The increased resistance of fish pathogens to conventional treatments has lead researchers to investigate the antibacterial properties of natural resources, such as essential oils (EOs) of plants, in an effort to find products that are less harmful to the environment. The objective of this review is to provide an overview of the studies, in vivo and in vitro, that addressed the use of EOs and their major compounds as antimicrobial agents in fish, to identify the best EOs and compounds to investigate considering feasibility of application and suggest possible future studies. To date, studies suggest that the use of EOs in the prevention and/or treatment of infectious diseases in fish may be a promising strategy to reduce the use of conventional antibiotics in aquaculture, since several EOs effectively reduce or avoid the effects of bacterial infections in fish. The use of EOs through nanotechnology delivery systems, especially in dietary supplementation experiments, is promising. This form of application of the EOs allows a potentiation and targeting of the desired effect of the EOs and also allows the protection of EOs active constituents against enzymatic hydrolysis, deserving further study.

This study evaluated the anesthetic potential of thymol and carvacrol, and their influence on acetylcholinesterase (AChE) activity in the muscle and brain of silver catfish (Rhamdia quelen). The AChE activity of S-(+)-linalool was also evaluated. We subsequently assessed the effects of thymol and S-(+)-linalool on the GABAergic system. Fish were exposed to thymol and carvacrol (25, 50, 75, and 100 mg/L) to evaluate time for anesthesia and recovery. Both compounds induced sedation at 25 mg/L and anesthesia with 50–100 mg/L. However, fish exposed to carvacrol presented strong muscle contractions and mortality. AChE activity was increased in the brain of fish at 50 mg/L carvacrol and 100 mg/L thymol, and decreased in the muscle at 100 mg/L carvacrol. S-(+)-linalool did not alter AChE activity. Anesthesia with thymol was reversed by exposure to picrotoxin (GABAA antagonist), similar to the positive control propofol, but was not reversed by flumazenil (antagonist of benzodiazepine binding site), as observed for the positive control diazepam. Picrotoxin did not reverse the effect of S-(+)-linalool. Thymol exposure at 50 mg/L is more suitable than carvacrol for anesthesia in silver catfish, because this concentration did not cause any mortality or interference with AChE activity. Thymol interacted with GABAA receptors, but not with the GABAA/benzodiazepine site. In contrast, S-(+)-linalool did not act in GABAA receptors in silver catfish.

This study evaluated anesthetic efficacy and possible effects of the essential oils (EOs) of Cunila galioides (EOC) and Origanum majorana (EOO) on ventilatory rate (VR) and ionoregulation in Rhamdia quelen. In the anesthesia assessments, 50, 100, 200 and 300 μL L -1 EOC and 50, 100, 200, 300, 400 and 500 μL L -1 EOO were tested, and time for induction to sedation and anesthesia stages, as well as recovery, were taken. A second trial employed lower concentrations of both EOs, 10, 25, 50 and 100 μL L . There was no significant difference between control and EO-treated groups regarding VR, but all fish subjected to 100 µL L -1 EOC died within 2 h of exposure. Overall, ionic loss declined in the presence of the EOs. The EOC at 200 -300 μL L -1 and EOO at 400 -500 μL L -1 present the potential to promote fast anesthesia in R. quelen.

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This study evaluated the activity of leaf essential oil (EO) from Hyptis mutabilis as well as its major constituent, (-)-globulol, in infections by the parasite Ichthyophthirius multifiliis (ich). Effects on hematological, biochemical, and immunological parameters of silver catfish, Rhamdia quelen, exposed to the same samples also were evaluated. In the first experiment, naturally infected fish were treated with EO (0, 10, and 20 mg · L) and ethanol, using several methods of exposure. Fish mortality and the number of trophonts per fish were assessed after 48 and 96 hr. Hour-long daily baths resulted in optimal survival, so this methodology was used for the second experiment, in which infected animals were exposed to (-)-globulol at 2.5 and 5 mg · L. The most effective concentrations in Experiments 1 and 2 were chosen for Experiment 3, in which healthy animals were subjected to hour-long daily baths with EO (20 mg · L) or (-)-globulol (2.5 mg · L). Additionally, an in vitro experiment was performed with EO and globulol at the same concentrations of the in vivo test. EO and (-)-globulol increased the survival of fish infected with ich and altered certain hematological and biochemical parameters. After 4 days, levels of hematocrit, erythrocytes, and leukocytes increased significantly in healthy animals exposed to EO. Exposure to (-)-globulol increased leukocyte number alone. No significant differences in nonspecific immunological parameters were detected when treated groups were compared to controls, but the leukocytosis observed in EO- and globulol-treated healthy animals indicates that EO and (-)-globulol increased innate immunity in these fish. An in vitro antiparasitic effect was observed in both samples.

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