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Feedlot study indicates no major links to antimicrobial resistance in humans

The use of antimicrobial drugs in Canadian cattle production is not currently a major contributor to the development of resistant bacteria that threaten human health.

Good news for the beef industry. "The most significant resistance concerns that we went into the project looking for, turned out not to be an issue." - Dr. Ron Read (pictured).

That's the indication of a much-anticipated, five-year study investigating antimicrobial resistance in Alberta feedlot cattle, led by the University of Calgary and Agriculture and Agri-Food Canada.

"The most significant resistance concerns that we went into the project looking for, turned out not to be an issue," says study leader Dr. Ron Read of the University of Calgary. Most notably, bacteria with resistance to vancomycin and methicillin, the top human health concerns speculatively linked to cattle production, were not found in Alberta feedlot cattle. Salmonella with multiple forms of resistance, widely thought to be in outbreak situations in food animals, were also not found. "Our study was helpful in closing the book on a number of resistance issues," says Read.

Only one form of resistance of potential concern for human medicine was found - E. coli strains with resistance to cephalosporins. However, the prevalence of this type of resistance in humans is extremely low and researchers consider the potential for relevant transfer from cattle to humans unlikely at this point. "We're in a situation where we've identified something that needs to be monitored," says Read. "We're fortunate that we have time to keep an eye on this situation and deal with it, because we're not seeing this resistance occurring in humans."

Speculation doesn't pan out

Antimicrobial agents are the drugs, chemicals and other substances used in medicine to kill or slow the growth of microbes, particularly those that cause disease. Over the past decade, a disturbing trend worldwide has been the emergence of microbe populations that are resistant to important antimicrobial agents used in veterinary and human medicine.

"It's a very serious issue," says Read, associate professor, Medicine and Microbiology and Infectious Diseases, University of Calgary, Faculty of Medicine. "Obviously, as more agents are rendered less effective due to increased populations of antimicrobial resistant microbes, humans and animals that depend on those agents to battle disease become more vulnerable."

The more widely and frequently an antimicrobial is used, the greater the risk of antimicrobial resistance developing. This is because antimicrobial use places selection pressure on target microbe populations to evolve survival mechanisms.

Antimicrobial resistance that threatens human health is primarily associated with antimicrobial use in human medicine and the role of hospitals as reservoirs of resistant organisms. However, there has been widespread concern that antimicrobial use in livestock production is also a contributing factor, with resistant microbes transferred to humans through direct contact, the environment - including water channels - and through food products.

The new study - the most comprehensive of its kind and the first to examine Canadian cattle - shows that's not the case currently in Canadian cattle production. "Going into the study, we had concerns about a whole bunch of resistance issues. What we found was most of these were, in fact, non-issues," says Read.

Monitoring recommendation

The one potential issue identified, cephalosporin resistance, was a surprise finding and its significance is unknown, he says. "I think the most important thing we can do is to continue to keep an eye on this phenomenon."

Larry Helland, an Alberta cow-calf operator, is Chair of the Canada Alberta Beef Industry Development Fund (CABIDF), which funded the antimicrobial resistance study. More information on CABIDF is available at www.albertabeef.org/CABIDF

Health Canada is currently establishing a surveillance system for antimicrobial resistant organisms in agriculture. Read and colleagues have proposed that the resistant microbes identified in their study be included in this monitoring program.

Production management changes may also be warranted, says Read. Risk factors identified for the resistance in the study included use of florfenicol or oxytetracycline at entry to the feedlot. "While further study is needed, our data would imply these antibiotics may be related to the persistence of this bug in cattle in the feedlot environment," he says. "As a precaution, it may be worth looking at ways to modify how those drugs are used, to reduce this problem."

A component of the study that evaluated feedlot workers for evidence of cattle-derived resistant organisms found no instances of resistance.

The Read study was funded in part by the Canada Alberta Beef Industry Development Fund (CABIDF). A multi-component study, it 1) investigated the presence of antimicrobial resistant microbes in cattle at four Calgary-area feedlots, 2) examined workers at those feedlots for evidence of transfer of resistant organisms, and 3) performed experiments in model feedlots to learn more about the roles of specific antimicrobials on the development and movement of resistance. More detailed information on the study is available on the CABIDF Web site.

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