Therapeutic use of sulfadimethoxine-ormetoprim for control of Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus) fry

Abstract

The therapeutic efficacy of sulfadimethoxine-ormetoprim (SDMX-OMP) against Streptococcus agalactiae (S.
agalactiae) infection in tilapia (Oreochromis niloticus) was examined in tilapia fry. Fish (3+0.44 g) underwent 3-hour
immersion challenge with 50% infective dose (2.58 x 106 CFU/ml) of S. agalactiae and were treated at 24 hours post
challenge with daily dosages of 30 and 50 mg SDMX-OMP/kg body weight (BW) feeding medication for 7 days. The
administration of medicated feed increased the survival of the infected tilapia fry. The survival rate of the challenged,
non-medicated group was 51.25+4.79%. The fish which were challenged with S. agalactiae and received the 30 and 50
mg treatment showed significantly improved survival rate at 82.50+2.89% and 92.50+2.89%, respectively. Streptococcal
bacteria were not recovered from the challenged survivors treated with SDMX-OMP (n=140) while the non-treated
survivors showed S. agalactiae isolation 8 out of 41 fish (19.51%). The substantial increase in the number of proper
survivors following the medication suggests that SDMX-OMP given at 50 mg/kg BW for 7 days could effectively
control streptococcal infection in tilapia fry. The application of this study is particularly important for fish hatchery to
control streptococcal infections affecting fish fry when they are premature for vaccination.