Tetracycline
Tetracycline is a prototype, short-acting **tetracycline antibiotic**. While historically used as a broad-spectrum antimicrobial, increasing bacterial resistance has shifted its clinical role. In modern veterinary medicine: * **Small Animal Practice**: Doxycycline is generally preferred for infectious diseases due to better tissue penetration and dosing frequency. However, tetracycline is frequently used in combination with **niacinamide** for its immunomodulatory effects to treat immune-mediated skin conditions (e.g., discoid lupus erythematosus, pemphigus). * **Large Animal Practice**: Oxytetracycline is typically preferred over tetracycline. **Clinical Pearls**: * It remains effective against atypical organisms like *Mycoplasma*, *Rickettsia*, spirochetes (e.g., Lyme disease), and *Chlamydia*. * It possesses significant **anti-inflammatory and immunomodulating properties**, making it valuable in veterinary dermatology.
Mechanism: Tetracycline is a **time-dependent, bacteriostatic antibiotic**. * **Antimicrobial Action**: Reversibly binds to the **30S ribosomal subunit** of susceptible organisms โ prevents binding of aminoacyl transfer-RNA to the ribosome โ inhibits bacterial protein synthesis. It may also bind to the 50S subunit and alter cytoplasmic membrane permeability. * **Immunomodulatory/Anti-inflammatory Action**: Suppresses antibody production and neutrophil chemotaxis. Inhibits key inflammatory mediators including **lipases, collagenases, prostaglandin synthesis**, and the activation of complement component 3.
Dosing by species
- Susceptible infections ยท 11 mg/kg, PO twice daily ยท PO ยท q12h ยท up to 5 days
- Soft tissue infections ยท 20 mg/kg PO q8h ยท PO ยท q8h ยท 21 days
- Hemotropic mycoplasmosis ยท 10-25 mg/kg PO q8-12h ยท PO ยท q8-12h ยท 21 days
- Bacteremia, systemic infections ยท 7 mg/kg IV , IM q12h ยท IV/IM ยท q12h ยท as long as necessary
- Rickettsial diseases ยท 16 mg/kg, PO three times daily ยท PO ยท q8h ยท 21 days
- Susceptible infections ยท 20 mg/kg PO q8-12h ยท PO ยท q8-12h ยท May give with food if GI upset occurs; avoid or reduce dose in animals with renal or severe liver failure; avoid in young, pregnant or breeding animals
- Susceptible infections ยท 22-33 mg/kg PO q8h ยท PO ยท q8h
- Susceptible infections ยท 25 mg/kg PO 2-3 times daily ยท PO ยท q8-12h
- Susceptible infections in calves ยท 11 mg/kg orally ยท PO ยท Not specified
- Susceptible infections in calves ยท 11 mg/kg, PO twice daily ยท PO ยท q12h ยท up to 5 days
Routes of administration
Contraindications
- Hypersensitivity to tetracyclines
- Pregnancy (last half) - retards fetal skeletal development and discolors teeth
- Young, growing animals (unless benefits outweigh risks)
Adverse effects
- Gastrointestinal distress (nausea, vomiting, anorexia, diarrhea)
- Discoloration of developing teeth and bones (yellow/brown/gray)
- Delayed bone growth and healing in young animals
- Superinfections (bacterial or fungal overgrowth)
- Photosensitivity
- Urolith formation (with long-term use in dogs)
- Hepatotoxicity and blood dyscrasias (rare)
- Cats: Colic, fever, hair loss, depression (poorly tolerated)
- Horses: Severe diarrhea (especially if stressed)
- Ruminants: Ruminal microflora depression and stasis (high oral doses)
- Injection site reactions (IM): Local necrosis and yellow staining
Drug interactions
- Atovaquone ยท Tetracyclines may decrease atovaquone levels.
- Beta-lactam or Aminoglycoside Antibiotics ยท Theoretical antagonism of bactericidal activity, though clinical significance is doubtful.
- Digoxin ยท May increase bioavailability of digoxin, potentially causing toxicity that persists for months.
- Divalent or Trivalent Cations (Antacids, Calcium, Iron, Magnesium, Zinc, Bismuth) ยท Chelation occurs, significantly decreasing the absorption of tetracycline. Separate doses by at least 1-2 hours.
- Methoxyflurane ยท Concomitant use can cause fatal nephrotoxicity.
- Sucralfate ยท May impair oral absorption of tetracycline; separate dosing by at least 2 hours.
- Warfarin ยท May depress plasma prothrombin activity; anticoagulant dosage adjustment may be needed.
Monitoring
- Adverse effects (GI signs, injection site reactions)
- Clinical efficacy
- Periodic renal, hepatic, and hematologic evaluations (with long-term use or in susceptible patients)
Overdose
Tetracyclines are generally well tolerated after acute overdoses. * **Oral Overdose**: Most likely associated with GI disturbances (vomiting, anorexia, diarrhea). Monitor fluids and electrolytes if severe emesis/diarrhea occurs. * **Chronic Overdose**: May lead to drug accumulation and nephrotoxicity. * **Ruminants**: High oral doses can cause ruminal microflora depression and ruminoreticular stasis. * **IV Overdose/Rapid Injection**: Rapid IV injection can induce transient collapse and cardiac arrhythmias, presumably due to chelation with intravascular calcium ions. If rapid IV administration is necessary, pre-treatment with intravenous calcium gluconate is recommended by some clinicians.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.