Feedlot antibiotics and their impact

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Antibiotics have been used in food animals for decades. The much-amended, but still in effect, Federal Food Drug and Cosmetic Act was passed in 1938, providing for the safety of drugs in the target species. Early use of antimicrobial drugs was very limited, and the introduction of antibiotics in commercial feed for cattle, pigs and chickens began in the early 1950s.

In 1951, an amendment to the Act created two drug categories, prescription and over-the-counter (OTC), explains Mike Apley, DVM, PhD, Kansas State University.” It is interesting to note that the sponsers of this amendment did not intend for prescription status to apply to drugs used in food animals. The attitude back then was that if you owned the animal you could do what you wanted with it. Thankfully, we now realize that we are raising food.” A 1962 amendment provided for the authority of the Food and Drug Administration Center for Veterinary Medicine (FDA-CVM) we are familiar with today.

“Regulatory responsibility was very little to none in the early days,” says Dan Upson DVM, PhD, professor emeritus of Kansas State University. “Residue concerns were minimal.” Mostly used were over-the-counter antimicrobials, recommended by veterinarians and others and administered by the owner. The primary antibacterial products for cattle treatment in the 1950–60s were the sulfonamides.

Soon after livestock producers began using antimicrobials in food-producing animals, scientists began studying the possible effects of their long-term use. The FDA first called for several restrictions on antimicrobial use in feed in 1977. The 1999 General Accounting Office Food Safety report noted that the use of antibiotics in agriculture is only one of several factors that contribute to antibiotic resistance in humans for pathogens that are not foodborne.

Since 1988, the FDA Center for Veterinary Medicine (CVM) has approved new therapeutic antimicrobials for use in food-producing animals as prescription-only products based on the need to assure the proper use of antimicrobials through precise diagnosis and correct treatment of disease and to avoid drug residues in food. The Animal Medicinal Drug Use Clarification Act (AMDUCA) of 1994 set the stage for regulations, codified in 1996, for the extralabel use of drugs by veterinarians.

Concerns about the use of antimicrobial products in food-producing animals have focused on human food safety because foods of animal origin have been identified as vehicles of foodborne disease in humans. As a result of treating animals with antibiotics, foodborne bacteria of animal origin may also have altered susceptibility to the antibiotics used to treat human disease.

These concerns changed the way new antimicrobial products are brought to market. Apley says in 2003 the FDA released a guidance document (#152) that outlines methods for evaluating human food safety with respect to the potential microbiological effects of antimicrobial new animal drugs on foodborne bacteria of human health concern.

Targets remain the same
Bacterial targets on the feedlot have remained roughly the same over the years. Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni are still the main targets for respiratory disease, with the more recent, and somewhat debated addition of Mycoplasma bovis, explains Apley. Fusobacterium necrophorum remains a challenge expressed as foot rot, liver abscesses, and diphtheria. E. coli, Streptococcus and Staphylococcus remain as “manure bugs” that get into places where they shouldn’t be, such as joints and the peritoneal cavity, and cause infections. Mycoplasma bovis and Histophilus somni remain as our most likely targets for arthirits. “Our nervous system targets remain Listeria monocytogenes and Histophilus somni,” says Apley. “Probably the target poised to cause the most trouble from a resistance standpoint is Salmonella.”

The means for taking on these pathogens in a therapeutic mode has expanded (in the United States) from the 1970s with the tetracyclines, spectinomycin, chloramphenicol, erythromycin, tylosin, Pen G, ampicillin/amoxicillin, sulfas and extralabel aminoglycosides to now include florfenicol, ceftiofur, enrofloxacin and danofloxacin (for BRD only), tilmicosin, and tulathromycin, Apley explains.

Apley believes the advent of single injection, subcutaneous products as one of the most important advances in antimicrobial therapy for cattle. “As I look back on these antimicrobials coming out, it is interesting how many of us looked at each new advance in hope this would finally solve our problems in treating and/or preventing our disease challenges,” he says. “We now have a more advanced, easier to manage antimicrobial tool box, but we still have extensive morbidity and mortality in some groups of cattle. We have been able to alter the equation with antimicrobials, but we haven’t been able to solve it yet.”

Feedlot antibiotic use
According to the 1999 NAHMS Feedlot Report, 83% of feedlots used some antimicrobials in feed or water. Almost 100% of feedlots of all sizes surveyed in the study used injectable antibiotics as part of a therapeutic regimen for BRD. Over all, 10.4% of cattle were treated metaphylactically to prevent clinical manifestations of BRD.

Almost three out of four feed lots provided formal training by qualified feedlot personnel, veterinary consultants, or drug company representatives in areas related to antimicrobial use. Nearly one-half of all feedlots included written guidelines with the formal training for both label use of antimicrobials and drug residue avoidance, while nearly one- half provided training on disease diagnosis without written guidelines.

On feedlots today, antibacterial products are prescribed, protocols given and extensive education of treatment personnel is provided by the veterinarian. “The responsibility for antibacterial use is much greater,” says Upson. “Today’s antibacterial treatments are much safer and more efficacious. Their use also contributes to more wholesome beef for consumers.” 


AVC’s aminoglycoside postition

In 1993, the AVC took a stand on aminoglycoside use in cattle with a position statement which was adopted or supported by other veterinary and livestock organizations. Supporting this statement were the facts that aminoglycosides do not meet AMDUCA criteria, FDA-CVM-approved products are currently available for use, food safety aspects of using these products have not been established by FDA, efficacy data is sparse, treatment cost should not be a primary reason for extra-label drug use, it’s not in the best interest of the beef industry and there are liability issues for the veterinarian.

“The position statement showed the AVC as a concerned and knowledgeable leader to the entire beef industry,” says Dan Upson, DVM, PhD. “It encouraged veterinarians to ‘think first, then prescribe’ better protocols.”

The AVC’s position statement, The Use of Aminoglycosides in Food Animals, reads: The systemic use of aminoglycoside antibiotics presents a potential conflict to the stated objectives of the AVC Standards of Practice because scientific justification for such use is limited and because it is known that identifiable residues in kidney tissue can result for an undetermined extended period of time.

Therefore, the AVC hereby resolves that until further scientific information becomes available alleviating safety and efficacy concerns, aminoglycoside antibiotics should not be used in cattle, except as specifically approved by the FDA. 



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