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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.
Keywords: Nile tilapia fry, streptococcal infection, sulfadimethoxine-ormetoprim
Introduction
Nile tilapia (Oreochromis niloticus) is a freshwater culture species which has gained in popularity in many countries as supplies for global food demand (Fitzsimmons, 2016). However, massive production leads to intensive farming practices which result in the raising of stocking densities, poor aeration and water quality. These unfavorable conditions induce stress and, consequently, increase susceptibility to diseases or infections (El-Sayed, 2006). Streptococcosis is now considered to be a frequent disease of farmed tilapia in different areas of the world such as Asia (China, Japan, Singapore, Taiwan and Thailand), the Middle East (Bahrain and Israel), North America (Canada, Mexico, the United States, and the Caribbean), and South America (Brazil) (Agnew and Barnes, 2007; Amal and Zamri-Saad, 2011; Ortega et al., 2018). Streptococcus iniae and Streptococcus agalactiae are two significant species that cause high mortality (up to 50-75%) and severe economic loss in tilapia aquaculture (Al-Harbi, 2016). According to previous studies, S. agalactiae is a prime etiological agent of streptococcosis in tilapia farming in Thailand (Wongtavatchai and Maisak, 2008; Suanyuk et...