There are two new studies out that confirm, once again, that drug-resistant staph or MRSA — normally thought of as a problem in hospitals and out in everyday life, in schoolkids, sports teams, jails and gyms — is showing up in animals and in the meat those animals become.
The strain of staph that shows up in farm animals, known as “livestock-associated” MRSA or MRSA ST398, first emerged in pigs in the Netherlands, and has been widely identified in retail meat in Europe. (You can find a long archive of posts on ST398 here, and more here.) But that same strain has been difficult to identify in the United States. That may be due in part to the US having a uniquely massive epidemic of community-associated MRSA, far larger than in any other country, which likely both obscures any animal epidemic from detection, and possibly also fills the ecological niche that livestock-associated staph might otherwise occupy. But, it must be said, there’s also remarkably little political will to look for livestock-associated MRSA (though the US Department of Agriculture has now funded a study).
First, and most important because of the group’s past research: In a pilot study, Blake Hanson and colleagues from the lab of Tara Smith at University of Iowa tested 165 samples of turkey, pork, chicken and beef bought at 22 food stores across the state of Iowa. They found staph on 27 samples and MRSA on two, both pork (1.2 percent); one of the two was a subtype (spa type t034) that falls under ST398. (The other was a spa type usually associated with human MRSA infections, t008.)
This finding is important because Smith and team are the people who first identified livestock-associated MRSA ST398 in pigs and farmworkers in the US. They’re also the recipients of that USDA grant funding the search for more of it. They probably know ST398 better than any other US team looking at the strain — and unlike the other research groups, they are embedded in pig country.
They found something else interesting in this research. One of the hallmarks of ST398 has been that — unlike human-associated MRSA strains — it is resistant to tetracycline. That’s an important signal. While human MRSA is multi-drug resistant, it is usually still susceptible to tetracycline, because tetracycline is not used in human MRSA infections and thus the bug has no exposure that encourages it to evolve resistance. However, tetracycline is very commonly given to animals raised for meat in confined conditions, which is presumably where ST398 worked up or picked up its tetracycline-resistance gene.
In this paper, all of the staph found in pork and turkey was tetracycline-resistant even when it was not MRSA (which, definitionally, is resistant to methicillin or its close analog oxacillin). Six of the seven staph-containing turkey samples were t034, and one was t337, which is associated with another MRSA strain that has emerged in pigs in China. “This suggests that turkeys, in addition to pigs, are a possible reservoir for both the ST398 and ST9 strains in the United States,” the authors say.
Tetracycline resistance is also an important signal in the second new paper, from researchers at the University of Maryland and the US Food and Drug Administration. They tested 694 samples of ground beef, ground pork and ground turkey bought in the Washington, DC area. Twenty-nine percent were staph; 17 percent each of the pork and turkey harbored MRSA. Here are the resistance profiles —
When tested for susceptibility to 22 antimicrobials, 69% of the S. aureus isolates were resistant to tetracycline, 26% to penicillin, 17% to ampicillin, 13% to methicillin, 8% to erythromycin, 4.5% to clindamycin, 1.5% to gentamicin, and 0.5% to chloramphenicol, oxacillin, cefoxitin, or quinupristin-dalfopristin
— once again, a lot of tetracycline resistance in staph, in meat from animals that would have received tetracycline as they grew. (The paper doesn’t sort the isolates by MLST or spa type, and thus doesn’t say whether any of the isolates were ST398.)
Given how consistently resistant staph, MRSA, and ST398 are showing up in US food products, you’d think we’d want to track down exactly where and how often these resistant strains are emerging. Unfortunately, no. A Government Accountability Report that was released in September to almost no notice scolds the federal health and food agencies for doing very little — despite an earlier GAO push in 2004 for them to do better. The report says:
…since GAO’s 2004 report, FDA began collecting data from drug companies on antibiotics sold for use in food animals, but the data do not show what species antibiotics are used in or the purpose of their use, such as for treating disease or improving animals’ growth rates. Also, although USDA agencies continue to collect use data through existing surveys of producers, data from these surveys provide only a snapshot of antibiotic use practices. In addition, agencies’ data on resistance are not representative of food animals and retail meat across the nation and, in some cases, because of a change in sampling method, have become less representative.. Without detailed use data and representative resistance data, agencies cannot examine trends and understand the relationship between use and resistance.
Clearly, agriculture isn’t going to offer this data on its own. And as the report makes clear, the US government (unlike those of Canada, the Netherlands or Denmark) isn’t pushing. Until it does, the search for MRSA and other resistant organisms in meat is likely to to proceed via papers like these two: incremental, local and subject to the initiative of individual investigators — but not able to tell the general public as much as they deserve to know about risks in their food.
- Hanson BM et al. Prevalence of Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) on retail meat in Iowa. Journal of Infection and Public Health, Sept 2011 (Vol. 4, Issue 4, Pages 169-174) DOI: 10.1016/j.jiph.2011.06.001
- Kelman A et al. Antimicrobial Susceptibility of Staphylococcus aureus from Retail Ground Meats. J Food Prot. 2011 Oct;74(10):1625-9.
- GAO. Agencies Have Made Limited Progress Addressing Antibiotic Use in Animals. Sept. 7, 2011. GAO-11-801.