The off-label use of veterinary medicines on dairy farms is surprisingly common. This article discusses the safe use of veterinary medicines and scenarios where these medicines are unintentionally used off-label.
There are many definitions associated with the safe use of veterinary medicines. The following are adapted from the APVMA and Dairy Australia websites:
With-holding period (WHP): The minimum time period between last administration of a veterinary medicine and the slaughter, collection, harvesting or use of the animal commodity for human consumption. WHPs are mandatory for domestic slaughter and are on the label of every registered product. In the dairy industry, WHPs apply to milk or milk products intended for human consumption and meat from bobby calves or mature animals. Withholding periods are set to ensure that when a drug is used in accordance with the directions on the label, no residues will exceed relevant Australian Maximum Residue Limits.
Maximum Residue Limit (MRL): The maximum concentration of a veterinary medicine that is permitted in food (for dairy farms this means milk or meat from cows or calves).
Minimum Dry Period (MDP): The time period that must elapse between treatment of a cow with Dry Cow Therapy and her calving date. The WHP for milk is the time period after calving before a cow’s milk can go for human or bobby calf consumption, provided the specified Minimum Dry Period has occurred.
Export Slaughter Interval (ESI): The period following treatment when cattle are unsuitable for processing for some export markets.
We need to be familiar with these definitions in order to use veterinary medicines safely and avoid drug violations in food products intended for human consumption. However, there have been recent reports of positive drug residues found in milk attributable to the use of off-label antibiotics, mainly lactating cow mastitis therapy.
Off-label use of veterinary medicines refers to situations when a registered veterinary medicine is used in a manner that is not specified on the individual product label.
What is unintentional off-label drug use?
Unintentional off-label dug use occurs when the farmer may believe that he or she is using the veterinary medicine as prescribed by the manufacturer or in accordance with veterinary advice. It is worth re-checking the dose rate, route of administration and duration of treatment for all veterinary medicines administered to cows and calves. Veterinary clinics sometimes have to switch to a different brand of a drug with the same active ingredient and this can result in a change in with-holding period. Any changes will be recorded on the drug label but if you don’t check, you may not notice it. If you are in any doubt please discuss this with your veterinarian prior to any treatment.
An inaccurate assessment of the animal’s body weight can lead to incorrect dosing of a veterinary medicine. Consequently, the animal may receive a completely inappropriate dose which will affect any with-holding periods. This commonly happens in calves who, in my experience, can receive up to three times the recommended dose rate for a given drug. This is not only inappropriate and unsafe use of veterinary medicines but may actually cause harm to the individual if the toxic dose is exceeded. Weigh tapes and scales can help estimate body weight to allow more accurate dosing.
Some drugs are designed to be administered by only one route. This means that if a veterinary medicine is prescribed to be given by the intramuscular route, then the with-holding period has been defined for this route only. If the same drug is given subcutaneously, then the withholding period may be affected. It is also worth noting that some drugs are irritant if given by a route that is off-label.
The duration of treatment should be followed as prescribed. Extended treatments are considered ‘off-label’ and will affect milk and meat with-holding periods. Always discuss extended treatments with your veterinarian beforehand and establish written with-holding periods for both milk and meat. If you are in doubt about a with-holding period, milk can be tested for drug violations prior to contributing to saleable milk.
With-holding periods are defined for a specific drug, made by a particular pharmaceutical company in a specific formulation. They are not interchangeable between pharmaceutical companies. They have been defined for the recommended dose rate provided by the manufacturer, to be administered in the described way and for the recommended duration of treatment. Thus two different brands of lactating cow mastitis treatment, both containing cloxacillin at the same dose can have different instructions for administration and very different with-holding periods. Likewise, two brands of injectable antibiotic, both containing oxytetracycline as the active ingredient can have differing dose rates, route of administration and with-holding periods.
Remembering all the different withholding periods for all the different veterinary medicines is almost impossible. Written treatment protocols for all veterinary medicines used in dairy animals will help avoid the need to recall this information from memory. These protocols also help train new staff and provide a written record of prescription and treatment practices during processor audits. Speak with your veterinarian to help you develop treatment protocols for the various disease conditions and potential treatments that can affect your cows and calves.
Gemma Chuck is a veterinarian and advisor with Apiam Animal Health.