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RESEARCH – for Oct. 4, 2010

It is a common assumption that using antimicrobials for livestock is an important cause of antimicrobial resistance in humans. This has led consumer and medical groups in the U. S. and Canada to ask their governments to restrict the use of antimicrobials for growth promotion in livestock. Ionophores such as Rumensin, Bovatec and Posistac are very widely used for growth promotion in the beef industry, but are not used in human medicine. Removing these antimicrobials would greatly increase production costs with no obvious benefi t for human health. Ongoing research is examining the broader issue of antimicrobial resistance in livestock.

Through the Canadian Integrated Program for Antimicrobial Resistance Surveillance (CIPARS) program, the Public Health Agency of Canada (PHAC) has monitored antimicrobial resistance inE. colifrom healthy cattle at packing plants since 2002 and in retail ground beef since 2003. Resistance to the antimicrobials that are the most important for human health has not exceeded one per cent and has not increased over the past eight years. The percentage ofE. colicultures that are resistant to five or more antimicrobials has never exceeded four per cent, and has not been increasing.

Antimicrobial resistance appears to be relatively uncommon in Canadian beef and cattle. However, the CIPARS program has found considerably higher levels of antimicrobial resistance in some other livestock species. This suggests that all livestock (including beef cattle) will remain in the antimicrobial resistance spotlight for some time.

Checkoff-funded research sponsored by the Beef Cattle Research Council and Alberta Beef Producers is gathering information to extend this surveillance program to the farm level. This will help determine whether modern on-farm beef production practices are contributing to antimicrobial resistance.

What they are doing

Dr. Sheryl Gow of the Public Health Agency of Canada leads a research team with partners in Alberta, Saskatchewan, Ontario and Colorado to develop a farm-level antimicrobial use and resistance surveillance program. Individual nasal and fecal samples are being collected from cattle on arrival and before leaving feedlots in southern Alberta. Nasal samples are cultured for Mannheimia haemolytica (a pathogen associated with bovine respiratory disease). If the nasal sample is positive for M. haemolytica, then the corresponding fecal sample is also cultured forE. coli.Composite manure samples from each pen of enrolled cattle are also collected at arrival to and exit from the feedlot and cultured forE. coliand Salmonella.

The bacterial cultures are exposed to a panel of 16 antimicrobials routinely used by CIPARS as well as the National Antimicrobial Resistance Monitoring System in the U. S. Bacterial genes coding for antimicrobial resistance are studied in detail. Particular attention is focused on the genes coding for resistance to tetracycline and extended spectrum cephalosporins (ESC). These genes may be linked in the bacterial genome. This means that exposure to tetracycline might indirectly select for bacteria that are also ESC resistant. Because ESC drugs are used to treat human illnesses that will not respond to other antimicrobials, they are of very high importance in human medicine.

This research has not been completed, but preliminary results from several thousand cattle have only identified oneE. coliculture from the individual samples and one from the composite samples with resistance to an ESC. No other resistance to other antimicrobials that are considered very important to human medicine has been found. Antimicrobial resistance has been rare for the M. haemolytica cultures, although some resistance to tetracycline and sulfasoxizole (both of which are of medium importance in human medicine) has been identified. Cases of resistance to two or more antimicrobials have been very rare in the cattle samples. The rates of multi-drug resistance are much higher in samples obtained from hospitalized humans. This may suggest that beef production is not one of the main sources of multiple drug resistance in human medicine.

Although individual animalE. colicultures have had somewhat higher levels of antimicrobial resistance at feedlot exit than at arrival, differences are less apparent for theE. colicultures from the composite manure samples. Additional sampling and analysis will need to consider animal health treatments before any definite conclusions can be drawn from this study.

What it means

These preliminary results indicate that antimicrobial resistance is not common in beef cattle at the present time. However, given the fact that antimicrobial resistance is considerably more common in other livestock species, antimicrobial use in livestock will likely continue to be a focus for public, media and legislative scrutiny. It is important that the beef industry continue to work with PHAC to monitor cattle and beef for signs that resistance may be developing or increasing. This information will also help to ensure that any potential regulatory changes to antimicrobial use in cattle are based on sound science.

It is also important that the beef producers continue to use these products responsibly. Producers enrolled on the Canadian Cattlemen’s Association’s Verified Beef Production program keep records to demonstrate that they follow proper antimicrobial storage, dosage, administration routes and disposal practices. For more information on Verified Beef Production, go to www.verifiedbeef.org/or call 306-737-2290.

ReynoldBergenisthesciencedirectorofBeefCattleResearch Council.

About the author

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Dr. Reynold Bergen is the science director of the Beef Cattle Research Council.

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