CategoryPublic Health - MRSA
Date Full Report Received08/03/2011
Date Abstract Report Received08/03/2011
Funded ByNational Pork Board
This study was conducted on ten conventional swine farms in Ohio to investigate the occurrence and prevalence of specific bacteria known to be resistant to methicillin called methicillin-resistant Staphylococcus aureus (MRSA) in pigs on-farm (before transportation), at lairage (after transportation), carcass swabs and retail pork. We also studied the relatedness of the MRSA isolates recovered across the pork production chain (farm-to-retail) using phenotypic and genotypic methods. We collected paired nasal and peri-anal swabs from finisher pigs on-farm (n=24/farm) and followed the same batch of pigs and sampled on arrival at lairage. Swab samples were collected from both anterior nares and the peri-anal region of corresponding pigs. Matching carcass swabs (24/farm) were collected at the post evisceration stage before chilling from those batches of pigs sampled on-farm and at lairage. Pork samples from the same batch of pigs were collected at retail market. Samples were examined for the presence of MRSA following conventional methods. Antimicrobial susceptibilities of the isolates were also determined at the USDA-BEAR using SensititreTM. Polymerase chain reaction (PCR) was performed for the detection of Staphylococcus aureus -specific gene (nuc), methicillin resistance marker gene (mecA) and staphylococcal cassette chromosome (SCCmec) typing. The relatedness of isolates was determined using a DNA finger printing method called pulsed-field gel electrophoresis (PFGE) and we also used the multi-locus sequence typing (MLST) to determine the sequence types of MRSA isolates across the sampling points. One or more MRSA positive pigs were detected in five of the ten herds (50%). The prevalence of MRSA in pigs was higher at lairage and ranged from 0% to 54.2% per farm compared to the same batch of pigs sampled on-farm (0% to 12.5%). MRSA was detected in 1.7% (4/235) of the carcass swab and 3.7% (5/135) of the retail pork samples. The MRSA isolates were multidrug resistant (resistance to three or more antimicrobial agents) and besides resistance to β-lactam antimicrobials (such as ampicillin, oxacillin, penicillin and gentamicin), resistance to tetracycline (76.4%), clindamycin (72.7%) and erythromycin (62%) was detected. MRSA isolates belonged to SCCmec-type II, III, V and others not belonging to any of the known types. Related isolates were detected across all stages of the pork production chain. Genotyping using MLST of selected isolates revealed that MRSA sequence type ST398 was detected from pigs on-farm, at lairage and retail pork samples. The present findings show that MRSA can be detected through out the pork production chain and suggests the need for further detailed studies in commercial swine farms in the US.