Laboratory Efficacy of Oxytetracycline for the Control of Streptococcus iniae Infection in Blue Tilapia

Experimental trials were performed to evaluate the efficacy of oxytetracycline in controlling Streptococcus iniae infection in blue tilapia Tilapia aurea. Minimum‐inhibitory‐concentration studies of oxytetracycline against multiple S. iniae isolates indicated a general sensitivity at concentrations of 0.25–0.50 μg/mL. The dose levels tested were 25, 50, 75, and 100 mg active ingredient per kilogram of fish body weight per day. Administration of medicated feed started within 4–5 h after infection by waterborne exposure to S. iniae (after skin scraping) and continued for 14 consecutive days, followed by a posttreatment period of 21 d. The 50‐mg treatment significantly increased the survival of infected fish from 7% in the nonmedicated group to 45%. The 75‐ and 100‐mg treatments had survival rates (85% and 98%, respectively) that were significantly higher than that of the 50‐mg treatment. There was no significant difference between the 75‐ and 100‐mg treatments and the uninfected, nonmedicated treatment (100% survival). Survivors from the 100‐mg treatment were not found to be carriers of the infection (negative bacterial isolation), whereas the bacterium was recovered from 10% of the survivors from the 75‐mg treatment.