Industry Summary
The bacterium Staphylococcus aureus is a commonly found on the skin and mucous membranes of a wide range of mammals; and usually exists in a non-clinical state. One subgroup of Staphylococcus aureus that has been particularly important clinically in healthcare and community settings is Methicillin-Resistant Staphylococcus aureus (MRSA). This bio-type has received significant attention from the medical community; and currently is a high priority for reduction in hospital environments by the Centers for Disease Control and Prevention (CDC website, 2013). Dutch researchers first identified MRSA ST398 as a clonal type in animals. This finding has raised the issue of MRSA ST398 as a potential human food safety issue in meats. One frequently raised assertion is: The presence of on-farm carriage of ST398 results in ST398 contamination of retail pork. Between the farm and retail sectors are a series of harvest and processing steps which may impact that connection. This study was designed to examine the impact of harvest and initial carcass handling on the presence of MRSA. It serves as an introductory foray into the potential impacts on MRSA carriage through lairage and initial carcass processing. In addition the relationship between skin swabs and nasal swabs on individual swine was studied to demonstrate correlative values for these different sampling strategies.
From the 15 observation dates, 58 lots complete sample sets representing four (4) production steps – receiving (skin swab), post lairage (pre-gondola skin swab), post-stun (shackle nasal swab), and pre-chill (carcass swab), were analyzed for the presence of MRSA using traditional bacteriologic methods. The first and most striking observation was the drop in carrier isolations pre-chill when compared to receiving, and post-lairage samples (1.2%, 29.3% and 65.5%, respectively). Such an observation is consistent with current plant-based HACCP procedures substantially reducing MRSA carriage from the live animal to pre-chill carcass stage. Because the lots were selected from direct site to abattoir loads, the entry rate may be used as an estimate for on-farm carriage rates. The post-lairage isolation rate was greater than at entry (29.3% vs. 65.5%), which supports the proposition that during the lairage period additional MRSA transmission between animals occurs due to skin contact and environmental contamination. Within this general rise in MRSA isolation, the results varied widely by date. Second, MRSA typing from a randomized draw of post-lairage and post-stun results demonstrated the presence of 21 ST398 and 12 ST5 patterns across all sampling dates. All but one ST398 isolate (20) were found in the March 27 cohort, while the ST5 isolates were scattered throughout the 15 sampling dates. Within the March 27th cohort all four lots had post-lairage positive ST398 isolates, but no carcass (pre-chill) swabs were ST398 positive, nor were any other MRSA bio-types identified pre-chill on that date. Conversely, the seven ST5 isolates on pre-chill carcasses were derived from four lots on three different days. Only one of the four receiving lots contained an MRSA positive sample. These observations lend credence to the lack of direct MRSA transfer from live swine to pre-chill carcasses. A probable source for the observed contamination at the pre-chill level could not found because of the project objectives limitations.
At the animal level individual post lairage and post-stun samples were collected to answer whether skin swabs taken post-lairage or nasal swabs taken post stun of individual animals would result in isolation compatibility. Skin or nasal swabs were obtained from 619 post lairage and 615 post-stun animals. A total of 589 post-lairage skin and post stun nasal swab pairs were available for analysis; with 351 pairs both negative and 72 both positive. The remaining 166 pairs were either post-lairage negative and nasal swab positive (88) or reversed designation (78). There was no statistically significant difference in the number of positive isolates each method detected within lots, but they did not statistically predict the same pairs as positive. McNemar test result (p value of 0.48) suggests no reason to suspect that the proportion of samples positive differed between the two locations.
In summary, this data supports observations reported elsewhere: 1) that a potential direct link between live swine MRSA and pre-chill contamination appears to be very low 2) that current HACCP-based interventions in processing provide significant protections from MRSA transfer at the pre-chill carcass level and 3) that skin swabs and nasal swab isolations do not detect the same individual animal carriage, but may be used interchangeably to determine group contamination rates. These data should be interpreted within the context that additional characterization of MRSA ecology in the peri-harvest environment is needed. However, in the interim, routine plant HACCP-based interventions appear capable of greatly reducing the transfer of Staphylococcus aureus from production sites to pre-chill carcass.
Key words: pork, swine, MRSA, peri-harvest, Staphylococcus aureus, lairage
Contact information: Dr. James McKean, DVM, JD; 2225 Lloyd Veterinary Medical Center, Iowa State University, Ames, Iowa, 50011-1250. Telephone: 515-294-8792; Email: [email protected].
