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MRSA in Conventional and Alternative Retail Pork Products

28 July 2012

Research in the US has confirmed the presence of Staphylococcus aureus on raw pork products in the United States, regardless of whether it was produced conventionally or from animals raised without antibiotics or antibiotic growth promoters.

The research from the University of Iowa, the Center for Emerging Infectious Diseases, the Department of Biostatistics and the Institute for Agriculre and Trade Policy found no statistically significant differences were observed when comparing S. aureau (whether MSSA or MRSA) positivity on conventional and alternative pork products.

This is in contrast to findings from the Netherlands, where a lower prevalence of MRSA in meat from “biological” reared chickens (raised using no growth promotants) and wild fowl and game was found, in comparison to the prevalence of MRSA found in conventionally raised poultry in the same study, suggesting a link between antibiotic use and prevalence of MRSA on meat products in Europe.

The research team of Ashley M. O'Brien, Blake M. Hanson, Sarah A. Farina, James Y. Wu1, Jacob E. Simmering, Shylo E. Wardyn, Brett M. Forshey, Marie E. Kulick, David B. Wallinga and Tara C. Smith reported that this was also shown in a study of US pigs, where MRSA was found in four of nine “conventional” pig farms, but was absent from pigs raised without antibiotics (Smith, unpublished findings).

It is possible that a difference in MRSA contamination between conventional and alternative meats does not exist in our sampling areas.

Alternatively, in the US, it is possible that a link between on-farm antibiotic use and MRSA on meat products may be obscured by human contamination of meat with MRSA post-slaughter, as human carriage of MRSA in the US is approximately 50 times higher than what has been reported in the Netherlands (1.5 per cent vs. .03 per cent).

However, given that 85.7 per cent (6/7) of the MRSA-positive alternative pork samples in our study came from the same retail chain in two different states and four from the same store, there could be other explanations for our findings. Both antibiotic claims used to identify alternative products for this study, “raised without antibiotics” and “raised without antibiotic growth promotants”, are not typically verified by an independent third party, unlike USDA-certified Organic meat products, which are also raised without antibiotics.

While certified organic pork products were available at some of the stores included in this study, they were not available in an appropriate form for our testing, e.g., they were smoked, frozen or pre-cooked.

Additionally, similar to meats raised conventionally, pork raised without antibiotics can be contaminated with MRSA at the processing plant, either from contaminated products processed on the same equipment or by colonised workers.

Processing equipment must be cleaned out between runs of certified organic and non-certified organic meats, which may help to prevent this type of cross-contamination the study said.

Two Dutch studies showed MRSA colonisation of slaughterhouse workers: one swine-based study that found MRSA ST398 and one broiler-focused study. In both studies transmission appeared to go from animals to humans, but exact transmission routes were not determined.

The research team said they did not beleive that any similar research had been published in the US.

In order to examine the prevalence of Staphylococcus aureus on retail pork, three hundred ninety-five pork samples were collected from a total of 36 stores in Iowa, Minnesota, and New Jersey. S. aureau was isolated from 256 samples (64.8 per cent, 95 per cent confidence interval [CI] 59.9 per cent–69.5 per cent). S. aureau was isolated from 67.3 per cent (202/300) of conventional pork samples and from 56.8 per cent (54/95) of alternative pork samples (labelled “raised without antibiotics” or “raised without antibiotic growth promotants”). Two hundred and thirty samples (58.2 per cent, 95 per cent CI 53.2 per cent–63.1 per cent) were found to carry methicillin-sensitive S. aureau (MSSA). MSSA was isolated from 61.0 per cent (183/300) of conventional samples and from 49.5 per cent (47/95) of alternative samples.

Twenty-six pork samples (6.6 per cent, 95 per cent CI 4.3 per cent–9.5 per cent) carried methicillin-resistant S. aureau (MRSA).

No statistically significant differences were observed for the prevalence of S. aureau in general, or MSSA or MRSA specifically, when comparing pork products from conventionally raised swine and swine raised without antibiotics, a finding that contrasts with a prior study from the Netherlands examining both conventional and “biologic” meat products.

In the present study spa types associated with “livestock-associated” ST398 (t034, t011) were found in 26.9 per cent of the MRSA isolates, while 46.2 per cent were spa types t002 and t008—common human types of MRSA that also have been found in live swine. The study represents the largest sampling of raw meat products for MRSA contamination to date in the US.

Overall, MRSA prevalence on pork products was similar to what has been found in other studies conducted in Canada, but higher than that previously identified in studies conducted in the US.

A significant proportion of MRSA isolates (26.9 per cent) were spa types associated with ST398 (t034, t011), a “livestock-associated” strain. An additional 23.1 per cent each were spa types t002 and t008, common human types that have also been found in live swine.

A limitation of this study is the research team's inability to determine the ultimate origin of these isolates - whether they are a result of contamination from the farming environment, human contamination post-slaughter, or both.

Also unknown is the role that MRSA contamination of raw meats in the US, now confirmed by several studies, may play in the overall ecology and transmission of this organism.

Bacterial contamination was not quantitated in the study; as such, the amount of initial colonies present on the pork samples is unknown.

Also unknown is the frequency of MRSA transmission to humans, via colonization or infection from food service professional and consumer handling and consumption of raw, undercooked and cooked MRSA-positive meat.

A study in the Netherlands examining MRSA carriage by food handlers did not find any MRSA-positive individuals, even though some had been handling meat products which were contaminated with MRSA. However, it is difficult to directly apply these findings to consumers, who may not receive as much training in proper food handling. Future studies are necessary to examine this aspect.

Further Reading

- You can view the full report by clicking here.
June 2012

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