HPRA Article - Use of Antibiotics in Dairy Cows

Background

Antibiotics are medicines that have anti-bacterial properties. Some share similar characteristics to others and are active against similar pathogens, while other classes have very specific properties and may be active against different organisms. Based on their mode of action they are classified into a number of different classes as follows:

Antibiotics behave in precise ways when they enter the animal’s bloodstream; some are distributed evenly throughout the body, while others are concentrated in specific locations e.g. the lungs. Antibiotics are subject to prescription control; the veterinary practitioner is best placed to prescribe the most appropriate antibiotic for the disease in question.

Resistance to antibiotics

Resistance to antibiotics leads to a phenomenon known as antimicrobial resistance (AMR). This may occur in the target pathogen itself, as well as in other bacteria. In the treated animal, resistance may be suspected where the antibiotic has not worked. However, because under field conditions most antibiotic use in cows takes place without prior susceptibility testing of the bacteria in question, it can be difficult to know for certain. Where an antibiotic does not work as intended, veterinary practitioners should file a report of lack of efficacy of the drug concerned to the HPRA or to the company responsible for marketing the product. Such reports help add to the emerging knowledge on AMR, and contribute to government and international policies to control AMR.

A growing public health issue is the impact of antibiotics on non-target bacteria in the animal’s body. Even when a drug eliminates the target bacterial pathogen, non-target bacteria are exposed to it and these may develop and propagate AMR. AMR is an increasing problem in human and veterinary medicine and government authorities are now engaged to address it[2].

Need for antibiotics in dairy cows

The three major production diseases in dairy cows are reproductive disorders, mastitis and lameness. Depending on their precise causes, antibiotics may be one tool in their treatment. In this case antibiotics should only be used based on the diagnosis of the prescribing veterinarian. However, in many cases, an individual clinical case might be regarded as a symptom of an underlying disease problem, which is better addressed by a root cause analysis and a herd health approach. Frequently this will involve an assessment of nutrition, genetics, biosecurity, vaccination, grassland management, milking techniques and animal husbandry.

Where antibiotics are to be used routinely, e.g. if deemed necessary for dry-cow therapy, the choice of antibiotic should be in consultation with a veterinary practitioner. The vet will assess factors such as:

• Previous culture results and antibiotics responses on the farm;
• Claimed cure rates of products for existing infections;
• Claimed period of protection for  new infections;
• Required minimum dry period;
• Milk withdrawal periods; and
• The option of using a combination of teat sealer and antibiotic dry cow treatment.[3]

Precautions in use of antibiotics in dairy cows

The dose of the antibiotic stated on the product labelling has been approved by the regulatory authority concerned. This is usually done on the basis of pre-clinical studies as well as clinical field studies and therefore follows a detailed scientific and evidence-based evaluation of the quality, safety and efficacy of each individual medicine. Deviations from those conditions of use established by the HPRA may invalidate the expected performance of the veterinary medicine under field conditions. In particular, users are reminded:

1. Utmost care is needed to ensure that cows that have been treated with an antibiotic are clearly marked and identified, to prevent milk being inadvertently used for human consumption before residues have depleted sufficiently to levels below the permitted limit.
2. To strictly adhere to the withdrawal periods stated on the product labelling and package leaflet.
3. That the use of an animal remedy at a dose above that stated on the product labelling might be expected to prolong the time needed by the animal to clear residues of the medicine beyond that given as the withdrawal period for the product.
4. That the use of an antibiotic concurrently with another medicine, e.g. a steroid or a different antibiotic, might lead to an interaction between both medicines, such that the withdrawal periods of both products is not sufficient to ensure that residues will be the legal limits at the end of the withdrawal periods.
5. That some antibiotics e.g. gentamicin have a prolonged withdrawal period (214 days for meat). Re-treatment of those animals during this period may result in an accumulation of drug residues necessitating an even longer withdrawal period.
6. Antibiotics carry particular risks for AMR when used off-label. Such use is restricted, by law, to veterinarians and only under specific circumstances.
7. The antibiotic screening tests routinely used by creameries might not be calibrated precisely in line with the European residue limits set for public health and may be exquisitely sensitive for certain classes of antibacterial drugs (e.g. penicillins and cephalosporins) while being less sensitive to other drugs. This can result in the withdrawal period stated on the product label to be insufficient to guarantee that the milk will pass the creamery standards for trading purposes (even though acceptable for public health on the basis of consumer health standards).
8. That milk from treated cows which contains antibiotic residues is expected to transfer AMR if fed to calves. Ideally, such milk should be disposed of into the slurry tank, where it is denatured and diluted.