Oxytetracycline in Fish: Uses, Dosing and Withdrawal
Oxytetracycline is a tetracycline antibiotic used against gram-negative bacterial fish diseases such as columnaris, with FDA- and Merck-based dosing and a 21-day withdrawal.
Overview
Oxytetracycline is a broad-spectrum tetracycline antibiotic. It works by interfering with the ability of bacteria to produce essential proteins, which stops bacterial growth and lets the host immune system clear the infection. In aquaculture it is most commonly delivered in medicated feed, and in some settings as an immersion (bath). As a tetracycline it is photosensitive, so medicated feed and stock solutions should be protected from light.
What it treats
Oxytetracycline is used against gram-negative bacterial diseases of fish. The FDA-approved in-feed product Terramycin 200 for Fish is approved to control mortality in catfish and freshwater-reared salmonids due to columnaris disease associated with Flavobacterium columnare, and the Merck Veterinary Manual also lists control of mortality in freshwater-reared salmonids due to coldwater disease associated with Flavobacterium psychrophilum, plus skeletal marking in Pacific salmon. The Merck Veterinary Manual notes oxytetracycline is fed to manage many gram-negative bacterial infections, including columnaris.
Administration and dosing
Medicated feed is the primary route; the Merck Veterinary Manual notes oxytetracycline is heat-labile, so it should not be mixed into hot gelatin when preparing medicated food. Sourced dose figures are summarized below.
| Route / product | Dose | Duration |
|---|---|---|
| In-feed, Terramycin 200 for Fish (FDA) | 3.75 g oxytetracycline per 100 lb fish per day | 10 consecutive days |
| In-feed, aquarium fish (Merck) | 55 to 83 mg/kg body weight per day | 10 days |
| Immersion / bath, aquarium fish (Merck) | 100 to 400 mg/L for 1 hour | Daily for 10 days |
The Merck Veterinary Manual notes that the effectiveness of immersion oxytetracycline is reduced in hard or marine water, because tetracyclines are chelated by polyvalent cations such as calcium and magnesium.
Safety, resistance and water chemistry
Because tetracyclines bind divalent and polyvalent metal cations, hard water can lower the activity of immersion treatments. Oxytetracycline is also light-sensitive and degrades on exposure to light. Oxytetracycline is one of the most heavily used antibiotics in aquaculture, and peer-reviewed work links its overuse to the spread of antibiotic-resistance genes, with consequences for environmental and public health, so it should be used only for diagnosed bacterial disease and at correct doses.
Regulatory status and withdrawal
In the United States, FDA-approved oxytetracycline in-feed uses require a 21-day withdrawal period for harvest-size food fish; for lobster (gaffkemia) the withdrawal period is 30 days. A tolerance of 2 ppm has been established for the sum of tetracycline residues in finfish muscle tissue. Legal status, approved species and withdrawal rules vary by country and apply to food fish; consult a fish-health veterinarian and the current label before use.