Antimicrobial use in food and companion animals

The vast literature on antimicrobial drug use in animals has expanded considerably recently as the antimicrobial resistance (AMR) crisis in human medicine leads to questions about all usage of antimicrobial drugs, including long-term usage in intensively managed food animals for growth promotion and disease prevention. Attention is also increasingly focusing on antimicrobial use and on bacterial resistance in companion animals, which are in intimate contact with the human population. They may share resistant bacteria with their owners, amplify resistant bacteria acquired from their owners, and act as a reservoir for human infection. Considerable effort is being made to describe the basis of AMR in bacterial pathogens of animals. Documentation of many aspects of use of antimicrobials in animals is, however, generally less developed and only a few countries can describe quantities of drugs used in animals to kg levels annually. In recent years, many national veterinary associations have produced 'prudent use guidelines' to try to improve antimicrobial drug use and decrease resistance, but the impact of guidelines is unknown. Within the evolving global movement for 'antimicrobial stewardship', there is considerable scope to improve many aspects of antimicrobial use in animals, including infection control and reduction of use, with a view to reducing resistance and its spread, and to preserving antimicrobial drugs for the future.     

Legal implications of the extra-label use of drugs in food animals

Although the Food and Drug Administration has never sanctioned the extra-label use of drugs in animals, it has not, until recently, objected when veterinarians deviated from label instructions as long as such use did not result in violative residues in food products derived from treated animals. However, because of the potential human health hazards associated with abuse of this position, the FDA has developed a less lenient policy toward the extra-label use of drugs in food animals. A brief review of the legislated responsibility and authority of the FDA with particular reference to the extra-label use of drugs in food animals is provided. Emphasis is placed on how anesthetics and anesthesia adjuncts in food animals can pose human health hazards.     

Human health aspects of antibiotic use in food animals: a review

Antibiotic use not only selects for resistance in pathogenic bacteria, but also in the commensal flora of exposed individuals. Veterinary surgeons regularly prescribe antibiotics for food animals to treat bacterial infections just as doctors do for human patients. In addition, however, animal feeds contain added antibiotics not for therapy but for economic reasons: to enhance the growth rate of these animals. Several of the antibiotics used as growth promoters are analogues of and fully cross resistant with important antibiotics used in human medicine. As a result of this high exposure to antibiotics, the prevalence of resistant bacteria in the faecal flora of these animals is high. These resistant bacteria can be directly and indirectly, via foods of animal origin, transferred to humans and either colonize the human intestinal tract or exchange their resistance genes with commensal bacteria of humans. As the intestinal flora functions as a reservoir of resistance genes for pathogenic bacteria and because many bacterial species of the intestinal flora are potential pathogens, the efficacy of antibiotic therapy in human medicine may be jeopardized.     

Fundamental safety requirements in the use of live vaccine in food animals]

Live vaccines have a number of advantages over inactivated ones--above all in respect of the stimulation of cell-mediated immune reactions. Various live vaccines, based on viruses, bacteria, fungi or parasites, have been approved for use in Germany in animals used as a source of food. Safety requirements obviously play a more important role for live vaccines, both in vaccine development and in batch testing, than with inactivated vaccines. Vaccine strains isolated from tissue samples must be clearly distinguishable from field strains. The safety of overdoses and the spread of the vaccine strain in the immunized animals have to be investigated, as well as shedding of the vaccine strain and its safety in non-target species. Any impact of a live vaccine strain on humans and the environment must be assessed. Live vaccines will remain an important research field in the long term, with efforts focused on developing deletion mutants and vector vaccines.     

Euthanasia of food animals

Humane euthanasia is considered to be the rapid and painless inducement of death. It is necessary for unconsciousness to occur quickly and be followed immediately by cardiac and/or respiratory arrest. Although many methods of euthanasia have been used, not all fulfill the criteria of humane euthanasia. Methods of euthanasia used in food animals are presented and evaluated as to their appropriateness.     

Drug residues in food animals

A total of 292 field investigative reports of drug residues in food animals for 1983 to 1988 were analyzed. The investigations had been conducted by the Food and Drug Administration (FDA) and the Virginia State Veterinarian's Office, in cooperation with the Center for Veterinary Medicine of the FDA, to trace residues reported by the USDA Food Safety and Inspection Service to the source of the animal and the administration of the drug. The analysis disclosed the following. (1) Antibiotic residues were most often associated with streptomycin, penicillin, oxytetracycline, and neomycin. Sulfamethazine was, by far, the most frequently cited sulfonamide. (2) Residues are being found predominantly in cows, veal calves, and market hogs (barrows and gilts). (3) The cause of drug residue most frequently cited by the field investigators was failure to observe the withholding time for the drug. Almost half of these investigations revealed that the individual responsible for the sale of the animal did not know the proper withholding time for the drug. Failure to maintain adequate records was also a contributing factor. (4) The producer was considered to be the responsible party in over 80% of the cases for which responsibility was determined. (5) Residues associated with injectable drugs were investigated most frequently. Long-acting and sustained-release products were most often associated with penicillin and oxytetracycline residues. (6) The 2 most common sources of purchase for the drugs involved in the investigations were the feed/farm supply store and the veterinarian. (7) Unapproved drug use was not a major cause of residues.     

Drug residues in food animals. III. Plasma and tissue kinetics of potassium penicillin G in young cross-bred swine

Twenty-two young cross-bred swine were treated either intravenously or orally with potassium penicillin G. The pharmacokinetics of penicillin G were determined in plasma and tissues. The plasma half-life of penicillin G in swine was found to be 19.45±1.69 min, and the distribution and elimination kinetics were found to fit a classical two-compartment model. The volume of distribution was found to be 0.53±0.12 1/kg, and the body clearance was found to be 19.06±5.06 ml/min/kg which exceeded the effective renal plasma flow of 16.50±2.73 ml/min/kg, suggesting that the drug was eliminated both by tubular excretion and glomerular filtration. The elimination rate constants (Beta) for the major organs were as follows: muscle, 0.00343 min^-1; lung, 0.0310 min^-1; fat, 0.0394 min^-1; and kidney, 0.0213 min^-1, which compared favorably with the elimination rate constant found in plasma (0.0320 min^-1). These values were found to be significantly similar at the level of P < 0.005 in muscle, spleen and fat, and at a level of P < 0.025 in lung tissue. The data indicates that blood plasma would be a satisfactory body fluid for estimating this drug in tissue.