The antibiotic-resistant staph infection known as MRSA — methicillin-resistant Staphylococcus aureus — kills more Americans than AIDS, and is widespread in the U.S. pig herd.
According to veteran food writer Tom Philpott, until the 1980s, MRSA infections were mainly limited to people who spent time in hospitals and nursing homes.
In the 1990s, MSRA cases began surfacing in the general population at about the same time the pork industry began escalating the factory production of hogs, forcing them into more compressed spaces (known as concentrated-animal feedlot operation or CAFO) while pumping them with massive doses of antibiotics.
The Canadian pork industry, also an adherent of the CAFO model, including the reliance on antibiotics, exported some 762 million pounds of pork into the U.S. annually.
In 2008, a Canadian researcher found MRSA in 10 percent of 212 samples of pork chops and ground pork bought in four Canadian provinces.
Philpott claims the Canadian researcher delivered his findings at a Center for Disease Control confab, which garnered no response from U.S. regulatory officials. According to Post-Intelligencer reporter Andrew Schneider, in 2008, the USDA, responsible for the safety of imported food, wasn’t testing for MRSA.
Schneider noted that even in light of the Canadian findings, the FDA failed to begin testing U.S. pork. However, in 2008, Philpott claims a researcher at the University of Iowa decided to do what U.S. authorities avoided: they tested U.S. CAFO-grown pigs for MRSA.
Assistant professor of epidemiology Tara Smith and her team of graduate students swabbed the noses of 209 pigs from 10 farms in Iowa and Illinois. They found MRSA in 70 percent of the porkers, and the results were the first-ever publicly released test of U.S. hogs for MRSA.
Philpott adds Smith and her researchers also tested 20 workers on Iowa hog farms. Nine of them carried the same MRSA strain as the pigs. And MRSA is contagious, meaning it can move from workers to their families and broader communities.
“The main possible concern is that people could get MRSA on their hands from raw pork, then touch their nose. The nose is the prime site for MRSA to live,” said one researcher.
And according to a Canadian researcher, “MRSA could also be in beef, chicken and lamb, but no one is checking.”
MRSA Found in U.S. Meat in Two New Studies
This month, Wired’s Maryn McKenna reports on two new studies out that confirm drug-resistant staph or MRSA is showing up in animals and in the meat those animals become. The pathogen was found in studies from May and this one from April.
We collected and tested a total of 136 meat and poultry samples from 5 US cities, encompassing 80 unique brands from 26 grocery stores. S. aureus contamination was most common among turkey samples (77%; 20/26), followed by pork (42%; 11/ 26), chicken (41%; 19/46), and beef (37%; 14/38). A subset of meat and poultry samples (10%; 14/136) was contaminated by multiple unique S. aureus strains as determined by MLST and susceptibility profiles, and a total of 79 unique isolates were used in subsequent analyses.
Ninety-six percent of the S. aureus isolates were resistant to at least 1 antimicrobial. Thirty-two unique susceptibility profiles were identified among the S. aureus isolates, with many resistant to multiple clinically important antimicrobial classes (Figure 1). Resistance (intermediate and complete) to tetracycline, ampicillin, penicillin, and erythromycin was highly prevalent, but resistance to other important antimicrobials was also observed, including quinupristin/dalfopristin, fluoroquinolones, oxacillin, daptomycin, and vancomycin (Figure 1).
Multidrug resistance, defined as intermediate or complete resistance to 3 or more antimicrobial classes, was common among the S. aureus isolates (52%) and most prevalent among S. aureus isolates from turkey (79%; 22/28), followed by those from pork (64%; 7/11), beef (35%; 6/17), and chicken (26%; 6/23).
Fifteen unique MLST sequence types were identified among the S. aureus isolates, but 2 sequence types—ST5 and ST398— dominated the collection due to their high prevalence among chicken (74%) and turkey (79%) samples (Figure 2). The most common sequence types among beef and pork samples were ST1159 (29%) and ST1 (55%), respectively. The 3 MRSA isolates belonged to sequence types ST8 (beef and turkey) and ST5 (pork).
During the trial, 380 isolates were submitted to CVM, of which 311 were confirmed as S. aureus. Of the 311 confirmed isolates, 32 (10%) contained mecA and were considered MRSA. Most of the isolates were SCCmec type VI (44%, 14/32). Furthermore, 38% (12/32) of MRSA were positive for PVL. By commodity, pork chops showed the highest prevalence of MRSA (44%, 14/32), followed by ground turkey (28%, 9/32). Ground beef and chicken breast were 16% (5/32) and 9% (3/32), respectively.
Resistant to Tetracycline
McKenna points out that Smith and the University of Iowa team found all of the staph found in pork and turkey was tetracycline-resistant even when it was not MRSA.
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.”
McKenna explains that one of the hallmarks of ST398 has been that, unlike human-associated MRSA strains, it is resistant to tetracycline, “and 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, McKenna says 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.
Researchers at the University of Maryland and the U.S. Food and Drug Administration tested 694 samples of ground beef, ground pork and ground turkey bought in the Washington, D.C., area. Twenty-nine percent were staph; 17 percent each of the pork and turkey harbored MRSA.
The resistance profiles indicate that when tested for susceptibility to 22 antimicrobials, 69 percent of the S. aureus isolates were resistant to tetracycline.
Twenty-six percent were resistant to penicillin, 17 percent to ampicillin, 13 percent to methicillin, 8 percent to erythromycin, 4.5 percent to clindamycin, 1.5 percent to gentamicin, and 0.5% to chloramphenicol, oxacillin, cefoxitin, or quinupristin-dalfopristin.
Nothing is Being Done to Address MRSA Problem
McKenna warns that despite the consistent presence of resistant staph, MRSA, and ST398 showing up in U.S. food products, nothing is being done to determine where and how often these resistant strains emerge.
A Government Accountability Report that was released in September to almost no notice chides the federal health and food agencies for doing virtually nothing despite earlier GAO warnings in 2004.