The aim of this study was to evaluate the effect of dietary supplementation with β-glucan on the survival of Nile tilapia challenged with Francisella orientalis (Fo), a pathogen of great importance for Brazilian and world aquaculture. Considering the immunomodulatory properties of β-glucan, Nile tilapia juveniles were supplemented with β-glucan and subsequently submitted to bacterial challenge with Fo. Changes in behavior, clinical signs and daily mortality were evaluated for 15 days after infection.
Tilapia is the main species of fish produced in Brazil; in 2021, production was 534 thousand tons, equivalent to 63.5% of the total Brazilian aquaculture (PEIXE BR, 2022). Francisella orientalis (Fo) is a pathogen of tropical fish that causes francisellosis (SOTO et al., 2009). No commercial vaccine is available against francisellosis in Brazil. Thus, one of the ways to prevent outbreaks of francisellosis in the country is using additives in the diet to increase the immune system of the fish (RAGHIANTE et al., 2017).
240 Nile tilapia juveniles (91.41 ± 7.6 g) were distributed in twelve tanks (250 L, 20 fish per tank) with aeration and continuous water flow. During the experimental period, water temperature, pH, dissolved oxygen were monitored and kept within than recommended for the comfort and well-being of the tilapia. The IA2203 strain (Francisella orientalis) was obtained from the strain collection of the Laboratório de Sanidade Aquícola (PREVET, Jaboticabal/SP.), which was isolated from Nile tilapia produced in the region of Santa Fe do Sul, SP. For the bacterial growth, it was seeded on agar plates enriched with cysteine ??at 24°C for 72 h. After this period, typical colonies were transferred to BHI broth and incubated at the same temperature.
A randomized design was used, with two supplementation levels (1 g kg-1 (T1, 0.1%) and 3 g kg-1 (T2, 0.3%)) of β-glucan (Macrogard® Lot QT19120, > 60%, Biorigin, Brazil) and a control diet (no β-glucan added) and four replications. After 15 days of acclimatization, fish began to be fed with the experimental diets (1.5% of live weight) split twice a day, for a period of 30 days. Two tanks containing fish from the control group (C) and four tanks from groups T1 and T2 were inoculated intracelomically with 0.1 mL of Fo bacterial suspension at a pre-established dose of LD50% (1.95 x 109 CFU.mL-1). The other two tanks of the control group were inoculated with 0.1 mL of PBS, denominated as C0. After bacterial inoculation, changes in behavior, clinical signs and mortality were recorded daily for 15 days to determine the survival rate and the cumulative mortality of each evaluated treatment. This study demonstrated that dietary supplementation with 3 g kg-1 β-glucan provided greater survival in fish after challenge with Francisella orientalis under the experimental conditions tested.