Sampling, prevalence and characterization of methicillin-resistant Staphylococcus aureus on two Belgian pig farms

This study investigated the spread of MRSA (methicillin-resistant Staphylococcus aureus) on two Belgian pig farms. Pigs of different ages (from farrowing to slaughter age and sows) as well as the barn environment were screened extensively on two occasions three months apart. A subset of MRSA isolates was tested for antimicrobial susceptibility to 16 antibiotics and was further characterized by pulsed-field gel electrophoresis. Ninety-five percent and 77% of the tested pigs on farm A and farm B, respectively, were colonized with MRSA. MRSA positive animals were detected in all age categories sampled on each sampling day. Piglets were already colonized in the farrowing unit with the same or other MRSA strains than their mother. The prevalence of MRSA colonized pigs increased significantly after weaning and decreased during the fattening period. Pigs carried MRSA mainly in the nares, followed by the perineum and skin and to a lesser degree the rectum. A pig could be contaminated or colonized with different MRSA strains at the same time. The barn environment was also found to be contaminated with different MRSA strains, including the air inlet and outlet. All isolates tested on both farms were resistant to both tetracycline and trimethoprim, while they were susceptible to rifampicin, mupirocin and linezolid. There was a significant difference in resistance prevalence between the two farms for the antibiotics gentamicin, kanamycin, tobramycin, tylosin, lincomycin and quinupristin/dalfopristin. Furthermore, several antibiotic resistance profiles were observed within one farm. This study clearly indicates that several MRSA strains circulate on one farm, from the nursery unit to the fattening unit. This is important to consider when attempts are made to remediate these farms.