Sulfadimethoxine-Ormetoprim (Romet) in Fish: Dosing and Withdrawal
Sulfadimethoxine-ormetoprim (Romet) is an FDA-approved potentiated sulfonamide given in feed against enteric septicemia of catfish and salmonid furunculosis, with sourced dose and withdrawal.
Overview
Sulfadimethoxine-ormetoprim is a potentiated sulfonamide, a fixed combination of the sulfonamide sulfadimethoxine and the diaminopyrimidine ormetoprim in a 5:1 ratio. In aquaculture it is the active combination in the FDA-approved medicated-feed premix Romet-30 (NADA 125-933), an antibacterial supplied as a premix containing 25% sulfadimethoxine and 5% ormetoprim and given by mouth in feed. The two drugs act on sequential steps of the bacterial folic acid pathway, so the combination is synergistic.
What it treats
Romet is approved to control bacterial infections in channel catfish caused by Edwardsiella ictaluri (enteric septicemia of catfish) and to control furunculosis caused by Aeromonas salmonicida in salmonids. It is an antibacterial and does not treat parasitic, fungal or viral disease.
Administration and dosing
Romet is administered in medicated feed (oral route). The following figures are from the FDA approval and the Merck Veterinary Manual.
| Item | Value | Source |
|---|---|---|
| Route | Oral, in medicated feed (Romet-30) | FDA |
| Dose | 50 mg total drug per kg fish body weight per day | FDA (NADA 125-933) |
| Duration | 5 consecutive days | FDA |
| Withdrawal, catfish | 3 days | Merck Veterinary Manual |
| Withdrawal, salmonids | 42 days | Merck Veterinary Manual |
The dose is calculated on fish body weight and delivered through feed, so an accurate biomass estimate and good feed acceptance matter; sick fish that stop feeding may not take in an effective dose, which is one reason early diagnosis is important.
Resistance and stewardship
As with other antibacterials, overuse or under-dosing of potentiated sulfonamides selects for resistant bacteria, and sulfonamide resistance is well documented. Romet should be reserved for diagnosed bacterial disease, given at the labeled dose for the full course, and combined with good husbandry, biosecurity and vaccination to reduce the need for antibiotics.
Regulatory status and withdrawal
Romet (sulfadimethoxine-ormetoprim) is FDA-approved for the indications above, with a 3-day withdrawal in catfish and a 42-day withdrawal in salmonids; medicated-feed antibiotics in the United States are used under veterinary oversight. Legal status, approved species, dose and withdrawal rules vary by country and apply to food fish; consult a fish-health veterinarian and the current label before use.