Historical Perspective on the Regulation ofAntimicrobial Residues in Food in the United StatesMargaret Ann Miller

From the Of® ce of Women’s Health, US Food and Drug Administration, 5600 Fishers Lane, Rockville, MD20857, USA

Correspondence to: Margaret Ann Miller, Of® ce of Women’s Health, US Food and Drug Administration, 5600Fishers Lane, Rockville, MD 20857, USA. Fax: » (301) 827-0926; E-mail: mmiller1@oc.fda.gov

Microbial Ecology in Health and Disease 2000; Suppl 1: 8–10

ORIGINAL ARTICLE

Introduction

In the US, antimicrobials used in animals for both thera-peutic and growth promotion purposes are consideredanimals drugs. Antimicrobial drugs are used in animals totreat and prevent disease, and to increase production.Before any animal drug can be legally marketed in the US,the drug’s sponsor must have a New Animal Drug Appli-cation (NADA) approved by the Food and Drug Adminis-tration (FDA). Within FDA, the Center for VeterinaryMedicine (CVM) is responsible for determining when ananimal drug is effective and safe for the animal, theenvironment and can be manufactured to uniform stan-dards of purity, strength and identity. When the animaldrug is intended for use in food-producing animals, thedrug sponsor must also demonstrate that edible productsfrom the treated animals are safe for consumers (1).

While CVM is responsible for performing the safetyassessment for animal drug residues, the United StatesDepartment of Agriculture (USDA), Food Safety Inspec-tion Service is responsible for testing the meat supply formicrobiological contamination and animal drug residues.Both FDA and USDA have extensive programs to ensurethat meat, milk and other animal products are safe andwholesome.

EVOLUTION OF FOOD SAFETY ASSESSMENTS

Whenever an animal is treated with an animal drug, someamount of the drug remains in edible tissue of the treatedanimal. Although food safety has always been a majorconcern of the FDA, the human food safety requirementsfor animal drug residues have changed over the years.Originally, animal drugs were approved based on a ‘NoResidue’ or ‘Zero Tolerance’ policy. While this approach iswidely endorsed by consumers, the zero tolerance actuallyrepresented the sensitivity of the analytical method used tomonitor drug residues—not a true zero value. As analyti-

cal methods improved, the zero tolerance was continuallybeing lowered. Therefore, the agency adopted a NegligibleTolerance Policy. Under this policy, the amount of toxicol-ogy data required for approval was limited to subchronicstudies in dogs and rats; and the tolerance was generallyassigned as 0.1 ppm (2). The main problem with both ofthese tolerance setting procedures was the disconnect be-tween residue levels and public health concerns.

CURRENT FOOD SAFETY ASSESSMENTS

To remedy this situation in the mid-1970’s the CVMadopted its current safety assessment procedure. Underthis procedure, tolerances are based upon toxicology,residue chemistry, and metabolism data. The humanhealth risk associated with an animal drug residue is theproduct of the harm of the drug and the exposure to thedrug. The agency regulates the human health risks associ-ated with animal drug residues by assessing the harm andcontrolling exposure to meet a risk standard of ‘reasonablecertainty of no harm’ (1).

The harm associated with an animal drug is assessedbased on a standard battery of toxicology tests (Table I).Each one of these tests is designed to determine a dose thatcauses an effect and a dose that produces a no-observedeffect for a speci�c toxicological endpoint. In determiningthe toxicological endpoints to be examined, the human

Table I

Standard battery of toxicology tests required for the human foodsafety assessment of new animal drugs

– A battery of short tem genetic toxicity tests.– A subchronic (90-day) feeding study both in a rodentspecies (usually the rat) and in a non-rodent mammalianspecies (usually the dog)– A two-generation reproduction toxicity study in the rat– A developmental toxicity study in the rat

© Taylor & Francis 2000. ISSN 1403-4174 Microbial Ecology in Health and Disease

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History of antimicrobial drug residue regulation in the US 9

food safety assessment generally focuses on the effect ofchronic low level exposure. Following completion of allthe toxicology studies, the no-observed effect level(NOEL) of the most sensitive effect from the most appro-priate toxicology study is divided by a safety factor todetermine an acceptable daily intake (ADI). The ADIrepresents the total drug residue; parent and all metabo-lites that can be safety consumed daily throughout one’slifetime. The amount of drug residue permitted in each ofthe edible tissues, i.e., the safe concentration depends uponthe quantity of tissue that is consumed by the 90th percen-tile consumer on a daily basis (1).

Once the safe concentration is determined, the drugsponsor conducts a total residue depletion study to deter-mine how the drug depletes from edible tissues of thetreated animal. The total residue consists of parent com-pound, free metabolites, and metabolites that are cova-lently bound to endogenous molecules and is very dif�cultto measure on a routine basis. Rather than measuring thetotal residues in all the edible tissues, FDA establishes amarker compound in one tissue, the target tissue, formonitoring drug residues. The tolerance, the level ofmarker compound in the target tissue, is established sothat when residues are below the tolerance in the targettissue the whole carcass is safe. If an animal drug productrequires time after the last treatment to deplete to thecalculated safe concentration, the FDA establishes a with-drawal time for the product. Information about the with-drawal time is included on the drug label (1).

FUNCTION OF HUMAN INTESTINALMICROFLORA

The standard battery of toxicology test performed toestablish the ADI for animals drug residues do not speci�-cally address the microbiological safety of drug residues.The indigenous bacteria that populate the gastrointestinaltract of humans provide resistance to infections, metabo-lize toxins and carcinogens, synthesize vitamins, and aid infood digestion (3, 4). The large bowel of mammals iscolonized by over 400 species of bacteria present at about1011 bacteria:g. Every species of animal has a unique �orathat has evolved with the host over time. Within a hostthere is variation in the �ora between individuals (3). Inthe large bowel anaerobic bacteria predominate over facul-tative bacteria by a ratio of 1000 to 1 (4). The control ofthe intestinal �ora is an important feature of health sincethe host must protect itself against the microbial insultsfrom the environment including its own �ora. Despitetremendous exposure to bacteria from external sources, theindigenous intestinal �ora remains relatively stable, withfew changes day-to-day (4). Therapeutic levels of someantibiotics perturb the gut ecosystem causing adversehealth effects (3, 4).

BACKGROUND ON THE MICROBIOLOGICALSAFETY OF RESIDUES IN FOOD

As early as 1970, FDA recognized that the use of antibi-otics in animals could result in microbiologically activeresidues in edible tissue that could adversely impact thehuman intestinal micro�ora (5). The interest at that timefocused only on the selection of resistant coliforms. Duringthe mid-1970’s, CVM conducted several studies developinga beagle dog model to determine the lowest dose ofantibiotic required to select for resistant bacteria. Rollinsand Gaines fed groups of beagle dogs residue doses ofoxytetracycline, dihydostreptomycin, penicillin or vir-ginamycin for 35–57 days and examined the selection ofresistant coliforms. For oxytetracycline, dose levels of 10ppm in the diet for 44 days resulted in a shift frompredominantly a susceptible population of lactose-ferment-ing bacteria to a resistant population. No effects were seenat 2 ppm (6). Generally, these studies suggested that thevery low tolerance level of antibiotic residues in foodwould not select for resistant bacteria. However, CVMhad no guidance on when to ask for microbiological safetydata, what endpoints to examine, and how to interpret theresults.

To correct this situation, in 1992, CVM and the AnimalHealth Institute co-sponsored a symposium entitled ‘Mi-crobiological Signi�cance of Drug Residues in Food’. Theobjectives of the symposium were to learn more about theintestinal micro�ora and the factors that in�uence thissystem, to determine if the current toxicology models wereadequately assessing the impact of antibiotic residues, andto discuss model systems appropriate for examining theeffect of antimicrobial agents on the intestinal micro�ora.

CURRENT GUIDANCE

Based on the information presented at the symposium,CVM published guidance entitled ‘Microbiological Testingof Antimicrobial Drug Residues in Food’. In this guid-ance, the Center identi�ed four areas of potential publichealth concern for antimicrobial residues. First, a changein the antimicrobial resistance pattern of the enteric �ora,particularly the anaerobes was considered an adverse effectbecause intestinal bacteria can cause human illness orcould pass resistance genes to bacterial pathogens. Second,a change in the metabolic activity of the intestinal mi-cro�ora was considered an adverse effect because normalmetabolic activity is required for vitamin synthesis andnutrition. Third, a perturbation of the colonization resis-tance properties of the intestinal micro�ora was deemed tobe an adverse effect because an intact bacterial ecosystemis vital for preventing an overgrowth or invasion of bacte-rial pathogens. Finally, a change in the bacterial popula-tion or composition was considered an adverse effectbecause an alteration in either of these parameters repre-sents a gross disruption of the normal intestinal ecologyrequired to maintain health (7).

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M. A. Miller10

The guidance described three categories of antimicrobialproducts that were not likely to be subject to additionalmicrobiological testing:

– antimicrobial drugs with ‘very low residues’ i.e. less that1.5 mg:person:day;

– antimicrobials that were extensively metabolized in theanimals such that residues had limited antimicrobialactivity when consumed; and

– antimicrobials that show no adverse effects on theintestinal micro�ora when tested at doses approved foruse in the target species.

Antimicrobial drugs seeking an ADI greater than 1.5mg:person:day would require additional microbiology test-ing. At the time the guidance was drafted, CVM recog-nized that the 1.5 mg:person:day threshold was basedupon the best information available at the time and mayneed to be modi�ed as new more potent antibiotics aredeveloped. Also, the Center noted that there were nomodel systems validated to evaluate the effect of low levelsof antimicrobials on the human intestinal micro�ora andthat such validation would require additional research.The Center did not, however, accept the MIC and formulaapproach utilized in Europe and by the World HealthOrganization because CVM found no scienti�c informa-tion the related the results of the Minimum InhibitoryConcentration (MIC) determined in vitro to the potentialpublic health harms identi�ed.

In 1995, FDA:CVM funded research to develop modelsystems for assessing the microbiological effects of antimi-crobial animal drugs that enter the food supply as residuesin edible tissues. Two model systems were selected forassessing the effect of low doses of antimicrobials on thehuman intestinal micro�ora—a continuous �ow single-chambered in vitro culture system (chemostat) and a germ-free mice associated with human intestinal micro�ora(HFA mice model).

The metabolic activity of the intestinal micro�ora, thedevelopment of resistant strains, the colonization resis-tance properties of the micro�ora, and counts of targetorganisms were evaluated in both models with both tetra-cycline and neomycin. These antibiotics were chosen be-cause of the extensive knowledge based on the adverseeffects of these drugs in humans. Extensive MIC data wasalso available for comparison. In addition to examiningthese two drugs in both systems, the effect of erthyromycinwas examined in the chemostat culture system and studieswith cipro�oxacin are completed for the chemostat culturesystem and ongoing for the HFA mice.

SUMMARY

Following the completion of the CVM funded research,CVM intends to make scienti�c decisions on how best toassess the microbiological safety of animal drug residuessuch that both animals and consumers are protected.CVM will reevaluate its guidance document on this topic.

REFERENCES

1. Friedlander LG, Brynes SD, Fernandez AH. The HumanFood Safety Evalution of new Animal Drugs, In: Tollefson L(ed.), The Veterinary Clinics of North America, W.B. SaundersCo., Philadelphia, March 1999: p. 1–11.

2. Teske RH. Microbiological Signi�cance of Drug Residues inFood: Welcome and Introduction. Veterinary and HumanToxicology 1993; 35 (supplement 1): 1–2.

3. Carman RJ, Van Tassell RL, Wilkins TD. The Normal Intesti-nal Micro�ora: Ecology, Variability and Stability. Veterinaryand Human Toxicology 1993; 35 (supplement 1): 11–4.

4. Gorbach SL. Perturbation of Intestinal Flora. Veterinary andHuman Toxicology 1993; 35 (supplement 1): 15–23.

5. FDA-CVM Guideline 18. Antibacterial Drugs in AnimalFeeds: Human Health Safety Criteria.

6. Rollins LD, Gaines SA, Pocurull DW, et al. Animal Model forDetermining the No-Effect Level of an Antimicrobial Drug onDrug Resistance in the Lactose-Fermenting Enteric Flora.Antimicrobial Agents and Chemotherapy 1973; 7 (5): 661–5.

7. FDA-CVM Guideline 52. Microbiological Testing of Antimi-crobial Drug Residues in Food. January, 1996.

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