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closeIs it pre- or post-harvest contamination?
Posted by kaltebe on 06 Feb 2012 at 20:18 GMT
The results of this study are, at least in part, somewhat unexpected. This investigators do not find lower prevalence of Staphylococcus aureus, or methicillin-sensitive (MSSA) strains, or methicillin-resistant (MRSA) strains of S. aureus in pork from “biologic” farms (alternative farms refraining from use of antibiotics for raising pigs) as compared to conventional farms that do use antibiotics. This is in contrast to European studies that did find a lower MRSA prevalence in “biologic” pork.
The fact that S. aureus or MSSA did not differ is not surprising, but that MRSA did not differ is unexpected. An interesting facet to this finding is not mentioned in the abstract, but only in results:
“Similar prevalences of MRSA-positivity were found in conventional pork (6.3% [19/300], 95%CI 3.9%—9.7%) and in alternative pork (7.4% [7/95], 95%CI 3.0%–14.6%). Six of the seven MRSA positive alternative pork samples came from the same retail chain (17.1% [6/35] of samples from the chain) in two different states and four from the same store (21.1% [4/19] of samples from the store), the largest number of MRSA isolates associated with anyone store in this study. Of the six MRSA isolates from this chain, five (83%) were spa type t008.”
This suggests the possibility that this difference is not due to contamination with MRSA on biologic farms, but that post-harvest contamination is the cause for the MRSA detection. Further evidence for this conclusion is the fact that the predominant strain found is a typical human-associated MRSA, but not livestock-associated MRSA genotype. This conclusion would be true for pork from biologic farms, but potentially also for a major percentage of MRSA found in conventional pork.
As good research usually does, this study raises more questions than it answers. It will certainly be critical in future studies to separate pre- from post-harvest MRSA contamination of pork, and to conclusively answer the question if antibiotic-free raising of pigs does not, or does, reduce the MRSA prevalence on the farm. We certainly do not know from this study if we humans, and not the animals, are the main, largely post-harvest, contaminators of pork with MRSA.
RE: Is it pre- or post-harvest contamination?
tarasmith replied to kaltebe on 09 Feb 2012 at 00:03 GMT
We do mention the possibility of human contamination and post-harvest contamination. It's very difficult to tell whether this is really "human" or not based on spa type alone, as we've found t008 in live pigs as well with collaborators (unpublished work). I agree with you that it certainly raises many questions.