Antimicrobial Use In Animal Feeds

Antibiotics were first discovered by Sir Alexander Fleming in 1928. However, it was not until more than 20 years later, in 1949, that animal scientists discovered that feeding a fermentation media to chickens and pigs stimulated growth.[1] Subsequently, an antibiotic (chlor-tetracycline) was isolated from the media and given the name aureomycin.

Within a few years after this discovery, the use of antibiotics in animal feeding programs was readily adopted by the livestock and poultry industries. Over the past 50 years, antibiotics and other antimicrobial agents have been widely used in swine and poultry feeds and, to a lesser extent, in beef and dairy feeds. These agents are commonly used at low (subtherapeutic) levels in feeds to enhance growth rate and efficiency of feed utilization in swine and poultry, to reduce mortality and morbidity in young pigs and dairy calves, to improve reproductive performance in swine, and to reduce liver abscesses in feedlot cattle. Antibiotics are also used at moderate to high levels (prophylaxis) for the prevention of disease in exposed animals, and at high (therapeutic) levels to treat diseases in animals.

Although many antibiotics have been discovered over the past six decades, relatively few have been approved by the Food and Drug Administration (FDA) for use in animal feeds. For example, only 12 antibiotics and five chemotherapeutics are approved by the FDA for inclusion in swine feeds.[2] These include the antibiotics apramycin, bacitracin (two forms), bambermycins, chlortetracycline, lincomycin, neomycin, oxytetracycline, penicillin, tiamu-lin, tylosin, and virginiamycin, and the chemotherapeutics arsanilic acid, carbadox, roxarsone, sulfamethazine, and sulfathazole. Certain ones of these antimicrobial agents are approved for combination usage (e.g., chlortetracy-cline, sulfamethazine, and penicillin; tylosin and sulfamethazine; neomycin and oxytetracycline), whereas certain others can only be fed alone and not with other agents. Usage is permitted only at levels approved by the FDA.[2] All of the chemotherapeutics approved for swine require withdrawal from the feed for specific periods of time before marketing the animal, but most of the antibiotics do not require withdrawal except when fed at certain therapeutic levels.[2]

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