Editor’s note: Dr. Scott Hurd is an Associate Professor and Director of the Food Risk Modeling and Policy Laboratory at Iowa State University’s College of Veterinary Medicine. He authors a blog, Hurd Health and writes The Gentle Vet column for Meatingplace. Dr. Hurd drafted this blog in response to a recent New York Times article, which highlighted a new study on MRSA and its correlation of antibiotic usage on farms.
Recently another paper was published about MRSA, further increasing public fear and blaming the livestock industry for increases in antibiotic resistance. In summary, much like the last scary article about MRSA that I wrote about, this paper is much ado about nothing. The title of this paper reads “Livestock-Associated Methicillin and Multidrug Resistant Staphylococcus aureus Is Present among Industrial, not Antibiotic-Free Livestock Operation Workers in North Carolina.” However, after careful digging, one can see the data upon which this title is based is one single sentence and one minor finding. On page 4, the authors state, “Multidrug-resistant MRSA was not detected among AFLO participants.” What is not clear without looking at the data is that that observation is from the only THREE people tested.
I should note, the methodology on this paper was very good. I greatly appreciate the fact that they compared MRSA prevalence between two type of exposures, workers on modern confinement operations (mostly pork) and antibiotic free operations (some poultry and swine). Too many studies of many topics fail to measure comparison groups. In this study, nasal swabs were collected from eligible individuals working at swine or poultry operations residing in North Carolina. “Those who worked at operations where animals were raised exclusively in confinement were classified as industrial livestock operation (ILO) workers. Those who reported working at operations that raised animals outdoors, on pasture, and did not administer antibiotics to raise animals were classified as antibiotic free livestock operation (AFLO) workers.” There were 99 ILO participants and 105 AFLO participants in the study. Samples were tested for antibiotic susceptibility, absence of the scn gene, and multi-locus sequence type.
There is much to be said about this paper, but let’s start simple as this is a holiday weekend (July 4). A careful look at the key data as shown in our Table 1 demonstrates some interesting, but not compelling, findings. First, both groups (ILO and AFLO) actually had equal rates of MRSA (~7%). The rates of MDRSA appear to be different (36% vs 19%) but they are not really. The 95% confidence interval shows the ratio of the two (36/19=1.895) could be less than 1.0. In other words, the prevalence in AFLO could actually be higher than ILO. As the famous epidemiologist Ken Rothman says, lack of statistical significance just means you need to collect more data.
The rate of tetracycline resistance and CC 398, however, were higher in ILO workers. This is not surprising. The rates of resistance on farms using antibiotics are usually higher. We understand use of antimicrobials increases resistance. Just because resistance is increasing does not mean there is a risk to human health. So, the question is, does the increase in resistance increase the risk to humans that are eating meat? The answer appears to be no.
This study is really about occupational risk to that small (<2%) of the population who works on a farm.
Additionally, for those that are working on the farm:
- MRSA in the nose doesn’t mean that these people are getting sick from MRSA.
- MRSA CC 398 appears to be a transient colonizer. There are studies showing (1) that within 24 to 48 hours it was gone in students, and (2) in the Netherlands when farmers went on vacation they came back and were no longer colonized.
- We still haven’t had a single reported case of livestock associated MRSA i