The efficacy and dosage of Mentha piperita essential oil in the control of Monogenean parasites in Oreochromis niloticus

This study determined the safe dosage of Mentha piperita essential oil in Oreochromis niloticus and tested its effectiveness against gill monogeneans. In the acute toxicity trial in fingerlings, seven oil concentrations (20, 25, 30, 35, 40, 45 and 50 mg/L) were evaluated with two controls, one positive (water) and one negative control (water with addition of 35 mL/L of absolute ethyl alcohol—99.8% PA). It was tested the lethal concentration for 48 h (LC50-48 h), and during this period we recorded the behavior and mortality of individuals. After the acute toxicity trial, histological analyzes were performed on the gills of specimens submitted to acute toxicity concentrations and controls. To test the efficacy of the essential oil against the parasite in the live host, it was performed an in vivo trial with 27 naturally infected juvenile Nile Tilapia. A three-replicate treatment was carried out, where the fish were submitted to 60-min baths with essential oil of M. piperita at a concentration of 35 mg/L (value determined by the acute toxicity trial), and two controls, one positive and a negative. The LC50 was determined using the Trimmed Spearman–Karber program with a 95% confidence interval. No behavioral changes were observed in the controls or in the 20 mg/L concentration, but it occurred in the other concentrations. Among the 270 individuals, 20 presented depigmentation. We found the following histological changes in the fingerlings gills: lamellar epithelium removal, partial lamellar fusion, generalized epithelial tissue proliferation, total lamellar fusion. In the in vivo trial, after 1 h of therapeutic bath at the concentration of 35 mg/L, no efficacy of the essential oil of M. piperita was verified against the monogeneans. According to this study, M. piperita essential oil demonstrated toxic to O. niloticus fingerlings at concentrations of 40, 45 and 50 mg/L, causing histological changes, changes in the color of the animal and death, and ineffectiveness against the monogeneans parasites after therapeutic bath in the highest concentration of essential oil tolerated by the studied fish specimens.