Introduction
Tilapia aquaculture worldwide is increasingly challenged by bacterial disease outbreaks, with streptococcosis representing one of the most relevant threats to production, especially under intensive farming conditions and elevated water temperatures. Streptococcal infections severely impair growth, feed efficiency, immune competence and survival, finally generating substantial economic losses and raising concerns about the sustainability of tilapia production. Among the nutritional strategies explored to support fish health and reduce disease susceptibility, dietary antimicrobial compounds, including botanical extracts, organic acids, and other bioactive feed additives, have shown potential to modulate gut microbiota, enhance innate immunity, and improve resilience against pathogenic bacteria (Kuebutornye e Abarike 2020; Radwan et al. 2022; de Aguiar et al. 2023; Liliana et al. 2024). This study evaluated the impact of different doses of a microencapsulated phytogenic blend (PHYTO) on Nile tilapia (Oreochromis niloticus) subjected to a cohabitation challenge with Streptococcus agalactiae.
Materials and Methods
A total of 675 Nile tilapia (average weight 16.85 ± 0.2 g) were stocked into 9 tanks (670 L) within a freshwater recirculating aquaculture system and allocated randomly to 3 experimental groups (3 tanks/treatment, 75 fish/tank). Treated groups received dietary supplementation of PHYTO at 0.5 or 1 kg/MT of feed via top-coating (PHYTO 0.5 and PHYTO 1.0, respectively), while the control group (CTR) received unsupplemented feed. Growth phase lasted 28 days, then a 31 days cohabitation challenge was performed by transferring 20 fish from each experimental tank into a challenge unit with 8 fish inoculated with S. agalactiae. Fish were bulk-weighed on day 28 of growth phase and on day 31 post-challenge (dpc) to assess performance parameters while survival was monitored daily. Additionally, 5 fish per tank (n = 15/treatment) were sacrificed to evaluate condition factor (K), hepatosomatic index (HSI) viscerosomatic index (VSI) and intestinal histopathological signs. Data were analyzed using one-way ANOVA with Tukey's post-hoc test or Kruskal–Wallis test for non-parametric data, survival data were analysed using Kaplan-Meier analysis. Significance was set at p ≤ 0.05, tendency was set at 0.05 ≤ p ≤ 0.1 (GraphPad Prism v.11.0.1; GraphPad Software, Inc., Massachusetts, USA).
Results
Statistical analysis of performance did not reveal any significant difference in growth and challenge phase for any treatment. Survival analysis of the challenge phase highlighted a tendency for relative percent survival improvement of PHYTO 1.0 group compared to both PHYTO 0.5 and CTR groups (+29% RPS) with the last mortality event in the treated group recorded 9 days prior to CTR. Regarding body composition all the diets experienced similar K-index in both growth and challenge phase, however regarding VSI and HSI, only PHYTO 1.0 was able to maintain pre-challenge body composition indexes. Other groups instead experienced an increase compared to pre-challenge levels suggesting an ongoing hepatomegaly process during the cohabitation trial although with different degrees of severity (PHYTO 0.5 +20% HSI; CTR +51% HSI). From a histological perspective, treated groups showed numerical decrease in all pathological alterations evaluated after challenge compared to CTR with a significant decrease of erythrocyte extravasation in the intestinal lining and a statistical trend for the mitigation of cellular inflammatory infiltration for all supplemented groups.
Conclusions
In conclusion, dietary supplementation with microencapsulated phytogenic compounds demonstrated potential as nutritional strategy to support Nile tilapia survival, resolve infection in shorter timing and finally mitigate clinical alterations during S. agalactiae challenge. These findings therefore support the integration of this phytogenic additive in tilapia feeds to promote innovative and sustainable intensive aquaculture nutrition practices. Moreover, further studies are necessary to optimize phytogenic dosages, application timing and finally confirm effectiveness against other aquatic pathogenic Streptococcus spp. strains.
References
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Kuebutornye, Felix K. A., e Emmanuel Delwin Abarike. 2020. «The Contribution of Medicinal Plants to Tilapia Aquaculture: A Review». Aquaculture International 28 (3): 965–83. https://doi.org/10.1007/s10499-020-00506-3.
Liliana, Petculescu Ciochina, Gabi Dumitrescu, David McCleery, et al. 2024. «Organic acids mitigate Streptococcus agalactiae virulence in Tilapia fish gut primary cells and in a gut infection model». Irish Veterinary Journal 77 (maggio): 10. https://doi.org/10.1186/s13620-024-00272-1.
Radwan, Mahmoud, Mahmoud Mahrous M. Abbas, Amaal Mohammadein, Jamila S. Al Malki, Said M. A. Elraey, e Mohammad Magdy. 2022. «Growth Performance, Immune Response, Antioxidative Status, and Antiparasitic and Antibacterial Capacity of the Nile Tilapia (Oreochromis Niloticus) After Dietary Supplementation With Bottle Gourd (Lagenaria Siceraria, Molina) Seed Powder». Frontiers in Marine Science 9 (giugno). https://doi.org/10.3389/fmars.2022.901439.