Illini DairyNet Papers
For over 50 years, antimicrobials have been used on dairy farms along with other drugs to treat and control disease, and to improve animal productivity. Recently, antibiotics have received an increasing amount of attention. Initially, regulatory agencies were concerned about chemical adulteration of milk and milk products with antibiotic residues. The public health concern was that these residues may induce allergic reactions in sensitized humans. The main concern today seems to focus on the concept that use of antimicrobials on farms may contribute to antimicrobial resistance. This concept is extended to imply that antimicrobial resistance observed in cattle pathogens may result in human health problems.
The Food and Drug Administration, Center for Veterinary Medicine (CVM) is concerned that the use of therapeutic antimicrobial drugs in food animals will create antimicrobial drug resistance that could contribute to drug-resistant human pathogen bacteria. These issues are being discussed with the U.S. Centers for Disease Control and Prevention (CDC). Both the CVM and CDC have agreed on the following points: 1) there is a legitimate need for both older and newer antimicrobial drugs in animal agriculture, 2) the bulk of Salmonella, E. coli O157, and Campylobacter infections in humans in the U.S. originate from food of animal origin, and 3) the use of antimicrobials in animals will cause resistance to develop, and there is a potential that resistant Salmonella, E. coli, and Campylobacter will be transferred to humans through food.
The Animal Medicinal Drug Use Clarification Act of 1994 (AMDUCA) spells out the rules and regulations veterinarians must follow when using drugs in an extra-label manner. The Act all but excludes drug-compounding activities by veterinarians and veterinary practices. Veterinary practitioners working with livestock species must keep these new regulations in mind when administering and dispensing medicinals. New antimicrobial drugs have been approved and others have been made illegal or been voluntarily banned. Practitioners must remain aware of these changing times. The integrity and leadership of the veterinary profession will determine whether any antimicrobial product will maintain approved for use in livestock.
Aminoglycoside use in cattle receives additional voluntary ban
A resolution to ban the use of aminoglycosides in cattle was passed by the American Veterinary Medical Association (AVMA) House of Delegates in July 1998. The AVMA has now joined the American Association of Bovine Practitioners, the Society for Theriogenology, and the state veterinary medical associations from Wisconsin, Iowa, Kansas, and Michigan. The resolution supports a voluntary ban on the extra-label use of aminoglycoside antibiotics, particularly gentamycin in cattle. The ban is to remain in effect until further scientific information becomes available.
The concern relates to the understanding that systemic use of aminoglycosides present a potential conflict to standards of practice because the scientific justification is limited. Residues can be found in renal tissue for an undetermined, extended period of time. In order to comply with AMDUCA regulations, extra-label drug use must include a sufficiently extended withdrawal period so that no residues are found in meat of milk products. Dairy producers using aminoglycosides and veterinarians prescribing the use of aminoglycosides will be in indefensible positions should residues be found.
Long acting tetracycline receives lactating cow clearance
The Food and Drug Administration (FDA) has approved the new animal drug application issued by Pfizer Animal Health for the use of Liquamyacin LA-200 injection in lactating dairy cattle. For the first time, dairy practitioners have an antibiotic approved for systemic use when treating acute metritis. Milk taken from cows during treatment with LA-200 and for 96 hours after the last treatment must not be used for food.
Coincident with this new approval comes an amended tolerance for the combination of oxytetracycline, chlortetracycline and tetracycline residues in milk. The Center for Veterinary Medicine (CVM) has reevaluated toxicity and metabolism data used to establish oxytetracycline tolerance levels. The previously accepted safe levels for oxytetracycline, chlortetracycline and tetracycline in milk were 30, 30, and 80 ppb respectively for a sum of residues of 140 ppb. The new combined tolerance level for tetracycline in milk is 300 ppb. With this increase, there are currently no FDA approved rapid screening tests for tetracyclines in raw co-mingled milk. This fact should be considered when evaluating milk from treated cows.
Fluoroquinolone receives approval from CVM
Enrofloxacin, a fluoroquinolone has been approved by the FDA, CVM for the treatment of bovine respiratory disease associated with Pasteurella and Haemophilus. The product, Baytril 100 Injectable Solution is produced and distributed by Bayer Corporation and remains a prescription product. Baytril is not for use in any class of dairy cattle including veal calves. Extra-label use of fluoroquinolones in food animals has been and is still prohibited by the FDA.
Fluoroquinolones are administered in human medicine for refractory cases where other antibiotics have proven of little or no use. Bayer will be conducting an intensive post-approval monitoring program to determine if use of the product contributes to development of bacterial resistance to fluoroquinolones in cattle. The company has voluntarily committed to the FDA to immediately take action, including removal of the Baytril from sale, should the FDA conclude that there is a risk to public health. Veterinarians with a cavalier attitude toward extra-label use of Baytril in lactating dairy cattle will not only be violating the AMDUCA law, they will be jeopardizing the use of the product for approved uses.
Nonsteroidal anti-inflammatory drugs prohibited as well
Phenylbutazone, known as "bute," is a veterinary drug only label-approved by the Food & Drug Administration for use by veterinarians in dogs and horses. It has been associated with debilitating conditions in humans and it is absolutely not permitted for use in food-producing animals. USDA/FSIS has conducted a special project to for this drug in selected bovine slaughter plants under federal inspection. An earlier pilot project by FSIS found traces less than 3% of the livestock selected for testing, sufficient cause for this special project. There is no tolerance for this drug in food-producing livestock, and they and their by-products are condemned when it is detected. Dairy producers must not use this drug in food-producing livestock and if it is found, those producers will be subject to FDA investigation and possible prosecution.
Topical Nitrofurans Are Now Prohibited
FDA is issuing an order prohibiting the extralabel use of topical nitrofuran animal and human drugs in food-producing animals. This order is based on evidence that extralabel use of topical nitrofuran drugs in food-producing animals may result in the presence of residues that are carcinogenic and have not been shown to be safe. A carbon-14 (C-14) radio-label residue depletion study conducted by the FDA showed that detectable levels of nitrofuran derivatives are present in edible tissues (milk, meat, kidney, liver) of cattle treated by the ocular (eye) route. The study indicates that use of these nitrofuran products may pose a risk to public health because residues of known carcinogens are present in edible tissues.
The current list of prohibited drugs includes furazolidone and nitrofurazone, but it contains the parenthetical statement (except for approved topical use). FDA plans to remove this parenthetical statement. Once this prohibition is in place, the revised list will state that the following drugs (both animal and human), families of drugs, and substances are prohibited for extra-label uses in all food-producing animals.
- Diethylstilbestrol (DES)
- Other nitroimidazoles
- Furazolidone, Nitrofurazone, other nitrofurans
- Sulfonamide drugs in lactating dairy cattle (except approved use of sulfadimethoxine, sulfabromomethazine, and sulfaethoxypyridazine)