Drug Classification I: Antimicrobials, Antiparasitics, and Vaccines

Drug Classification I: Antimicrobials, Antiparasitics, and Vaccines

Veterinary pharmacology is a core competency for both veterinary assistants and credentialed veterinary technicians. Understanding how drugs are grouped, how they work, and when they are used safely protects patients, clients, and the public food supply.

Antimicrobials: Families and Mechanisms

Penicillins (e.g., amoxicillin, ampicillin, penicillin G) inhibit bacterial cell-wall synthesis by binding to penicillin-binding proteins, causing cell lysis. They are bactericidal and work best against gram-positive organisms, though amoxicillin-clavulanate extends coverage to many gram-negatives. Adverse effects include allergic reactions and, rarely, gastrointestinal upset.

Cephalosporins share the same cell-wall mechanism as penicillins and are organized into generations (first through fifth) with progressively broader gram-negative coverage. Cefazolin (first-generation) is commonly used perioperatively; cefpodoxime (third-generation) treats skin and urinary infections in small animals. Cross-reactivity with penicillin allergy is possible but uncommon.

Fluoroquinolones (e.g., enrofloxacin, marbofloxacin) inhibit bacterial DNA gyrase and topoisomerase IV, blocking DNA replication. They are bactericidal with broad spectrum. Important adverse effects include cartilage damage in young, rapidly growing animals — enrofloxacin is avoided in dogs under one year of age — and retinal toxicity in cats at high doses. Fluoroquinolones are critically important antimicrobials; their use should be reserved for culture-confirmed cases.

Tetracyclines (e.g., doxycycline, oxytetracycline) inhibit the 30S ribosomal subunit, blocking protein synthesis (bacteriostatic). They treat Rickettsia, Chlamydia, Leptospira, and Mycoplasma infections. Tetracyclines bind calcium, so they should not be given with dairy products or antacids; they can cause tooth discoloration in young animals and are avoided in pregnant animals.

Macrolides (e.g., azithromycin, tulathromycin, tylosin) bind the 50S ribosomal subunit and inhibit protein synthesis. They are used for respiratory pathogens, Mycoplasma, and, in food animals, bovine respiratory disease (BRD). GI motility effects are noted in some species.

Sulfonamides (e.g., trimethoprim-sulfamethoxazole) inhibit folate synthesis at two sequential steps, producing a synergistic bactericidal effect. They treat urinary tract infections, protozoal infections (e.g., coccidiosis), and some respiratory infections. Adverse effects include keratoconjunctivitis sicca (dry eye) in dogs with prolonged use, as well as bone marrow suppression.

Antifungals: Azoles (e.g., itraconazole, fluconazole) inhibit ergosterol synthesis in fungal cell membranes. Amphotericin B binds ergosterol directly, increasing membrane permeability — it is nephrotoxic and reserved for serious systemic mycoses. Griseofulvin disrupts fungal mitosis and is used for dermatophytosis.

Antiparasitic Agents

Endectocides (e.g., ivermectin, doramectin, selamectin) are macrocyclic lactones that potentiate GABA-mediated nerve transmission in invertebrates, causing paralysis and death of both internal and external parasites. They are safe for most mammals because the mammalian blood-brain barrier excludes them; however, dogs with the MDR1 (ABCB1) gene mutation (common in collies and related breeds) lack this protection and can suffer serious neurotoxicity at doses safe for other dogs. Always check breed before administering.

Coccidiostats (e.g., ponazuril, amprolium) target the intracellular protozoan Eimeria spp. Amprolium is a thiamine analogue that disrupts coccidia metabolism. These drugs are important in poultry, cattle, and small ruminants.

Anthelmintics: Benzimidazoles (e.g., fenbendazole) inhibit tubulin polymerization in parasites. Pyrantel pamoate depolarizes neuromuscular junctions in nematodes. Praziquantel disrupts cestode (tapeworm) tegument. Resistance to anthelmintics is a growing concern, particularly in small ruminants.

Biologics and Vaccines

Core vaccines are recommended for all animals in a species regardless of lifestyle because the diseases are severe, widespread, or zoonotic. Canine core vaccines include distemper, adenovirus, parvovirus, and rabies (DA2PP + rabies). Feline core vaccines include panleukopenia, herpesvirus, calicivirus, and rabies (FVRCP + rabies). Equine core vaccines include Eastern/Western equine encephalomyelitis, tetanus, West Nile virus, and rabies.

Non-core vaccines are given based on individual risk assessment (geographic location, lifestyle, exposure). Examples include Bordetella, Lyme, leptospirosis, and canine influenza for dogs; feline leukemia virus (FeLV) for cats with outdoor exposure.

Modified-live vaccines (MLV) contain attenuated live organisms and produce strong cell-mediated immunity but must be handled carefully — they lose potency if exposed to heat, light, or disinfectants. Killed vaccines are safer in immunocompromised or pregnant animals but typically require adjuvants and boosters.

Antimicrobial Stewardship

Inappropriate antibiotic use — wrong drug, wrong dose, wrong duration — selects for resistant bacteria. Culture and sensitivity (C&S) testing identifies the causative organism and the antibiotics to which it is susceptible, intermediate, or resistant. Always select the narrowest-spectrum effective agent. Critically important antimicrobials (fluoroquinolones, third- and fourth-generation cephalosporins) should be used only when justified by C&S results.

Withdrawal Times and Food Animal Safety

When drugs are used in food-producing animals, residues can enter the human food supply through meat or milk. A withdrawal time is the interval that must pass after the last dose before the animal can be slaughtered for food or its milk can be sold. Extralabel drug use (ELDU) — using a drug in a way not on the label — is legal in the US only under veterinary supervision and may require an extended, estimated withdrawal time. The Food Animal Residue Avoidance Databank (FARAD) provides scientifically derived withdrawal interval recommendations for extralabel situations. Failure to observe withdrawal times is a federal violation and a serious public health risk. Always record drug name, dose, route, date of last treatment, and calculated withdrawal date on a treatment record.