Use of antimicrobials in Equine Practice across North America: a case-based survey
designated PBP2a, which present a very low affinity for betalactam antibiotics. The mecA gene is located on a mobile genetic element, named staphylococcal cassette chromosomemec (SCCmec) inserted in the S. aureus chromosome. In addition, MRSA are frequently resistant to a wide range of additional antimicrobial classes. From January 2007 to June 2011, 912 S. aureus isolates were obtained from various clinical samples. Identification of MRSA was confirmed by amplification of the nuc (specific of S. aureus) and mecA genes. MRSA strains were typed by pulsed field gel electrophoresis (PFGE) and by Multilocus Sequence Typing (MLST). SCCmec elements (types I to VI) and genes encoding virulence factors Panton-Valentine Leukocidin (PVL), Toxic Shock Staphylococcal Toxin (TSST1) and Enterotoxin A were characterized by using a PCR approach. Susceptibility testing of MRSA isolates was performed using the agar diffusion method and following drugs were tested: erythromycin (ERY), tetracycline (TET), streptomycin (STR), gentamicin (GEN), rifampin (RIF), cotrimoxazole (TMP-SULF), chloramphenicol (CHL) and ofloxacin (OFL). Genes conferring resistance to macrolides [erm(A), erm(B), erm(C)], gentamicin [aac(6’)-aph(2’’)], and tetracyclines [tet(M)] were amplified by PCR. Among the 912 S. aureus isolates between 2007 and 2011, 19 strains were characterized as MRSA. Their prevalence increased until 6% in2010 and4% in thefirst sixmonths of 2011. 9 types could be distinguished by PFGE. MLST revealed thatMRSA isolated belonged to two sequence types, ST8 and ST398. All ST398MRSA (n1⁄47)were detected in 2010 including two isolates originating from the same farm. These results suggested possible cross-transmission of MRSA. The isolates, except two, possessed SCCmec IV and did not harbor the genes for the virulence factors PVL, TSST1, and enterotoxin A. All isolates were multiply resistant to other antimicrobials. PCR experiments showed that resistance to quinolone was active efflux, while to gentamicin and tetracycline was due to the presence of the aac(6’)-aph(2’’) and tet(M)genes, respectively. The increase in the prevalence of MRSA in horse infections is related to the emergence and spread of the ST398 type. This type is known to cause outbreaks in horses, to be responsible of cross-colonization and eventually infect humans. Farm hygiene and antimicrobial use contribute to MRSA occurrence in animals. Hygiene measures should, therefore, be maintained or reinforced to limit the transmission of S. aureus between horses and also between horses and personnel.