Overview and Clinical Importance
Antimicrobial pharmacology is fundamental to veterinary practice and represents a significant portion of Domain 2 on the BCSE examination (28-32 questions total for Pharmacology, Physiology, and Toxicology combined). This study guide covers all major antimicrobial classes including antibacterials, antifungals, antiparasitics, and antivirals.
Understanding mechanisms of action, spectrum of activity, pharmacokinetics, adverse effects, and species-specific considerations is essential for both examination success and clinical competency as an entry-level veterinarian.
Section 1: Beta-Lactam Antibiotics
[Include Image: Figure 1. Beta-lactam ring structure and mechanism of cell wall synthesis inhibition] Source: https://commons.wikimedia.org/wiki/File:Beta-lactam_antibiotics_example_1.svg
Beta-lactams are the most widely used antimicrobials in veterinary medicine. They share a common four-membered beta-lactam ring structure that is essential for their bactericidal activity. This class includes penicillins, cephalosporins, and carbapenems.
Mechanism of Action
Beta-lactams inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), which are transpeptidase enzymes essential for cross-linking peptidoglycan chains. This prevents proper cell wall formation, leading to osmotic instability and bacterial cell lysis. Beta-lactams are bactericidal and exhibit TIME-DEPENDENT killing, meaning efficacy depends on maintaining drug concentrations above the minimum inhibitory concentration (MIC) for extended periods.
MEMORY AID - BETA-LACTAM Mechanism
"PBP = Prevents Building Peptidoglycan" - Beta-lactams bind Penicillin-Binding Proteins to prevent cell wall synthesis.
Penicillins
MEMORY AID - Penicillin Classes
"A POEM for Penicillins": A = Aminopenicillins (Ampicillin, Amoxicillin), P = Potentiated (with clavulanate), O = Oxacillin-type (anti-staphylococcal), E = Extended spectrum (Piperacillin), M = Mother of all = Natural penicillins (Pen G)
Cephalosporins
Cephalosporins are classified by generations, with increasing gram-negative coverage and decreasing gram-positive coverage as generations progress. Third-generation cephalosporins are considered critically important antimicrobials by WHO.
MEMORY AID - Cephalosporin Generations
"Gram-Positive Goes Away as Generations Go up": 1G = Best gram-positive; 3G/4G = Better gram-negative. Think of it as a see-saw: as one goes up, the other goes down.
Carbapenems
Carbapenems (imipenem, meropenem) have the broadest spectrum of any beta-lactam class and are resistant to most beta-lactamases. They are reserved for severe, multi-drug resistant infections and are considered critically important antimicrobials. They require parenteral administration and are typically hospital-based drugs.
Beta-Lactam Resistance Mechanisms
Resistance to beta-lactams occurs through three main mechanisms: (1) Production of beta-lactamases that hydrolyze the beta-lactam ring, (2) Alteration of penicillin-binding proteins (e.g., MRSA/MRSP), and (3) Decreased permeability or increased efflux. Extended-spectrum beta-lactamases (ESBLs) and AmpC enzymes confer resistance to third-generation cephalosporins.
MEMORY AID - Beta-Lactam Resistance
"BEP" for resistance: B = Beta-lactamases (destroy drug), E = Efflux pumps (pump drug out), P = PBP changes (target modification)
Section 2: Aminoglycosides
[Include Image: Figure 2. Aminoglycoside mechanism of action at bacterial ribosome] Source: https://commons.wikimedia.org/wiki/File:Aminoglycoside_mechanism.svg
Aminoglycosides are bactericidal antibiotics that inhibit protein synthesis by binding irreversibly to the 30S ribosomal subunit. They exhibit CONCENTRATION-DEPENDENT killing and have a significant post-antibiotic effect (PAE), meaning bacterial killing continues even after drug concentrations fall below the MIC.
Key Aminoglycosides
Dosing and Toxicity
Once-daily (extended interval) dosing is preferred for aminoglycosides because: (1) Concentration-dependent killing is optimized with high peak concentrations, (2) Post-antibiotic effect allows drug-free intervals, (3) Nephrotoxicity is reduced by allowing drug-free periods for renal tubular regeneration. The main toxicities are NEPHROTOXICITY (renal tubular damage) and OTOTOXICITY (vestibular and cochlear damage). Risk increases with prolonged therapy, concurrent nephrotoxins, and dehydration.
MEMORY AID - Aminoglycoside Toxicity
"Amino-GLYCO-sides are NOT sweet to the kidneys and ears" - Remember NEPHROTOXICITY and OTOTOXICITY. Also: "A Mean Nasty Group" = Aminoglycosides cause Nephro- and Oto-toxicity
Section 3: Fluoroquinolones
[Include Image: Figure 3. Fluoroquinolone inhibition of DNA gyrase and topoisomerase IV] Source: https://commons.wikimedia.org/wiki/File:DNA_gyrase.svg
Fluoroquinolones are synthetic bactericidal antibiotics that inhibit bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, essential enzymes for DNA replication, transcription, and repair. They exhibit CONCENTRATION-DEPENDENT killing with a significant post-antibiotic effect.
Veterinary Fluoroquinolones
Spectrum and Resistance
Fluoroquinolones have excellent gram-negative coverage including Pseudomonas (variable), good gram-positive coverage, and activity against Mycoplasma, Rickettsia, and Chlamydia. They are ineffective against most anaerobes (except pradofloxacin). Resistance develops through mutations in DNA gyrase (GyrA) or topoisomerase IV genes, and through efflux pumps.
MEMORY AID - Fluoroquinolone Toxicities
"CART" for FQ toxicities: C = Cartilage damage (growing animals), A = Arrhythmias (QT prolongation), R = Retinal toxicity (cats with enrofloxacin), T = Tendon problems (rare)
Section 4: Tetracyclines
Tetracyclines are broad-spectrum BACTERIOSTATIC antibiotics that inhibit protein synthesis by binding reversibly to the 30S ribosomal subunit, preventing attachment of aminoacyl-tRNA to the ribosome.
MEMORY AID - Tetracycline Absorption
"DOXY is Different" - Doxycycline absorption is NOT significantly affected by dairy/calcium unlike other tetracyclines. It is also eliminated hepatically (safe in renal disease).
Adverse Effects and Contraindications
Key adverse effects include: GI upset (common), photosensitivity, teeth discoloration in young animals (binds calcium), esophageal strictures in cats (give with water or food), and hepatotoxicity at high doses. Tetracyclines can cause fatal diarrhea in horses and disrupt rumen flora in adult ruminants. They are contraindicated in pregnancy due to fetal bone/teeth effects.
Section 5: Macrolides and Lincosamides
Macrolides and lincosamides share a similar mechanism of action: both are BACTERIOSTATIC agents that inhibit protein synthesis by binding to the 50S ribosomal subunit. They share cross-resistance patterns known as MLSB (Macrolide-Lincosamide-Streptogramin B) resistance.
Macrolides
MEMORY AID - Rhodococcus equi Treatment
"RAM" for R. equi: Rifampin + Azithromycin/clarithromycin (Macrolide). This combination is the mainstay treatment for R. equi pneumonia in foals.
Lincosamides
MEMORY AID - Clindamycin Contraindications
"CHHRG" (sounds like CHIRP) - Clindamycin Harmful to Horses, Rabbits, Guinea pigs (and other hindgut fermenters)
Section 6: Sulfonamides and Potentiated Sulfonamides
[Include Image: Figure 4. Folate synthesis pathway and sites of sulfonamide and trimethoprim inhibition] Source: https://commons.wikimedia.org/wiki/File:Folate_synthesis_pathway.svg
Sulfonamides are synthetic BACTERIOSTATIC antibiotics that inhibit folic acid synthesis by competitively inhibiting dihydropteroate synthase, an enzyme that incorporates para-aminobenzoic acid (PABA) into the folic acid pathway. When combined with trimethoprim or ormetoprim (diaminopyrimidines), which inhibit dihydrofolate reductase, the combination becomes BACTERICIDAL through sequential blockade of the folate pathway.
MEMORY AID - Sulfonamide Mechanism
"PABA Problem" - Sulfonamides compete with PABA. Combined with Trimethoprim = "Two Steps of Folate Blocked = BACTERICIDAL"
Common Potentiated Sulfonamides
Trimethoprim-sulfamethoxazole (TMS) and trimethoprim-sulfadiazine are the most commonly used combinations. The typical ratio is 1:5 (trimethoprim:sulfonamide). These drugs have broad spectrum coverage including many gram-positive and gram-negative bacteria, some protozoa (Coccidia, Toxoplasma), and Nocardia.
Adverse Effects
Important adverse effects include: Keratoconjunctivitis sicca (KCS/dry eye) in dogs (may be irreversible), crystalluria (ensure adequate hydration), bone marrow suppression, hepatotoxicity, immune-mediated reactions (polyarthritis, skin eruptions), and thyroid suppression. Sulfonamides are contraindicated in Doberman Pinschers due to increased risk of immune-mediated reactions.
MEMORY AID - Sulfonamide Side Effects
"SULFA-DKBT" - Sulfonamides cause: D = Dry eye (KCS), K = Kidney crystals, B = Bone marrow suppression, T = Thyroid suppression
Section 7: Antifungal Agents
[Include Image: Figure 5. Antifungal drug targets: cell membrane (azoles, polyenes) and cell wall (echinocandins)] Source: https://commons.wikimedia.org/wiki/File:Antifungal_targets.svg
Antifungal agents target unique components of fungal cells, primarily the cell membrane (ergosterol) and cell wall (beta-glucan). The three major classes used in veterinary medicine are azoles, polyenes, and echinocandins.
Azole Antifungals
Azoles inhibit lanosterol 14-alpha-demethylase (CYP51), a cytochrome P450 enzyme essential for ergosterol synthesis. Ergosterol is the main sterol in fungal cell membranes (analogous to cholesterol in mammalian cells). Azoles are FUNGISTATIC against most organisms.
MEMORY AID - Azole Antifungals
"FIK-VP" = Fluconazole (CNS/Crypto), Itraconazole (Blasto/Histo), Ketoconazole (Cushings), Voriconazole (Aspergillus), Posaconazole (Mucor)
Polyene Antifungals
Polyenes (amphotericin B, nystatin) bind directly to ergosterol in the fungal cell membrane, creating pores that cause leakage of cellular contents. Amphotericin B is FUNGICIDAL with the broadest antifungal spectrum but is limited by significant NEPHROTOXICITY. Lipid formulations reduce nephrotoxicity but are expensive. Nystatin is too toxic for systemic use and is only used topically.
MEMORY AID - Amphotericin B Toxicity
"AmphoBterrible for Kidneys" - Amphotericin B causes significant nephrotoxicity. Pre-hydration with saline helps reduce toxicity.
Echinocandins
Echinocandins (caspofungin, micafungin, anidulafungin) inhibit beta-1,3-D-glucan synthase, preventing synthesis of the fungal cell wall. They are called the "penicillin of antifungals" because they target the cell wall like beta-lactams target bacterial cell walls. They are FUNGICIDAL against Candida and FUNGISTATIC against Aspergillus. Limited use in veterinary medicine due to IV-only administration and cost.
Section 8: Antiparasitic Agents
[Include Image: Figure 6. Mechanisms of action of major antiparasitic drug classes] Source: https://commons.wikimedia.org/wiki/File:Parasite_treatment.svg
Benzimidazoles
Benzimidazoles (fenbendazole, albendazole, mebendazole, oxfendazole) bind to parasite beta-tubulin, preventing microtubule polymerization. This disrupts glucose uptake and energy metabolism, leading to parasite starvation and death. They have broad spectrum activity against nematodes and some cestodes.
Macrocyclic Lactones (Avermectins and Milbemycins)
Macrocyclic lactones (ivermectin, selamectin, moxidectin, milbemycin) potentiate glutamate-gated chloride channels in invertebrate nerve and muscle cells, causing hyperpolarization, paralysis, and death. They are effective against nematodes and arthropods. These channels are not present in mammals, providing a high safety margin (except in animals with MDR1 mutations).
MEMORY AID - MDR1 Mutation Breeds
"White Feet, Don't Treat" (with high-dose ivermectin) - Collie-type breeds with white markings often carry MDR1 mutation. Better yet: "COLLIES" = Can Only tolerate Low Levels of Ivermectin-type Ectoparasiticides Safely
Other Antiparasitics
MEMORY AID - Antiparasitic Drug Classes
"BIG MIP FP" - Benzimidazoles, Isoxazolines, avermectins (Glutamate channels), Macrocyclic lactones, Imidacloprid, Pyrantel, Fipronil, Praziquantel
Section 9: Antiviral Agents
Antiviral drugs are limited in veterinary medicine compared to antibacterials. Most target viral nucleic acid synthesis. Only a few antivirals are licensed for veterinary use, with many others used off-label from human medicine.
Nucleoside Analogues
MEMORY AID - Antiviral for Cats
"FAM is a Friend to Feline Herpes" - FAMciclovir is safe and effective for FHV-1. "ACYclovir is AWFULLY TOXIC in Cats"
Interferons
Feline interferon-omega (Virbagen Omega) is the only interferon licensed for veterinary use (in some countries). It has immunomodulatory and antiviral properties. It is used to reduce mortality in canine parvovirus and to improve clinical signs in cats with FeLV or FIV. Human recombinant interferon-alpha has also been used off-label.