M.H. Beleau, DVM, MS; A. MacDonald, PhD
The commercial channel catfish industry, as with any food animal production industry, must have an adequate supply of effective, approved drugs and chemicals for use in disease control. Antibacterial agents suitable for administration in feeds are necessary for treating systemic infections. The systemic bacterial disease resulting from infection by Edwardsiella ictaluri, enteric septicemia of catfish (ESC) is the most serious. This pathogen has cost catfish farmers millions of dollars in losses.
Until 1986, Terramycin (Pfizer, Inc.) was the only antibacterial agent approved by the U.S. Food and Drug Administration (FDA) for incorporation into commercially prepared catfish feeds. In May 1986, the potentiated sulfonamide Romet-30 (Hoffmann- LaRoche, Inc.) was approved by the FDA for use in catfish for treating ESC. Romet-30 is a 5:1 combination of sulfadimethoxine (SDM) and the potentiator, ormetoprim (OMP). Prior to FDA approval and a ter the drug was commercially available, several studies were conducted to determine tissue and environmental residues resulting from field use of the compound.
For one study, Romet-30 was fed for the recommended treatment period of 5 days to pond reared channel catfish (x=635 g) held in a 0.4-ha pond. The drug was incorporated into a sinking feed to provide 58.4 mg Romet-30/kg fish per day when fed at 3% b.w. feeding rate. Six sampling stations were designated at specific locations in the pond for collection of water and mud. Fish were collected by rod and reel for tissue samples.
In 3 separate studies a 0.04-ha pond was used with 3 designated sample sites for mud and water. Romet-30 was incorporated into floating feed and administered at 50 mg, 50 mg, and 150 mg/kg fish/day for 5 day treatments. Average fish weights were 266 g, 305 g, and 804 g. In these studies the relative amount of feed in the gut in samples of fish was determined.
The depletion of SDM and OMP from the edible portion of catfish which had been fed Romet-30 medicated feed was very rapid. Following a 5 day treatment at a dose rate of 58.4 mg Romet-30/kg fish, SDM and OMP returned to non-detectable levels (<0.05 mg/1) within 2 days after withdrawal from the drug. When fed at 150 mg Romet-30/kg fish, the depletion pattern was unchanged. There was no indication of persistence of the drug in the pond water or mud nor was there any effect on phytoplankton communities. Visual examination of relative amounts of feed in the intestinal tracts of fish did not indicate differences in consumption of medicated and non-medicated feed.