Clinical trials used to set antimicrobial withdrawal times

It’s becoming common to see the words, “raised without the use of antibiotics,” on the meat we buy in the grocery store or on the menu of many popular restaurants. 


That phrasing is much more preferable to the sometimes used “antibiotic free,” which describes certain animal proteins. In Canada, we are very certain that all of the meat we buy is free of antibiotics whether or not the animal has ever received any antibiotic treatments.


Antimicrobial (antibiotic) resistance is an important public health concern, and animal agriculture is certainly under greater scrutiny about how we use antimicrobials. 


However, there is little evidence that antimicrobial use by the beef industry is contributing to resistance in human health.


We primarily use antimicrobials in the cow-calf industry to treat bacterial diseases such as respiratory diseases, pink-eye and foot rot. We want to maintain the effectiveness of these antimicrobial products to treat sick cattle and so it is important that we are good stewards of these drugs and use them only when necessary. 


There is also a significant animal welfare component to having antimicrobials available for treating sick cattle. Prevention of disease would obviously be ideal, and we need to ensure we use vaccination programs, nutrition and other health and low stress management techniques to minimize the number of animals we need to treat.


When an animal does need treatment with antimicrobials, it is important to use an appropriate antibiotic and to read and follow the label instructions. 


Your veterinarian can help to design appropriate treatment protocols for common diseases. This will help you to choose the most appropriate antimicrobial drug and also give you guidance on which animals are most likely to benefit from treatment.


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Label instructions are often printed in a font that is too small for me to read without my reading glasses, but they always contain a very important statement about withdrawal time. 


The Verified Beef Production manual says that “the withdrawal time is the minimum time from the last treatment of a pharmaceutical product to the earliest time when meat from beef cattle should be consumed. Essentially it is the time required before cattle are safe to ship and is usually measured in days”. 


Every licensed antibiotic that is given to an animal is eventually metabolized or broken down by the body, and the withdrawal time reflects the time that it takes an animal to clear the antimicrobial from its system. 


These withdrawal times must be determined for every species of animal on which the antimicrobial is licensed to be used. 


A withdrawal time may be very different in cattle than in sheep for the same drug. For dairy cows, withdrawal times are usually determined for both milk and for meat.


To get an antimicrobial licensed for use in cattle, the pharmaceutical company must prove its effectiveness against a specific disease in the species of interest, such as beef cattle. This is usually done through clinical trials and experiments. 


In addition, it will have to demonstrate safety of the product through various dose-response trials and safety studies. 


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Most importantly, it will have to provide evidence to Health Canada about the human health safety of using a drug in a food-producing species.


These health risk assessments for new drugs will include studies on the metabolism of the drug in cattle and in laboratory animals. 


An acceptable daily intake level will be determined for safety in humans, and this will contain significant safety margins for a variety of uncertainties. 


A maximum residue level will eventually be set for animal tissues, which will ensure human safety with significant safety margins built in. These MRLs will be used to eventually determine a withdrawal time based on the data that was developed on the metabolism of the drug.


Canada has a low number of violations with antibiotic residues in beef cattle. This means the vast majority of producers are doing a good job of following the withdrawal times specified on the drug labels they use in their cattle. 


This requires keeping good records on which animals were treated and which antimicrobial drugs were used. 


It means following the label instructions about how the antibiotic is administered and following the label dosage. It also means working with a veterinarian to minimize antimicrobial use by using vaccines and other preventive strategies. 


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The Verified Beef Production program is Canada’s on-farm food safety program, which provides a way for you to document that you are taking these responsibilities seriously. Consider registering your operation in the VBP program.

John Campbell is head of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.