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Extra-Label Antibiotic Use and By Erin Cuthbert, Provincial Dairy Inhibitor-Positive Risks
 Technologist, BC Ministry of Agriculture
While tests for somatic cell count and individual bacteria count serve as indicators of milk quality, milk safety tests go well beyond that, to detect even the slightest presence of antibiotics (inhibitors) in milk. In British Columbia, every load of milk reaching the processing plant is tested for the presence of beta-lactams, sulfa, and tetracyclines. Additionally, random antibiotic testing occurs on farm once per month for each bulk tank. In the unfortunate event that a milk test result is positive for antibiotics at the processing plant, the contaminated milk is destroyed. It is at this time that the producer can expect a call from Ministry of Agriculture milk inspectors to discuss the infraction.
When a milk sample tests positive for an inhibitor, it is because the concentration of antibiotic in the milk has exceeded the maximum residue limit (MRL) for that specific antibiotic. Through the Veterinary Drug Directorate, Health Canada sets an MRL for every antibiotic available, and these MRLs are used for the regulatory testing of milk.
Antibiotics are used “on-label” and “extra-label” with a veterinary prescription, and producers need to adhere to withdrawal times to avoid positive inhibitor results and the associated penalties. Extra-label use refers to the practice of using an antibiotic for purposes outside its intended use. These uses are often not well defined in terms of exact dose, application, and withdrawal times, thus carrying higher risks. Producers should review their antibiotic protocols on a yearly basis to ensure they are up to date and that record keeping is complete. Luckily, with biannual Ministry inspections and the Dairy Farmers of Canada’s proAction program, this is becoming second nature! However, even with detailed SOPs, training, and record keeping, accidents do happen.
Case Study
Once the producer was notified that their bulk tank had tested positive for antibiotics, the process of elimination began to determine the source of the inhibitor and prevent more contaminated milk from entering the bulk tank. With no animals in the milking herd being treated for mastitis, they initially thought the wrong cow was treated with a dry-off product. It wasn’t until the type of antibiotic in the milk sample was classified as a tetracycline that the cause was identified – the tetracycline used for treating digital dermatitis (DD). Milk from these cows was immediately diverted from the bulk tank, and individual milk samples were taken to determine inhibitor status.
The extra-label use of tetracycline for treating DD for dairy cattle is well-known and very common. Herd veterinarians provide appropriate protocols to guide the use. In this specific herd of 120 milking cows, 15 cows had been treated with tetracycline – seven with powder, eight with paste (one cow had both back hooves treated). Six cows treated with powder tested positive up to 72 hours after treatment. The
one cow with paste treatment on both hind feet tested negative.
In most cases, inhibitor positives are determined at the processing plant (and confirmed at a Ministry designated laboratory) and the producer is notified within hours. This case was unique because the milk truck that arrived at the processing plant tested negative for antibiotics. It wasn’t until the next day when lab results for the individual producer’s bulk tank tested positive for antibiotics that a problem was identified. The reason for discrepancy between the results was due to dilution.
Dilution is an important factor when considering milk quality because the milk from an individual animal is co- mingled with the herd, and then the entirety of the milk in the milk truck. In the same way a high SCC cow can be masked by the low SCC of the entire bulk tank, a positive inhibitor cow’s milk will be diluted to some extent in the herd’s bulk tank. Multiple cows treated with tetracycline will increase the concentration of the antibiotic in the milk tank and the dilution factor won’t be as strong. It must be emphasized that no antibiotic residues should ever be allowed to reach the bulk tank.
In this case study, the dilution of antibiotic positive milk with untreated milk was not great enough to reduce the concentration of the bulk tank below 100ng/ml (MRL) which resulted in a positive test. However, once comingled with an additional 20,000L of milk on the bulk truck, the concentration of the antibiotic was below the MRL and resulted in a negative test at the plant.
Whether milk tests positive at the farm level or at the processing plant, the producer is subject to the penalties outlined in Section 112.4 of the Milk Industry Standards Regulation and administered by the Ministry of Agriculture. It’s a rare occurrence when an inhibitor infraction isn’t an accident – mistakes occur, human error gets the best of us – and they serve as a reminder to producers to be vigilant to prevent unintended consequences from on-label and extra-label antibiotic use. Whether using antibiotics on- label or extra-label, multiple factors need to be considered, including dosage, number of cows being treated, method of administration, cow characteristics (i.e. body weight, milk production), and the severity of the illness, along with understanding the milk withdrawal variable. Extra-label use of antibiotics is inherently riskier when trying to prevent residues in the bulk tank.
Always work with your veterinarian to develop the best guidelines and follow all precautions for safe use and proper milk withdrawal to ensure no antibiotic residues reach the bulk tank. With the sensitive nature of inhibitor tests, an added measure of safety may be well advised to prevent a positive tank test. Everyone understands the critical importance of only antibiotic-free milk leaving the farm and reaching the consumer for trust and food safety.
 Extra vigilance is needed to avoid unintended consequences when using any antibiotics, especially off-label.
Photo: World of Hooftrimming (WOHT)
Photo Credit: World of Hooftrimming (WOHT)
The Milk Industry Standards Regulation (Section 16.1) states that a dairy farmer must administer veterinary drugs and medicated feed only (2a) as prescribed by a veterinarian, or (2b) if the veterinary drug or medicated feed is authorized for sale without a prescription. Furthermore, a dairy farmer must (4a) clearly identify every dairy animal that has been treated with veterinary drugs or medicated feed, and (4b) withhold raw milk from that dairy animal for a period of time set (i) by the veterinarian who prescribed the veterinary drug or medicated feed, or (ii) if the veterinary drug or medicated feed is authorised for sale without prescription, the manufacturers instructions on the label.
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