Erythromycin in Fish: Bacterial Kidney Disease and Regulatory Status
Erythromycin is a macrolide used against bacterial kidney disease and gram-positive infections in fish, including broodstock injection, but it is not FDA-approved for US food fish.
Overview
Erythromycin is a macrolide antibiotic that is mainly active against gram-positive bacteria. It inhibits bacterial protein synthesis by binding the 50S ribosomal subunit and is generally bacteriostatic. In fish it is used chiefly against bacterial kidney disease and other gram-positive infections, but its regulatory status for food fish is restricted.
What it treats
Erythromycin is used against bacterial kidney disease (BKD) caused by Renibacterium salmoninarum, a slow-growing gram-positive bacterium of salmonids, and against some streptococcal (gram-positive) infections. A notable use is injection of broodstock to reduce vertical transmission of BKD from infected females to their eggs.
Administration and dosing
Erythromycin is delivered in medicated feed for active infections and by injection in broodstock. Sourced figures are summarized below; BKD is difficult to clear because the organism is intracellular and slow-growing, so courses are long.
| Route | Dose | Notes (source) |
|---|---|---|
| In-feed (BKD treatment) | 100 mg/kg fish/day for 21 to 28 days | Peer-reviewed / Merck |
| Broodstock injection (reduce vertical transmission) | about 11 to 25 mg/kg body weight, intraperitoneal, before spawning | Peer-reviewed (chinook salmon; rainbow trout broodstock study) |
In a rainbow trout broodstock study, intraperitoneal injection of about 25 mg/kg given three times before spawning (roughly 21 days apart) reduced Renibacterium salmoninarum to undetectable levels, although vertical transmission to progeny still occurred in some cases, illustrating that injection reduces but does not guarantee elimination of transmission.
Regulatory status
Resistance and stewardship
Reduced susceptibility of Renibacterium salmoninarum to macrolides has been reported, so erythromycin should be used only for diagnosed, appropriate infections, at correct doses and durations, and under veterinary guidance. Because BKD is hard to eliminate, prevention through biosecurity, broodstock screening and avoiding movement of infected stock is central to control, alongside, not instead of, judicious antibiotic use.
Legal status, approved species and any residue or withdrawal requirements vary by country and apply to food fish; consult a fish-health veterinarian and current regulations before use.