Prevalence and antimicrobial susceptibility of methicillin-resistant Staphylococcus aureus among pigs in Belgium.

The prevalence, distribution, and antimicrobial susceptibility of methicillin-resistant Staphylococcus aureus (MRSA) in Belgian pig farms has been investigated. To that end, nasal samples were collected from 1,500 pigs on 50 farms randomly selected over Belgium. Both closed (breeding or farrow-to-finish) and open (fattening) farms were included. Within closed farms different age groups were investigated. A total number of 663 (44%) pigs belonging to 34 (68%) farms carried MRSA. According to their management practice, MRSA was detected on 94% of the open farms and 56% of the closed farms. Focusing on the in-herd prevalence among fattening pigs for both management systems, a significantly higher rate was found in open farms (72%) compared to closed farms (26%). Within the closed farms, piglets (41%) showed a higher MRSA prevalence than sows (26%) and fattening pigs (26%). All strains tested were ST398 and showed mainly spa-type t011, as commonly found on pig herds in Europe. Less dominating spa-types were t034, t567, and t2970. The MRSA strains carried two SCCmec-types, type IVa or V. All 643 MRSA strains were resistant to tetracycline and additional resistances to trimethoprim (97%), lincosamides (73%), macrolides (56%), aminoglycosides (48%), and fluoroquinolones (32%) were found. Multiresistance (defined as resistance to four or more non-β-lactam antimicrobial classes) was found in 63% of the tested strains. In conclusion, a high prevalence of MRSA was found in Belgian pig farms, with the highest prevalence in open farms. In accordance with other European countries, age-related and management-related differences in MRSA prevalence were observed that should be considered when control strategies are outlined.