Introduction
Aquaculture is continuously expanding as a consequence of the demand for healthy and nutritious food, which, along with the global saturation of wild fishery production, has encouraged considerable growth of the sector (Tacon et al., 2025). However, aquaculture is plagued by a plethora of bacterial pathogens, and the rearing of fish at high population densities in aquaculture systems makes them particularly susceptible to disease outbreaks, causing massive mortalities (Behringer et al., 2020). The aquaculture sector has suffered major economic tragedies due to infectious diseases, estimated at around $6 billion annually (Maezono et al., 2025). The effective control of bacterial diseases, including streptococcosis, through the use of antibiotics has become increasingly challenging in aquaculture as a consequence of antimicrobial resistance (Manage, 2018). Therefore, alternative antimicrobial agents, such as phytogenics, are being sought after to reduce reliance on antibiotics (Bhanja et al., 2023). Oregano essential oil (OEO), has demonstrated an unwavering number of benefits as an aquafeed additive, including well-documented efficacy in improving health, disease resistance, and performance of various aquaculture species (Alagawany et al., 2020). Anpario's Orego-Stim Powder (OSP) is a phytogenic feed additive containing 100% natural, single-source OEO which contains over 100 active compounds, with the primary highly-functional compounds being carvacrol, thymol, p-cymene, β-caryophyllene, γ-terpinene, and linalool. These OEO compounds work in synergy to provide a complex mode of action, including antioxidant, anti-inflammatory, immunomodulatory, antimicrobial, and growth-promoting functions (Alagawany et al., 2020), highlighting OSP's potential as an effective nutritional strategy to mitigate disease-related challenges and improve resilience in aquaculture species.
Materials and Methods
This independent feed trial was conducted for 85 days by the State University of Western Paran�� in Brazil. A total of 200 juvenile Nile tilapia (Oreochromis niloticus), with average initial weight of 50 g, were equally and randomly distributed in 20 conical tanks, each with a usable water volume of 500 L. The trial consisted of five dietary groups with four replications being evaluated; one basal control diet, used as a negative control (NC) during the feeding phase and as a positive control (PC) during the 10day Streptococcus agalactiae challenge, and four experimental diets supplemented with either OSP or another phytogenic product at 1 or 3 kg/t of feed. For the purposes of this abstract/presentation, results from three dietary groups are reported, representing a subset of the full dataset.
At the end of the trial, zootechnical performance parameters were analysed, including final length, final weight, weight gain, specific growth rate, feed conversion, hepatosomatic index, feed intake, condition factor, and cumulative mortality. The remaining parameters, including digestibility, carcass composition, intestinal enzyme activity (protease, amylase, and lipase), haematology (white blood cell count, lysozyme activity, and antioxidant enzymes), liver and intestinal histology, and intestinal microbiota are still under analysis.
Preliminary Results
The results in Fig. 1 demonstrate a significant effect of dietary groups on cumulative mortality (p < 0.05). Among the dietary groups, only OSP 1 kg/t showed a significant effect, indicating an approximate reduction of 86.5% in the risk of death compared with the PC group.
Additionally, greater individual resilience was observed in the OSP 1 kg/t group, with a mean time to death (MTD) of 5.0 ± 1.41 days, higher than that observed in the PC group (MTD = 3.75 ± 0.96 days). Thus, the protective efficacy of OSP 1 kg/t is corroborated by the lower specific mortality rate (SMR) observed in the experiment (SMR = 1.82), indicating a slower progression of the pathogen in this group.
On the other hand, the OSP 3 kg/t diet showed a trend towards statistical significance (p = 0.067), with a longer MTD (7.0 ± 2.58 days) and an approximate 60% reduction in mortality relative to the PC.
b b
ab ab
a a
Figure 1. Cumulative mortality percentage of juvenile O. niloticus fed a PC diet or diets supplemented with OSP at 1 or 3 kg/t of feed following a S. agalactiae challenge.
Different superscripts indicate significant differences (p < 0.05).
References
Alagawany, M., Farag, M. R., Salah, A. S., and Mahmoud, M. A. (2020). The role of oregano herb and its derivatives as immunomodulators in fish. Reviews in Aquaculture, 12(4), 2481-2492. https://doi.org/10.1111/raq.12453
Behringer, D. C., Wood, C. L., Krko��ek, M., and Bushek, D. (2020). Disease in fisheries and aquaculture. Marine disease ecology, 183, 183-216. Oxford University Press. https://doi.org/10.1093/oso/9780198821632.001.0001
Bhanja, A., P., and Mandal, B. (2023). Phytobiotics: Response to Aquaculture as Substitute of Antibiotics and other Chemical Additives. South Asian Journal of Experimental Biology, 13(5). https://doi.org/10.38150/sajeb.13(5).p341-355
Maezono, M., Nielsen, R., Buchmann, K., and Nielsen, M. (2025). The current state of knowledge of the economic impact of diseases in global aquaculture. Reviews in Aquaculture, 17(3). https://doi.org/10.1111/raq.70039
Manage, P. M. (2018). Heavy use of antibiotics in aquaculture; emerging human and animal health problems–a review. https://doi.org/10.4038/sljas.v23i1.7543
Tacon, A. G., Metian, M., Parsons, G. J., and Shumway, S. E. (2025). A global aquaculture and aquafeed production update: 2010 to 2023. Reviews in Fisheries Science & Aquaculture, 1-10. https://doi.org/10.1080/23308249.2025.2568830