Edwardsiellosis, caused by the bacterium Edwardsiella tarda, is one of the most devastating diseases in global aquaculture, leading to millions of dollars in economic losses and high mortality rates in species such as the Nile tilapia (Oreochromis niloticus). Traditionally, the industry has relied on antibiotics, such as oxytetracycline, to control these outbreaks. However, their indiscriminate use has accelerated the emergence of resistant bacteria, posing a serious risk to public health and the sustainability of the sector.
In this context, the search for natural, safe, and effective alternatives is a priority. A recent study published in the Journal of Fish Diseases by researchers from the Federal University of Santa Catarina (UFSC) and the Federal Institute of Santa Catarina evaluates the efficacy of a combination of monoterpenes—thymol and carvacrol—as a treatment in Nile tilapia juveniles experimentally infected with E. tarda, directly comparing its results with oxytetracycline. The findings are encouraging and open a new door for health management in fish farming.
Key conclusions
- A dietary dose of 0.8 g/kg of monoterpenes (thymol and carvacrol) eliminated mortality (0%) in tilapia infected with E. tarda, proving to be as effective as oxytetracycline (3% mortality).
- The monoterpenes functioned as a treatment (applied after infection), not just a preventive measure, by alleviating the clinical signs of Edwardsiellosis.
- The highest dose of monoterpenes significantly improved key immune system parameters, including an increase in the total count of leukocytes and lymphocytes in the fish.
- These phytogenic additives are emerging as a viable, residue-free alternative to replace the use of antibiotics, helping to combat the growing problem of antimicrobial resistance in aquaculture.
What are monoterpenes?
Monoterpenes are natural compounds found in the essential oils of aromatic plants like rosemary, eucalyptus, clove, and cinnamon, possessing antioxidant and antimicrobial properties. According to Danilo Vitor Vilhena Batista, a doctoral candidate in the Aquaculture Graduate Program (PPGAQI) at UFSC and the study’s coordinator, these substances can be compared to medicinal herbs, but in this case, applied to fish. “Just as teas or plant extracts are used to improve human health, monoterpenes in feed can be used to treat and prevent diseases in farmed fish,” Batista explains.
“The overuse of antibiotics in aquaculture generates bacterial resistance, meaning the bacteria ‘learn’ not to respond to the treatment. This represents a health problem not only for the fish but also for humans, as this resistance can spread among microorganisms,” Batista clarifies. “Furthermore, antibiotic residues in the water and fish can affect the environment and food safety. Therefore, finding natural alternatives, like monoterpenes, is essential to ensure a more sustainable and healthy production,” he adds.
How was the efficacy of monoterpenes tested?
The study compared different concentrations of monoterpenes in the feed supplied to control groups and a group treated with antibiotics. The results indicated that higher concentrations of monoterpenes significantly reduced mortality and improved hematological and immunological parameters in the infected tilapia.
To evaluate this phytogenic alternative, the researchers conducted a 22-day experimental challenge. First, they infected 300 Nile tilapia juveniles with the E. tarda strain (ATCC 15947) via a gastric tube. Once the fish showed the first clinical signs of the disease, a 10-day treatment was initiated.
The fish were divided into five groups, each with four replicates:
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- Negative control: Received a commercial feed with no additives.
- Positive control: Were fed a diet medicated with oxytetracycline (OTC).
- Three treatment groups: Were given a feed supplemented with a mixture of monoterpenes (MTs) at different concentrations: 0.4, 0.6, and 0.8 grams per kilogram of feed.
During and after the treatment, cumulative mortality was recorded, and blood and immunological parameters were analyzed to determine the health status of the surviving fish.
Drastic reduction in mortality
The most compelling finding of the study was the ability of monoterpenes to control mortality in a dose-dependent manner. While the control group (without treatment) reached a mortality rate of 53% in 10 days, the group treated with the antibiotic oxytetracycline showed a significantly lower mortality of 3%, confirming its effectiveness.
Notably, the treatments with monoterpenes demonstrated exceptional performance:
- The 0.4 g/kg dose reduced mortality to 13%.
- The 0.6 g/kg dose lowered it further to 8%.
- The highest dose, 0.8 g/kg, achieved 0% mortality, slightly outperforming the oxytetracycline treatment.
It is important to highlight that after the 10-day treatment period ended, no further deaths were recorded in any of the treated groups (neither with OTC nor MTs) during an additional 10-day observation period, indicating that the treatments successfully reversed the clinical signs of the disease.
Impact on the immune system and fish health
Beyond survival, the study analyzed the blood of the fish to understand how monoterpenes influenced their physiological response. The analyses revealed that the 0.8 g/kg dose of MTs led to a significant increase in the total count of leukocytes, as well as lymphocytes, monocytes, eosinophils, and basophils. This increase in defense cells suggests a strong activation and modulation of the immune system, enabling the fish to combat the bacterial infection more effectively.
Furthermore, the capacity of the blood plasma to fight the bacteria was evaluated. The antimicrobial titer, which measures the inhibition of bacterial growth, was significantly higher in all groups treated with monoterpenes compared to the control group and even the oxytetracycline group. This reinforces the idea that these compounds not only act directly against the bacteria but also enhance the fish’s natural defenses.
Although some alterations in red blood cells were observed, such as a decrease in hematocrit and hemoglobin levels in the treated groups compared to the control, the positive immunological results prevail as the main indicator of the treatment’s efficacy.
A Sustainable Alternative for Modern Aquaculture
The results of this study are clear: the mixture of monoterpenes thymol and carvacrol, administered in the feed at a dose of 0.8 g/kg, is a treatment as effective as oxytetracycline for combating Edwardsiellosis in Nile tilapia.
This research is innovative because it focuses on the curative use of phytogenics, not just their preventive application—a less explored but highly potential field. Offering producers a natural tool that can be administered once the disease is detected represents a significant advancement in health management.
Substituting antibiotics with natural products like monoterpenes offers crucial advantages:
- It reduces the risk of antimicrobial resistance.
- It avoids the accumulation of antibiotic residues in fish muscle, water, and sediments, protecting consumer health and the environment.
- It promotes a more sustainable aquaculture aligned with the demands of a market that values “clean” and safe products.
In conclusion, monoterpenes such as thymol and carvacrol are established as a powerful and viable strategy for treating bacterial diseases in aquaculture, offering a solution that protects both fish health and public health.
Contact
Marco Shizuo Owatari
Laboratory of Algae Cultivation – LCA, Aquaculture Department, Federal University of Santa Catarina (UFSC)
Florianópolis, SC, Brazil
Email: owatarimarco@hotmail.com
Reference (open access)
Batista, D. V. V., A. Jatobá, A. V. da Silva, et al. 2025. “Therapeutic Efficacy of Monoterpenes in Nile Tilapia Infected With Edwardsiella tarda: A Phytogenic Alternative to Oxytetracycline.” Journal of Fish Diseases e70032. https://doi.org/10.1111/jfd.70032.
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
