environmental surfaces in proximity to animals; cloths were cultured using a broth enrichment protocol. We described isolates by genotype and antimicrobial susceptibility phenotype. None of the samples (nasal or environmental) were positive from the pleasure horse farm or the cattle farm. On the racehorse farm, 8/13 (61%) nasal and 5/7 (71%) environmental samples were positive for MRSA at the first visit. Isolates found were indistinguishable by pulsed-field gel electrophoresis (PFGE) genotype. We observed significant positive correlation between nasal carriage of MRSA in animals and our ability to isolate MRSA from dry surface samples of their local environments. The methods presented here may aid in surveillance efforts for equine and other animal MRSA. This study successfully applies existing MRSA surveillance methods for indoor, high animal density settings to outdoor and low-density farms.

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    • "This PFGE clone has been frequently reported in horses as an endogenous MRSA strain and has been classified as a nosocomial or hospital-acquire MRSA (HA-MRSA) [31,32]. These results provide further evidence that among the constellation of MRSA strains circulating in humans, certain strains are more likely to circulate in horses, their environments and their human contacts as has been previously suggested [4,6,32,33]. In contrast, other strains classified in this study as USA100, USA300 and USA800 are clones frequently reported in the US human population, either as a cause of nosocomial infections (HA-MRSA, USA100 and USA800) or present in the general population (also known as community-acquired MRSA [CA-MRSA], USA300) [34,35]. "
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    • "No known risk factor following the national MRSA guideline was present [1]. However, she mentioned regular contact with horses and lived in a rural area, which are sometimes mentioned as a risk factor for MRSA carriage [14-16]. All hospital contacts of known patients positive for MRSA t011 (n = 39) were checked for contact with the HCW in the period between negative and positive status (October 2008 and April 2009). "
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