Chlortetracycline

Overview

Chlortetracycline (Aureomycin®) is a tetracycline-class antibiotic used in veterinary medicine primarily in feed or water medications and as a topical ophthalmic preparation. It has broad antimicrobial activity against a variety of Gram-positive and Gram-negative bacteria, as well as atypical organisms such as Mycoplasma, Rickettsia, spirochetes, and Chlamydia species.

In small animal practice, chlortetracycline is most commonly used topically for ophthalmic infections, including conjunctivitis, keratitis, and blepharitis caused by susceptible bacteria. It may also be used in the treatment of feline chlamydial conjunctivitis and mycoplasmal conjunctivitis, although doxycycline is generally considered the preferred systemic treatment for these infections.

Mechanism of Action (MOA): Chlortetracycline acts as a bacteriostatic antibiotic by reversibly binding to the 30S ribosomal subunit of susceptible microorganisms. This action inhibits bacterial protein synthesis and prevents translation of messenger RNA, thereby stopping bacterial growth. Tetracyclines may also alter cytoplasmic membrane permeability in susceptible organisms.

Chlortetracycline shares a similar antimicrobial spectrum and cross-resistance pattern with other tetracyclines such as oxytetracycline and tetracycline. However, resistance among many bacterial species has increased over time, limiting its effectiveness against some organisms, particularly certain strains of Staphylococcus and Gram-negative bacteria.

Indications

Chlortetracycline is used in veterinary medicine primarily for treatment of ocular infections and certain atypical bacterial pathogens. Although resistance among many common bacteria has increased, the drug may still be useful against susceptible organisms such as Mycoplasma, Chlamydia, Rickettsia, and spirochetes.

• Feline chlamydial conjunctivitis: May be used for treatment of conjunctivitis caused by Chlamydia felis, although oral doxycycline is generally considered the treatment of choice.
• Mycoplasmal conjunctivitis: Used as a topical ophthalmic antibiotic for infections associated with Mycoplasma species affecting the conjunctiva.
• Bacterial ophthalmic infections: Authorized for treatment of keratitis, conjunctivitis, and blepharitis caused by susceptible organisms including Staphylococcus, Streptococcus, Proteus, and Pseudomonas species.
• Canine spontaneous chronic corneal epithelial defects: May be used topically for its immunomodulatory properties that may support healing of chronic corneal epithelial defects in dogs.

Dosage (Reference)

Dog

In dogs, chlortetracycline is primarily administered as an ophthalmic ointment for treatment of bacterial eye infections. The medication is applied directly to the affected eye, allowing local antimicrobial activity at the site of infection.

Cat

In cats, chlortetracycline ophthalmic ointment is used similarly to dogs for treatment of bacterial conjunctivitis and other susceptible ocular infections. The dosing regimen is the same as that recommended for dogs.

Warnings & Precautions

Chlortetracycline should be used with caution in certain patient populations due to the potential for developmental effects, organ toxicity, and species-specific sensitivities. Careful patient selection and monitoring are recommended when this drug is administered.

• Hypersensitivity: Contraindicated in animals with known hypersensitivity to chlortetracycline or other tetracycline-class antibiotics.
• Pregnancy risk: Tetracyclines can cross the placenta and may delay fetal skeletal development and cause permanent discoloration of deciduous teeth. Use only when potential benefits outweigh fetal risks, particularly during the latter half of pregnancy.
• Renal and hepatic disease: Use cautiously in patients with renal insufficiency or hepatic impairment. Dose adjustments and careful monitoring of renal and hepatic function may be necessary.
• Concurrent nephrotoxic or hepatotoxic drugs: Avoid or use extreme caution when chlortetracycline is administered with other medications that may impair kidney or liver function.
• Young animals: Use cautiously in growing animals because tetracyclines may cause discoloration of developing teeth and may affect bone growth.
• Species sensitivity: Chlortetracycline should not be used in guinea pigs because it may induce clostridial enterotoxemia.

Drug Interactions

Drug interactions involving chlortetracycline are primarily related to reduced gastrointestinal absorption due to chelation with cations, alterations in activity of other antimicrobial agents, or pharmacologic effects on certain medications. Careful monitoring or dose adjustment may be required when chlortetracycline is used concurrently with the following drugs.

• Beta-lactam or aminoglycoside antibiotics: Because tetracyclines are bacteriostatic, they may interfere with the bactericidal activity of penicillins, cephalosporins, and aminoglycosides, although the clinical significance of this interaction remains somewhat controversial.
• Cholestyramine: May reduce gastrointestinal absorption of chlortetracycline when administered concurrently.
• Digoxin: Tetracyclines may increase the bioavailability of digoxin in some patients, potentially increasing the risk of digoxin toxicity.
• Divalent or trivalent cations (e.g., aluminum, calcium, iron, magnesium, zinc, bismuth, sucralfate): These agents can chelate tetracyclines in the gastrointestinal tract and significantly decrease absorption. Oral doses should be administered at least 1–2 hours before or after products containing these cations.
• Lanthanum: May decrease the absorption of tetracyclines when administered concurrently.
• Retinoid acids (e.g., acitretin, vitamin A): Concurrent use may increase the risk of intracranial hypertension and should be avoided.
• Warfarin: Tetracyclines may reduce plasma prothrombin activity, which can enhance the anticoagulant effects of warfarin and may require dose adjustment.

Side Effects & Overdose

Side Effects

Adverse effects associated with chlortetracycline are generally related to gastrointestinal irritation, effects on developing tissues, and alterations of normal microbial flora. The severity of these reactions may increase with higher doses or prolonged administration.

• Gastrointestinal effects: Nausea, vomiting, anorexia, and diarrhea may occur in small animals receiving tetracycline antibiotics.
• Tooth and bone discoloration: Administration to young animals may cause permanent yellow, brown, or gray discoloration of developing teeth and may affect bone development.
• Delayed bone growth: High doses or prolonged therapy may interfere with normal bone growth and healing.
• Superinfections: Long-term tetracycline therapy may allow overgrowth of nonsusceptible bacteria or fungi.
• Photosensitivity: Some animals may develop sensitivity to sunlight during treatment.
• Hepatic effects: Rarely, tetracyclines may cause hepatotoxicity or blood dyscrasias.
• Species-specific reactions: Cats may poorly tolerate oral tetracyclines and may develop signs such as colic, fever, hair loss, or depression.

Overdose

Acute overdoses of tetracyclines are generally well tolerated; however, excessive dosing or chronic administration may result in gastrointestinal disturbances and potential organ toxicity due to drug accumulation.

• Gastrointestinal toxicity: Vomiting, anorexia, and diarrhea are the most likely signs following oral overdose.
• Renal effects: Chronic excessive dosing may lead to drug accumulation and nephrotoxicity, particularly in patients with preexisting renal impairment.
• Ruminant complications: High oral doses may suppress ruminal microflora and cause ruminoreticular stasis.
• Rapid IV administration: Very rapid intravenous administration of tetracyclines may lead to cardiovascular collapse or cardiac arrhythmias due to chelation with circulating calcium.
• Management: Treatment of overdose is primarily supportive and may include fluid therapy, correction of electrolyte imbalances, and monitoring of renal and hepatic function.

Key Notes

Practical clinical considerations that may help optimize the use of chlortetracycline in veterinary patients:

• Antimicrobial resistance: Many bacterial isolates have developed resistance to older tetracyclines, so culture and susceptibility testing should be considered when possible before selecting this drug.
• Similar activity among tetracyclines: Chlortetracycline shares a very similar antimicrobial spectrum and cross-resistance pattern with oxytetracycline and tetracycline.
• Administration timing with food: Oral tetracyclines are best administered on an empty stomach to improve absorption when possible.
• Dairy and mineral interactions: Milk, dairy products, and mineral supplements containing calcium or iron may reduce absorption when given orally.
• Monitoring during prolonged therapy: In long-term treatments or susceptible patients, periodic evaluation of renal, hepatic, and hematologic parameters may be advisable.
• Food animal considerations: When used in food-producing animals, labeled withdrawal times must be followed to avoid drug residues.