Chlortetracycline
Chlortetracycline is a broad-spectrum, first-generation **tetracycline antibiotic**. While it shares a nearly identical spectrum of activity with oxytetracycline and tetracycline, it is primarily utilized in veterinary medicine as a feed or water additive for food-producing animals (cattle, swine, poultry). Key clinical features include: * **Broad-spectrum activity:** Effective against many *Mycoplasma*, spirochetes (including *Borrelia*), *Chlamydia*, and *Rickettsia*. * **Increasing resistance:** Many gram-positive and gram-negative bacteria (e.g., *E. coli*, *Klebsiella*, *Pseudomonas*) have developed resistance over time. * **Exotic animal use:** Frequently used in avian medicine (especially for chlamydiosis) and in certain small mammals, though it is strictly contraindicated in guinea pigs due to fatal enterotoxemia. > **Clinical Pearl:** Because of its formulation as a feed/water additive, it is highly practical for herd or flock treatment, but individual dosing in companion animals is less common compared to newer tetracyclines like doxycycline.
Mechanism: Chlortetracycline is a **bacteriostatic** antibiotic that inhibits bacterial protein synthesis. * **Primary Mechanism:** Reversibly binds to the **30S ribosomal subunit** of susceptible organisms → blocks the binding of aminoacyl transfer-RNA to the mRNA-ribosome complex → halts peptide chain elongation. * **Secondary Mechanism:** May also reversibly bind to **50S ribosomes** and alter cytoplasmic membrane permeability. * **Mammalian effects:** At very high concentrations, it can inhibit protein synthesis in mammalian cells, leading to antianabolic effects.
Dosing by species
- Susceptible infections ยท 25 mg/kg PO q6-8h ยท PO ยท q6-8h
- Bacterial conjunctivitis, keratitis, blepharitis, and spontaneous chronic corneal epithelial defects (SCCEDs) ยท 0.5-2 cm of ointment to affected eye ยท topical ยท q6h ยท 5 days ยท Used for immunomodulatory mechanism in SCCEDs.
- Chlamydiosis (Small birds) ยท 0.05% concentration in food ยท PO ยท Continuous
- Chlamydiosis (Larger psittacines) ยท 1% CTC concentration in food ยท PO ยท Continuous
- Susceptible infections (Ratites) ยท 15-20 mg/kg PO three times daily ยท PO ยท TID
- Susceptible infections (Pigeons) ยท 50 mg/kg PO q6-8h; or 1000-1500 mg/gallon drinking water ยท PO ยท q6-8h or continuous ยท In warm weather mix fresh every 12 hours. Calcium inhibits absorption; withhold grit and layer pellets during treatment.
- Susceptible infections ยท 25 mg/kg PO q6-8h ยท PO ยท q6-8h
- To prevent recurrence of mycoplasma or chlamydial conjunctivitis in large catteries ยท 50 mg per day per cat ยท PO ยท q24h ยท 1 month ยท Administer soluble chlortetracycline powder in food.
- Feline chlamydial conjunctivitis, mycoplasmal conjunctivitis, bacterial keratitis/blepharitis ยท 0.5-2 cm of ointment to affected eye ยท topical ยท q6h ยท 5 days ยท Oral doxycycline is the treatment of choice for feline chlamydial conjunctivitis.
Routes of administration
Contraindications
- Known hypersensitivity to chlortetracycline or other tetracyclines
- Guinea pigs (causes fatal clostridial enterotoxemia)
Adverse effects
- Gastrointestinal distress (nausea, vomiting, anorexia, diarrhea)
- Discoloration of developing teeth and bones (yellow, brown, or gray)
- Delayed bone growth and healing (at high doses)
- Hepatotoxicity
- Increased BUN (antianabolic effect)
- Ruminal microflora depression and stasis (in ruminants)
- Intravascular hemolysis and hemoglobinuria (with rapid IV injection of propylene glycol-based products)
- Colic, fever, and hair loss (in cats)
- Severe diarrhea (in stressed horses)
- Superinfections (bacterial or fungal overgrowth)
- Photosensitivity
Drug interactions
- Beta-lactam Antibiotics ยท Bacteriostatic action of tetracyclines may interfere with the bactericidal activity of penicillins and cephalosporins.
- Aminoglycosides ยท May interfere with bactericidal activity.
- Digoxin ยท May increase the bioavailability of digoxin in a small percentage of patients, potentially leading to toxicity.
- Divalent or Trivalent Cations (Antacids, Calcium, Iron, Magnesium, Zinc, Bismuth) ยท Chelation occurs, significantly decreasing the oral absorption of the tetracycline. Separate administration by at least 1-2 hours.
- Warfarin ยท May depress plasma prothrombin activity; anticoagulant dosage may need adjustment.
Monitoring
- Clinical efficacy (resolution of infection)
- Gastrointestinal signs (vomiting, diarrhea, anorexia)
- Renal function (BUN, creatinine) and hepatic function, especially with prolonged use or in compromised patients
- Fluid and electrolyte status if severe GI distress occurs
Overdose
Tetracyclines are generally well tolerated after acute overdoses. * **Oral Overdose:** Most likely associated with gastrointestinal disturbances (vomiting, anorexia, diarrhea). If severe emesis or diarrhea develops, monitor and replace fluids and electrolytes. * **Chronic Overdose:** May lead to drug accumulation and nephrotoxicity. * **Ruminants:** High oral doses can cause ruminal microflora depression and ruminoreticular stasis. * **Intravenous Overdose:** Rapid IV injection of undiluted propylene glycol-based products can cause intravascular hemolysis and hemoglobinuria. Rapid IV injection can also induce transient collapse and cardiac arrhythmias due to chelation with intravascular calcium ions.
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.