Antibiotic use in farmed livestock
Introduction
Antibiotics are vital medicines used for the treatment of bacterial infections in both humans and animals. The emergence of antibiotic resistance as a serious problem in human medicine has prompted concerns about the public health implications of antibiotic use in agriculture. Antibiotics have been used for over 40 years in farm animals for 3 main purposes:
- Therapy, to treat an identified illness
- Prophylaxis, to prevent illness in advance
- Performance enhancement, to increase feed conversion, growth rate or yield.
Treatment and prevention of disease
Antibiotics, used responsibly, are an essential element in the fight against animal diseases caused by bacteria. However, in animals, as in humans, a significant proportion of those treated for infectious disease would recover without antibiotics. Bacterial diseases cause pain, distress and economic loss. Antibiotics reduce this suffering and distress and speed recovery in infected animals. Since the animal cannot be allowed to suffer the alternative is to kill the animal. The removal of antibiotics from veterinary medicine would cause great welfare problems.
Antibiotics are sometimes used to prevent diseases that might occur in a herd or group of animals. In some situations when the proportion of animals suffering a disease during a defined time period reaches a threshold value, all animals in the herd or group are treated as the probability of most or all of the animals getting infected is high.
In both treatment and prevention of disease medicines are administered over a defined, preferably short, period of time and are prescribed by a veterinary surgeon. Antibiotics are administered by the most convenient and effective routes. With intensively reared stock, such as poultry and pigs, individual treatment usually is not feasible and mass oral medication (in feed or water) is the only practical method of treatment as injections and handling are stressful for them. Sick cattle and sheep are usually treated individually.
Antibiotics for growth promotion
The use of antibiotics as growth promoters has been prohibited in the EU since January 1, 2006. Antibiotics had been added in low doses to the feed of farm animals for decades worldwide because they improved the growth rate and efficiency of conversion of feed into carcass meat in pigs, poultry and cattle. Their use increased average daily growth and food conversion ratios by 3 per cent to 11 per cent depending on species. The ban was the final step in the phasing out of antibiotics used for non-medicinal purposes in the EU and antibiotics are now only allowed to be added to animal feed for veterinary purposes. Antibiotics are still used widely as feed additives for growth promotion in many countries outside the EU.
Current regulatory framework
All antibiotics currently used in the EU have been registered for their current uses on the basis that they are effective and safe for both human and animals. This safety and efficacy is kept under review. Use for the purposes for which they are registered should therefore pose little or no hazard to the public, to those who administer them and to workers involved in the preparation of food. The Veterinary Medicines Directorate issues the product license where the product has been found to be safe, effective and of sound quality. The Veterinary Products Committee advises with respect to safety, quality and efficacy in relation to veterinary use of any substance or article.
When medicines are used for food animals, studies must be carried out to assess the time needed for any residues of a substance or its metabolites which may still be present in an animal's body to fall below the level shown to be safe. Once this has been determined, the withdrawal period can be established. The withdrawal period is the minimum time required between the last treatment and the collection of meat or milk for human consumption.
It is a requirement of the Veterinary Medicines Regulations that animal feedingstuffs containing antibiotics or any other POM-V medicines can only be supplied to the keeper of animals on receipt of a medicated feedingstuffs prescription (MFSp) completed in writing by a veterinary surgeon. In the context of the Regulations, persons who feed the products to their animals are called keepers of animals. The Regulations are revoked and replaced every year.
Antibiotic residue
Antibiotic residues may be found in milk or meat when products from animals have been sent for processing within the withdrawal period of a therapeutic antibiotic or when animals have had access to excreted residues in their environment. An independent advisory committee, the Veterinary Residues Committee, oversees the UK's surveillance for residues of Veterinary Medicines (www.vet-residues-committee.gov.uk/home.htm).
The Veterinary Medicines Directorate (www.vmd.gov.uk) oversees the authorisation and surveillance of veterinary medicines in the UK. The Veterinary Medicine Directorate publishes all the results for residues surveillance quarterly in VMD's Medicines Act Veterinary Information Service (MAVIS - www.vmd.gov.uk/Publications/MAVIS/mavis.htm). The detailed results of the statutory and non-statutory surveillance programmes are available on the Veterinary Residues Committee website. In 2006 of the results from the antimicrobial screening test which covers residues of tetracyclines, penicillins, sulphonamides and streptomycin/dihydrostreptomycin 99.9% were less than the Limit of Quantification or the action level for the antimicrobial - red meat 4824/4831 samples; poultry meat 1405/1406 samples; milk 680/681samples (Annual Report on Surveillance for Veterinary Residues in the UK, 2006, Veterinary Residues Committee).
Antibiotic resistance
To avoid elimination by antibiotic substances bacteria develop resistance by mutation or by acquisition of genes from other bacteria. Horizontal gene transfer between bacteria is a common event and an important factor in microbial evolution. The consequences of antibiotic resistance are seen as an increase in morbidity and mortality due to bacterial diseases.
In a report on 'The Medical Impact Of The Use Of Antimicrobials In Food Animals' the World Health Organisation acknowledged that in general, there is little doubt that treatment problems in humans due to resistant bacteria are primarily related to the prescribing practices of health workers and to medication-taking practices of patients. In its Report (August 1999) the Advisory Committee on the Microbiological Safety of Food (ACMSF) Working Group on Microbial Antibiotic Resistance in Relation to Food Safety stated that 'It is highly probable that the greatest stimulant to the production of resistant bacteria in humans is the use of antibiotics in human medicine.....' However, the Report also states 'We believe that it (the evidence) shows conclusively that giving antibiotics to animals results in the emergence of some resistant bacteria which infect humans.'
Humans may eat antibiotic resistant bacteria on their food. Transmission of resistant bacteria between different species occurs, although the resulting colonisation may be transient. Transmission of resistance genes can and does occur in a variety of surroundings. The question that remains is how often successful transfer cause clinical problems.
Resistance genes can persist in the environment both in their original bacterial hosts and in environmental organisms. There was resistance to antibiotics long before antibiotics were used in medicine e.g. 2,000 year old antibiotic resistant bacteria have recently been isolated from glaciers in the Canadian Arctic. In the presence of antibiotics, resistant bacteria will have a competitive advantage.
Resistance becomes a problem when bacteria causing a disease withstand antibiotic therapy. Increased resistance to antibiotics may therefore cause clinical problems and may shorten the useful life span of some antibiotics. Cases of human salmonellosis caused by multiple resistant Salmonella typhimurium definitive phage type 104 are occurring in the UK and the majority of these arise from contact with animals or contaminated food. Antibiotics are only used to treat Salmonella in a small minority of cases in compromised patients but their use in those cases can be a case of life and death. Campylobacters, Clostridia and Yersinias may also develop resistance. It is not clear whether any of the vancomycin resistant enterococci found in man and causing disease belong to species or strains derived from animals but at present the suggestion is that they are not.
Responsible use of medicines
Animal medicines play an important role in the control and prevention of disease and animal suffering but have the potential to cause harm if not used properly. Ultimately it is farmers and vets who are responsible for ensuring that antimicrobials are used in a safe, responsible and effective way. The Royal College of Veterinary Surgeons Guide to Professional Conduct makes specific and detailed reference to the use of pharmaceutical products. In 1998 the British Veterinary Association published a Code of Practice on Medicines for their members.
Farmers are legally obliged to keep a record of all medicines administered to food-producing animals, including those administered by veterinary surgeons or in-feed. The record must be kept available for inspection by authorised persons for at least 3 years. Auditors from the farm assurance schemes also check compliance with legal requirements. Guidelines on responsible use and practical strategies to reduce the need to use antimicrobials on farms have been developed by the Responsible Use of Medicines in Agriculture Alliance (RUMA) and can be downloaded from their website (
www.ruma.org.uk).
Acknowledgements
Prepared for BSAS by Derek Armstrong MVB MRCVS, AHDB Senior Veterinary Scientist, Veterinary Department, Agriculture and Horticulture Development Board. For further information please contact the author at the Veterinary Department, Agriculture and Horticulture Development Board, PO Box 44, Winterhill House, Snowdon Drive, Milton Keynes, MK6 1AX; Tel. 0044 (0)1908 844216
Email - derek.armstrong@ahdbms.org.uk
Updated: 1 July 2008
For further information please contact the author.
