Study Notes on Antibiotics Use in Food Animals

ANTIBIOTICS USE IN FOOD ANIMALS

Presented by: Eoin Ryan MVB MVM DECBHM
Contact: shumphreysdvm@gmail.com

LEARNING OBJECTIVES

  • 1) Understand the basic principles of antibiotic selection.

  • 2) Differentiate between MIC (Minimum Inhibitory Concentration) and Kirby-Bauer method.

  • 3) Review the fundamentals of each pertinent antibiotic class, which includes:

    • Mechanism of Action (MOA)

    • Spectrum of activity

    • Time vs concentration dependency

    • Tissue penetration (hydrophilic vs lipophilic)

    • Adverse side effects

    • Special restrictions (where applicable)

PRINCIPLES OF ANTIMICROBIAL SELECTION

  1. Are antibiotics indicated?

  2. What is the known/suspected organism?

  3. What antibiotics are the known/suspected organism susceptible to?

  4. Can therapeutic concentrations be achieved and maintained at the site of the infection?

  5. Work within the limitations of the label (or legal Extralimital Drug Use - ELDU).

ARE ANTIBIOTICS INDICATED?

  • Questions to assess:

    • Is there an infectious agent involved in the disease process?

    • Investigate through diagnostics.

    • Are antimicrobials necessary to clear the infection?

    • Does the animal have a reasonable chance of clearing the infection with antibiotics?

    • Are there any alternative treatments that can be implemented concurrently with antibiotics?

    • Quote: "The best antibiotics are a scalpel and a drainage needle."

WHAT IS THE KNOWN/SUSPECTED ORGANISM?

  • Ideal scenario: Take actions to identify the infectious agent involved.

    • Easy and cost-effective methods:

    • Cytology

    • Gram stain

    • Best approach (though costly):

    • Culture

    • PCR (Polymerase Chain Reaction)

    • Reality: The infectious agent is often unknown.

    • Collect samples for identification at initial evaluation.

    • If treatment must begin before identification:

    • Predict likely organisms based on experience and knowledge.

WHICH ANTIBIOTICS ARE THE BACTERIA SUSCEPTIBLE TO?

  • Ideal scenario: Identify the organism known to be susceptible to certain antibiotics.

    • Qualitative susceptibility testing methods:

    • Kirby-Bauer (Disc diffusion susceptibility test)

    • Quantitative susceptibility testing method:

    • Minimum Inhibitory Concentration (MIC)

  • Reality: Often the antimicrobial susceptibility is unknown.

    • Submit samples for susceptibility testing.

    • In the interim, predict likely susceptibility based on geographical patterns and past cases.

WILL OUR ANTIBIOTICS REACH AND WORK AT THE SITE OF INFECTION?

  • This factor is influenced by the physiology and pathological state of the infection site.

    • Examples:

    • Encapsulated abscess

    • Fibrin accumulation

    • Inflamed or “leaky” central nervous system (CNS)

  • Pharmacokinetics/Pharmacodynamics (PK/PD) optimization is essential to ensure therapeutic concentrations are maintained at the site of infection.

PK/PD OPTIMIZATION

  • Pharmacokinetics: Refers to how the body affects the drug, including:

    • Absorption

    • Distribution

    • Metabolism

    • Elimination

  • Pharmacodynamics: Indicates how the drug acts within the body, encompassing:

    • Mechanism of Action (MOA): bactericidal vs bacteriostatic

    • Concentration-dependent vs time-dependent effects

    • Duration of post-antibiotic effect

    • Toxicity considerations

TIME- VS CONCENTRATION-DEPENDENT KILLING

  • Time-dependent killing:

    • Goal: Maintain plasma concentrations above the MIC for more than 50% of the dosing interval.

  • Concentration-dependent killing:

    • Goal: Achieve peak plasma concentrations that are 8-10 times higher than the MIC.

    • Post-antibiotic effect allows trough concentrations to fall below MIC.

    • Monitor for potential toxicity associated with sustained high trough concentrations.

MECHANISMS OF ANTIMICROBIAL AGENTS

  • Cell Wall Synthesis Inhibitors:

    • Beta Lactams:

    • Penicillins

    • Cephalosporins

    • Carbapenems

    • Monobactams

    • Vancomycin

    • Bacitracin

    • Folate Synthesis Inhibitors:

    • Sulfonamides

    • Trimethoprim

  • Nucleic Acid Synthesis Inhibitors:

    • DNA Gyrase: Quinolones

    • RNA Polymerase: Rifampin

  • Protein Synthesis Inhibitors:

    • 50S subunit:

    • Macrolides

    • Clindamycin

    • Linezolid

    • Chloramphenicol

    • Streptogramins

    • 30S subunit:

    • Tetracyclines

    • Aminoglycosides

COMMON BACTERIA BY GRAM STAINING

  • Gram + Aerobes:

    • Streptococcus

    • Staphylococcus

    • Corynebacterium*

    • Listeria*

    • Enterococcus

    • Trueperella*

  • Gram - Aerobes:

    • E. coli

    • Pasteurella multocida

    • Pseudomonas

    • Manheimia hemolytica

    • Histophilus somni*

    • Enterobacteriaceae

    • Actinobacillus

  • Gram + Anaerobes:

    • Clostridium

    • Peptostreptococcus

    • Actinomyces

  • Gram - Anaerobes:

    • Bacteroides

    • Fusobacterium

    • Porphyromonas

    • Prevotella

  • Atypical:

    • Mycoplasma

    • * = facultative anaerobe

ANTIBIOTICS FOR LIVESTOCK

  • Gram + Aerobes:

    • Beta-lactams

    • Cephalosporins*

    • Macrolides

    • Potentiated sulfas*

    • Tetracyclines

    • (Fluoroquinolones* – for Staph)

    • Rifampin

  • Gram - Aerobes:

    • Beta-lactams

    • Cephalosporins*

    • Fluoroquinolones*

    • Potentiated sulfas*

    • Tetracyclines

    • Florfenicol

    • Aminoglycosides

  • Gram + Anaerobes:

    • Beta-lactams

    • Cephalosporins*

    • Tetracyclines

    • Macrolides

    • Florfenicol

    • Rifampin

    • Metronidazole

  • Gram - Anaerobes:

    • Beta-lactams

    • Cephalosporins*

    • Tetracyclines

    • Macrolides

    • Florfenicol

    • Rifampin

    • Metronidazole

  • Atypical:

    • Tetracyclines (+/-)

    • Macrolides

    • Florfenicol

    • Fluoroquinolones

    • * = limited use in livestock

CONCLUSIONS

  • Thorough diagnostic efforts are critical prior to treatment to optimize outcomes in clinical cases involving antibiotics. The importance of understanding antimicrobial resistance, patient history, and adaptations in treatment strategies is emphasized.

QUESTIONS

  • Open-ended questions discussed throughout the presentation for further insight into approaches to treatment and management of cases involving antibiotics in food animals.