Oxytetracycline (Ophthalmic)
**Oxytetracycline** is a broad-spectrum bacteriostatic antibiotic belonging to the tetracycline class. * **Veterinary Use:** In veterinary ophthalmology, it is most frequently utilized in cats for the treatment of *Chlamydia felis* and *Mycoplasma* conjunctivitis, as well as for nonspecific or symptomatic therapy of undiagnosed conjunctivitis. * **Formulation:** It is commercially available as an ophthalmic ointment in combination with **Polymyxin B**, which extends its spectrum to include many gram-negative organisms (such as *Pseudomonas*). > **Clinical Pearl:** While topical oxytetracycline is effective for managing clinical signs of chlamydial conjunctivitis, it often fails to clear the systemic carrier state. Systemic therapy (e.g., oral doxycycline) is generally preferred for complete clearance of the organism.
Mechanism: Oxytetracycline is a bacteriostatic antibiotic that inhibits bacterial protein synthesis. * It reversibly binds to the **30S ribosomal subunit** of susceptible organisms. * This binding blocks the attachment of aminoacyl-tRNA to the mRNA-ribosome complex → **inhibits the addition of amino acids to the growing peptide chain** → halts bacterial growth and replication. * The addition of Polymyxin B (in commercial ointments) disrupts the bacterial cell membrane of gram-negative bacteria, altering permeability and causing cell death.
Dosing by species
- Chlamydial/Mycoplasma keratoconjunctivitis · Apply 4 times daily · topical ophthalmic · q6h · 3-4 weeks · Dramatic improvement should be noted in 3-4 days, but treatment should continue for 3-4 weeks for Chlamydia to break the reproductive cycle. Better treatment is oral doxycycline 25 mg PO twice daily for three weeks.
- Susceptible infections (when no other agent is suitable) · 10 mg/kg · IM/SC · q24h · Not specified · Only use in cats when no other agent is suitable.
- Susceptible infections · 10 mg/kg · IM/SC · q24h · Not specified
- Susceptible infections · 50 mg/kg loading dose, then 25 mg/kg · PO · q12h · up to 5 days · Give oral dose on an empty stomach.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Known hypersensitivity to tetracyclines or polymyxin B
- Use of concentrated cattle/sheep depot formulations in small animals
- Pregnancy (last 2-3 weeks)
- Neonates (first month of life)
- Use in cats unless absolutely necessary
Adverse effects
- Local irritation
- Hypersensitivity reactions (rare)
- Potential for overgrowth of non-susceptible organisms (e.g., fungi) with prolonged use
- Vomiting
- Diarrhoea
- Depression
- Hepatotoxicity (rare)
- Fever
- Hypotension (following IV administration)
- Anorexia (especially in cats)
- Superinfections (with prolonged use)
- Renal tubular functional changes
- Teeth staining in developing animals
Drug interactions
- Penicillins · Bactericidal action of penicillins may be inhibited by oxytetracycline · moderate
- Antacids (Mg, Ca, Al) · Binds tetracycline, significantly reducing its GI absorption · major
- Food or Milk products · Binds tetracycline, reducing its absorption · major
- Methoxyflurane · May increase the nephrotoxic effects · major
- Theophylline · GI effects of tetracyclines may be increased · moderate
Monitoring
- Resolution of clinical signs (conjunctival hyperemia, chemosis, ocular discharge)
- Signs of local irritation or hypersensitivity
- Renal function (BUN, Creatinine, Urinalysis)
- Hydration status
- Gastrointestinal signs
Overdose
Topical overdose is highly unlikely to cause systemic toxicity. If severe local irritation occurs, flush the eye with sterile saline and discontinue use.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.