Veterinary Medicine

Antimicrobial Therapy

Principles of antimicrobial therapy Practical usage of antimicrobial drugs Antimicrobial resistance

Principles of Antimicrobial Therapy

Definition

Antimicrobial therapy: The use of drugs to kill or inhibit microorganisms (bacteria, fungi, protozoa).

Antibacterials are the most common in veterinary medicine.

Key Principles:
  • Correct Diagnosis
    • Distinguish between bacterial, viral, parasitic, or non-infectious causes.
    • Avoid unnecessary antibiotic use in viral or non-bacterial conditions.
  • Choice of Drug
    • Consider spectrum of activity (broad vs. narrow).
    • Base on culture and sensitivity testing when possible.
    • Take into account tissue distribution (e.g., aminoglycosides don’t penetrate CNS well).
  • Dosage and Duration
    • Correct dose ensures therapeutic levels without toxicity.
    • Underdosing > treatment failure + resistance.
    • Duration must be adequate to clear infection but not excessive.
  • Host Factors
    • Age, species, pregnancy status, liver/kidney function.
    • Withdrawal times in food animals (residues in milk, meat, eggs).
  • Drug Interactions & Toxicity
    • Example: Macrolides + ionophores (in cattle) result in toxicity.
    • Aminoglycosides → nephrotoxic/ototoxic.
Case Example

A cow with mastitis is treated empirically with intramammary penicillin. Milk culture later shows E. coli resistant to penicillin but sensitive to ceftiofur. Therapy adjusted accordingly → illustrates importance of culture/sensitivity.

Discussion Questions
  • Why is it dangerous to use antibiotics without confirming bacterial infection?
  • How does host species affect choice of antimicrobial drug?


Practical usage of antimicrobial drugs

Routes of Administration

Oral: practical for groups (poultry, pigs), but reduced absorption in ruminants.

Parenteral: IM, IV, SC — ensures therapeutic levels quickly.

Local: intramammary, intrauterine, topical applications.

Common Antimicrobial Classes in Veterinary Medicine:

Beta-lactams (penicillins, cephalosporins): broad use, safe, time-dependent killers.

Aminoglycosides (gentamicin, amikacin): powerful, but nephrotoxic, poor tissue penetration.

Tetracyclines (oxytetracycline, doxycycline): broad spectrum, useful for tick-borne diseases.

Macrolides (tylosin, tulathromycin): respiratory pathogens in cattle/pigs.

Fluoroquinolones (enrofloxacin): reserved for serious infections; banned/restricted in food animals in some regions.

Sulfonamides + Trimethoprim: urinary and respiratory infections.

Factors Affecting Efficacy:

Bacterial load: higher load → harder to eradicate.

Biofilms: reduce antibiotic penetration.

Drug distribution: lipophilic drugs (macrolides, fluoroquinolones) reach lungs well; hydrophilic drugs (aminoglycosides) less so.

Case Example

A poultry flock develops Mycoplasma gallisepticum infection. Treatment with tylosin in water improves flock health. Shows group therapy application.

Discussion Questions
  • Why are fluoroquinolones considered “last-resort” antibiotics?
  • What factors influence the route of administration chosen by a veterinarian?


Antimicrobial resistance

Definition

AMR: The ability of microorganisms to survive and grow despite exposure to drugs that normally kill/inhibit them.

Mechanisms of Resistance:
  • Genetic mutations altering drug targets (e.g., ribosome changes).
  • Enzymatic inactivation (e.g., beta-lactamases).
  • Efflux pumps expel drugs out of cells.
  • Reduced permeability of bacterial cell walls.
Drivers of AMR in Veterinary Medicine:
  • Overuse and misuse (treating viral infections, prolonged treatment).
  • Subtherapeutic doses (growth promoters in feed).
  • Poor compliance with withdrawal times and guidelines.
  • Cross-resistance between human and animal health (One Health issue).
Consequences:
  • Treatment failures.
  • Zoonotic transfer of resistant bacteria (Salmonella, E. coli, MRSA).
  • Public health crisis > fewer effective drugs.
Control & Stewardship:
  • Strict adherence to antimicrobial use guidelines.
  • Use culture and sensitivity whenever possible.
  • Implement biosecurity and vaccination to reduce need for antibiotics.
  • Educate farmers on responsible drug use.
Case Example

A pig farm routinely adds tetracycline to feed for growth promotion. Over time, resistant E. coli emerges, causing diarrhea unresponsive to treatment. Demonstrates selection pressure due to misuse.

Discussion Questions
  • What role does veterinary medicine play in the global fight against AMR?
  • How can farmers and vets work together to reduce unnecessary antimicrobial use?