Low prevalence of methicillin resistant Staphylococcus aureus but high prevalence of multidrug resistant Staphylococcus aureus in pigs in the USA

It has been suggested that the emergence of livestock associated MRSA in swine may be a consequence of feeding antibiotics and/or zinc to control swine diseases or promote growth. However, the dynamics and health implications of interspecies transmission of MRSA from pigs to humans is still incompletely understood. To address these questions, better understanding is required of the broader ecology of S. aureus in pigs, including MRSA. To obtain more representative data on the prevalence and characterization of S. aureus, including MRSA, we collected nasal samples from growing pigs on 36 swine farms across 11 states in the USA. All isolates were spa typed, and MLST typing was conducted on selected isolates of each spa type. Antimicrobial and zinc susceptibility testing was performed on a subset of 130 isolates selected purposively to maximize the diversity of isolates by farm and spa type. S. aureus was detected on 35 of the 36 farms, and from 76% of pigs sampled, but no MRSA were found apart from one farm of known status that was included as a MRSA positive control. A total of 33 spa types were detected, with the most prevalent being t337 (ST9), t034 (ST398) and t002 (ST5). Antimicrobial resistance testing for 18 antibiotics showed that resistance was most common to spectinomycin (100%), tetracycline (94%), clindamycin (75%) and penicillin (72%), and a majority of isolates were resistant to 5 or more antibiotics (multidrug resistant S. aureus, MDRSA). Only ST398 MRSA isolates from the positive control farm were positive for the czrC gene, although phenotypic resistance to zinc was observed in some MSSA isolates. These results support earlier studies indicating a relatively low prevalence of MRSA in pigs in the USA, but a high prevalence of MDRSA. The data also concur with the hypothesis that zinc resistance may be more strongly associated with MRSA in swine reservoirs than is resistance of antibiotics such as tetracyclines.