Using whole genome sequencing, scientists have found conclusive evidence that a type of methicillin-resistant Staphylococcus aureus (MRSA) that caused human infections in Denmark came from livestock – adding to concerns that food animals are a significant MRSA reservoir for human infections.
While previous research has suggested that MRSA transfers from animals to humans, including a U.S. study last year that looked at “pig-MRSA,” the study published this week in EMBO Molecular Medicine provides the strongest evidence to date that this phenomenon is occurring and provides fodder to those advocating for greater limits on antibiotics in agriculture.
For this particular study, scientists at various research institutions in the UK and Denmark, including the University of Cambridge, looked at two Danish cases – a 53-year-old woman (Patient A) whose blood and nasal passage was positive for mecC-MRSA and a 69-year-old woman (Patient B) with a wound infected with mecC-MRSA. Both lived on farms – Patient A had two cows, two horses and a dog, and Patient B had a flock of ten sheep.
For Patient A, the isolates from her blood and nasal swab and the isolate from a cow on her farm differed by a total of five single nucleotide polymorphisms (SNPs), or DNA variations, which, according to the researchers, suggests that a transmission event had occurred between the two but the direction of the transmission – whether human to animal or animal to human – “remains unclear.”
For Patient B, two isolates from two sheep and the patient’s wound isolate each differed by five and three SNPs, or DNA variations. Isolates from other sheep were found to be slightly different and the scientists concluded that the level of diversity between the isolates suggests that strains of MRSA may have been circulating among the sheep for an extended period of time, or had been introduced multiple times. They concluded “it is most likely that the direction of transmission was from sheep to human.”
While there are a number of caveats to the study’s findings – which infectious disease journalist Maryn McKenna describes in detail here – some experts believe the research is undeniable proof that antibiotic-resistant bacteria are indeed being transferred from livestock to people.
“[The study] is a more detailed, refined level of analysis, so it provides even stronger evidence that this is happening,” said Dr. Jim Johnson, a professor of medicine at the University of Minnesota and a physician at the Minneapolis VA hospital, who noted that he believes there has long been convincing evidence that antibiotic-resistant bacteria can jump from food animals to humans.
“This study refutes the counterargument that’s been out there – that the animals and humans could have independently acquired MRSA,” explained Johnson, who works on antimicrobial resistance issues for the Infectious Disease Society of America, an association of infectious disease physicians. Johnson said that with the level of detail provided by whole genomic sequencing in the latest study there is little doubt that the MRSA in these cases transferred animal to human.
The researchers argue that, considering the recent science on the matter, “surveillance of S. aureus and other animal pathogens from livestock and wildlife should be undertaken to monitor the emergence of new clones, and to further improve our understanding of bacterial pathogen evolution.”
Those advocating for limiting antibiotics in agriculture say the study is another sign that antibiotic-resistance is a growing health problem and the widespread use of antibiotics in food animal production is a major contributing factor.
Congresswoman Louise Slaughter (D-NY), who also happens to be a microbiologist, responded to the study this week by sending a letter to the U.S. Food and Drug Administration urging swift action.
“This study adds to the extensive scientific research supporting the connection between the overuse of antibiotics – 80% of which are used on otherwise healthy animals in the United States – and the development of antibiotic resistant bacteria,” said Slaughter, in the letter addressed to FDA Commissioner Margaret Hamburg. “Evidence that resistant bacteria transfer from animals to humans should be a clear call for significant, meaningful action to protect one of the greatest advancements in medical history – the development of antibiotics.”
“This study ends any debate,” said Slaughter in a statement Wednesday. “The extreme overuse of antibiotics in livestock is endangering human health.”
Gail Hansen, a senior officer at the Pew Charitable Trusts, which actively advocates for restricting the use of antimicrobials in agriculture, said she was not surprised by the study’s findings: “Science and technology advances, but the ultimate conclusion remains the same: using antibiotics to produce meat and poultry breeds dangerous superbugs.”
Lance Price, a professor at George Washington University who has done extensive research on antimicrobial resistance at Translational Genomics Research Institute in Arizona, said the study presents “compelling evidence” that mecC-MRSA is jumping from livestock to humans.
“It also shows that these host jumps are not limited to MRSA ST398 (the strain that we study),” he said (Price was a lead researcher on “pig-MRSA” study mentioned above), adding that the new research also demonstrates how useful whole genomic sequencing is as a tool to “resolve transmission events like these.”
On the other hand, Scott Hurd, a veterinarian and professor at Iowa State University criticized the study and called it inconclusive.
Hurd, who served as Deputy Undersecretary for Food Safety at the U.S. Department of Agriculture, said, in an email to Food Safety News that, “It is irresponsible science to make conclusions that one came from another in the absence of some other information.”
“We could just as easily argue that people gave the genome to the animal strains of MRSA. Or better BOTH got it from a common bacterial source. Note, we do all live the same environmental, bacterial ecosystem,” he said. “For some reason our friends in Denmark have a strong desire to “eradicate” their livestock production, and then export those costly practices to the rest of us.”
So will this study end the debate, as Rep. Slaughter suggests? Not likely, said Johnson.
“I don’t think the debate is happening because of any shortage of evidence,” said Johnson, who predicts certain animal agriculture and veterinary pharmaceutical interests will always say the science is inconclusive or incorrect and that public health and consumer advocacy interests will always say it’s absolutely clear antibiotic abuse in agriculture is contributing to the problem. “The debate will never end.”
This story has been updated with comments from Scott Hurd.© Food Safety News