Antibiotic use in cattle
An antibiotic is a pharmaceutical drug that is used to treat infections caused by bacteria and other microorganisms. The earliest antibiotics were naturally produced by one microorganism to selectively kill or prevent the growth of another microorganism. Synthetic antibiotics have since been produced that are chemically similar to natural antibiotics and perform similar functions.
Antibiotic resistance is the ability of bacteria to resist the effects of an antibiotic.
· Antibiotics are given to food-producing animals for three reasons: 1. When animals are sick; 2. When animals are not sick but are susceptible to illness. In these cases, antibiotics are administered at high doses for a brief period of time; 3. When low levels of antibiotics are added to animal feed on a long-term basis to aid in growth of the animal. This practice provides ideal conditions for bacteria to develop resistance over time.
· Bacteria become resistant through several mechanisms. Certain bacteria have the ability to transfer resistance genes and can confer antibiotic resistance on previously susceptible bacteria. Some bacteria have the ability to neutralize the effect of the antibiotic; others can release the antibiotic before it can have an effect.
· Microorganisms can acquire resistance to a single antibiotic, to a group of related antibiotics (cross-resistance), and to several unrelated antibiotics (multi-drug resistance; MDR).
· Disease causing microorganisms that are resistant to currently available antibiotics are increasingly becoming a human health problem. Many diseases such as, tuberculosis, gonorrhea and ear infections have become difficult to treat with antibiotics. Food producing animals such as beef and dairy cattle are given antibiotics for important therapeutic, disease prevention, or production reasons. Using these antibiotics on animals can make the microorganisms more resistant to drugs used to treat human illnesses, making some human illnesses difficult to treat.
The primary threat to human health from the use of antibiotics in animal production stems from the emergence of resistant human pathogens and not necessarily from antibiotic residues in food products. Health Canada establishes withdrawal periods for veterinary drugs such as antibiotics. A withdrawal period is a waiting time that must be observed after administering drugs to an animal before it can be used for food production. Withdrawal periods are based on the residue depletion data for residues in specific tissues. Milk that is collected from a treated animal is subject to a withholding time. During this time the milk collected must be discarded and is not available to consumers. Labels on veterinary drug products must include complete directions including the withdrawal period and any other restrictions of use or warnings that are deemed important for the handler of the drug.
Antibiotic resistant bacteria occur commonly among Canadian livestock. Studies conducted at the Health Canada Laboratory for Foodborne Zoonoses have documented the widespread occurrence of antibiotic resistance and multi-drug resistance among Salmonella strains isolated from food-producing animals throughout Canada.
Resistant bacteria can be transferred from animals to humans in three ways: via food, meat in grocery stores is widely contaminated with antibiotic resistant bacteria; via working with animals, people working in the livestock industry handle animals, feed and manure and can become contaminated with resistant bacteria and transfer it to other people; and via the environment, ground water, surface water and soil are contaminated with manure from livestock. Resistant bacteria in livestock can be shed through their feces which can then contaminate the environment.
Resistance of human pathogens to antibiotics commonly used to treat humans is especially worrisome. There are many factors that contribute to the incidence of antibiotic resistant bacteria in animals and humans. Once humans are exposed to resistant bacteria, the bacteria must be able to survive and grow in, or on, humans, or must be able to transfer its resistance to bacteria that can survive and grow in, or on, humans. The bacteria must have the ability to cause disease in humans because without disease, there is no health risk. Reducing unnecessary or inappropriate use of antibiotics in both humans and animals will decrease the opportunity for resistance development in the treated microorganisms. It is important that the efficacy of antibiotics is maintained for as long as possible.
The Canadian Food Inspection Agency (CFIA) and Health Canada are in charge of policy and surveillance with regards to chemical residues from drugs such as antibiotics in food. Two especially important guidelines are the maximum residue limits (MRL) for residues in animal tissues, and the observance of withdrawal/withholding periods for the use of veterinary drugs in animals.
Maximum residue limits are established by the Veterinary Drug Directorate (VDD) of HealthCanada. According to the VDD, an MRL “is a level of residue that could safely remain in the tissue or food product derived from a food-producing animal that has been treated with a veterinary drug. This residue is considered to pose no adverse health effects if ingested daily by humans over a lifetime”. The basis for the evaluation of drugs includes: the metabolism of the drugs in the animal; the toxic and carcinogenic effects; and, the excretion time. Residue levels are determined in a variety of tissues, including muscle, organs, fat, and in products such as milk and eggs. Once approved, the MRL becomes part of the Canadian Food and Drug Act and Regulations. A residue limit of zero is tentatively assigned to drugs that are not currently being evaluated for an MRL. The evaluation process conducted by Health Canada is also used to establish withdrawal periods for veterinary drugs.
Canadian Food Inspection Agency. (2008). Chemical residues. Retrieved fromhttp://www.inspection.gc.ca/english/anima/heasan/man/crrc/crrce.shtml
Centre for Disease Control. (n.d.). Get Smart: Know When Antibiotics Work on the Farm. Educational Activities to Promote Appropriate Use of Antimicrobial Agents in Animals Retrieved from, http://www.cdc.gov/narms/get_smart.htm
Health Canada. (2008). Health Canada Veterinary Drug Directorate. Retrieved fromhttp://www.hc-sc.gc.ca/dhp-mps/vet/index_e.html