Evaluation of difloxacin for shrimp aquaculture: In vitro minimum inhibitory concentrations, medicated feed palatability, and toxicity to the shrimp penaeus vannamei

Standard in vitro minimum inhibitory concentrations (MICs) were determined for difloxacin and compared with the MICs of several other antimicrobials against a standardized battery of 13 gram-negative bacterial isolates associated with shrimp disease. The palatability and safety (toxicity) of difloxacin to the shrimp Penaeus vannamei were also evaluated during 15 d of medicated feeding at 1 × (100 mg/kg of feed), 2×, and 4× treatment levels to give doses of approximately5, 10, and20 mg difloxacin/kg body weight. A significant reduction (P < 0.05) in difloxacin-medicated feed palatability was noted in the 2× and 4× trials. However, differences were still acceptable, because more than 80% of the feeds were consumed in both treatments relative to the control diet. Shrimp mortality rates increased with difloxacin level from 7% for the control treatment to 20% for the 4× treatment. Differences in percent survival were not significant (P> 0.05)by the Williams test however, analysis of mean survival time indicated that difloxacin significantly (P ≤ 0.05) reduced survival time at the highest dose. Signs of animal stress, characterized by extreme lethargy, were noted in the 4× treatment. An actual therapeutic dose for difloxacin in penaeid shrimp is unknown; however, the 1× treatment (100-mg/kg) was acceptable with respect to both palatability and toxicity, where as 400 mg/kg of feed or more may be unpalatable and toxic to shrimp.