Over the last 10 years, concerns have grown about the possible importance of livestock reservoirs as a source of methicillin resistant Staphylococcus aureus (MRSA). S. aureus is a normal bacterial inhabitant of healthy people as well as many animal species including pigs. However, S. aureus is also an important cause of human infections, and among the most significant problems related to antibiotic resistance in human medicine. A small number of studies have confirmed the presence of several variants of MRSA in pigs (types ST398; ST5; ST9) in the USA, but the epidemiology of S. aureus in pigs in the US has not been well documented. We conducted a study of 36 farms located in 11 states of the US to describe the prevalence and types of S. aureus (including MRSA) in the swine reservoir. One additional farm, previously known to harbor MRSA, was included as a positive control farm. We also determined patterns of antimicrobial resistance and the presence of genes producing toxins that can cause food poisoning in people. Because zinc resistance has been implicated in the emergence of ‘livestock associated” MRSA in Europe, we also tested isolates for resistance to zinc and for the presence of a specific gene (czrC) linked to ST398 MRSA in Europe.
Overall, 739 pig nasal swabs were collected, of which 558 (76%) were culture positive for S. aureus from 35 of the 36 farms (97%). Except the positive control farm, on which all 20 pigs tested MRSA positive, no MRSA were detected in any of the pigs. Among the 35 S. aureus positive farms there was considerable diversity found with 33 spa types detected within 4 MLST sequence types. The most prevalent spa types (sequence type) were t337 (ST9), t034 (ST398) and t002 (ST5) which together accounted for 59% (627 of 1070). Antimicrobial resistance testing showed resistance was most common to spectinomycin (100%), tetracycline (94%), clindamycin (75%) and penicillin (72%), and 89% (116/130) of isolates were resistant to 5 or more antibiotics (multidrug resistance SA, MDRSA). ST398 (t034) MRSA isolates from the positive control farm were positive for the czrC gene, but no other isolates tested were positive.
The most striking finding of the study was that none of the 36 study farms were positive for MRSA. Although much higher prevalence (up to 30%) has been previously reported in smaller, and more geographically limited, studies in the USA, relatively low herd prevalence was also reported in larger and geographically diverse studies in the USA and Canada. Our observations on the czrC gene are also aligned with other studies indicating that zinc and other selective factors (e.g., disinfectants and therapeutic use of cephalosporins) are likely to have had a much greater role in the emergence of these organisms than growth promotant usage. Our observation that multiple antimicrobial resistant is prevalent in S. aureus isolates from pigs is both unsurprising and consistent with other studies. However, current evidence suggests that pig adapted S. aureus rarely harbor major virulence factors associated with human disease. Hence the human health significance, if any, of multiple resistance in this group of organisms is yet to be established.