Evaluation of benzocaine and eugenol as anesthetics and antiparasitic agents in a freshwater fish model

Abstract

Anesthetic agents with antiparasitic properties offer dual advantages in aquaculture and experimental research by facilitating fish handling and potentially reducing parasite mean abundance (MA) and prevalence (P). Eugenol and benzocaine are commonly used anesthetics, yet their combined anesthetic and antiparasitic effects remain underexplored, especially in Neotropical species. This study assessed these effects in Cnesterodon decemmaculatus, a sentinel species in ecological and toxicological studies. In November 2023 and February 2024, 251 specimens were collected from a lake in La Plata, Argentina, and exposed to eugenol (10, 20, 30, 40, 50 mg L⁻1) and benzocaine (20, 50, 80, 100, 150 mg L⁻1), with the controls groups. Induction and recovery times were recorded. Parasite indices MA and P were calculated for Echinostomatidae (gills), Ascocotyle sp. (heart, mesentery), Saccocoelioides kirchneri (intestine), and Trichodina sp. (tegument). Echinostomatidae MA was analyzed via log-transformed linear models, while other taxa were assessed with generalized linear models, including sex, total length, standard length, and weight as covariates. Results demonstrated that eugenol 50 mg L⁻1 induced anesthesia more rapidly with increasing concentrations, whereas recovery time remained unaffected. Benzocaine yielded significantly faster recovery at 100 mg L⁻1, with SL significantly influencing induction time. Eugenol at 50 mg L⁻1 significantly decreased Echinostomatidae MA (P = 0.05), while benzocaine significantly reduced Trichodina sp. prevalence (P < 0.001). Parasite MA and P were modulated by sex and morphometric variables. These findings underscore the dual utility of eugenol and benzocaine as anesthetic and antiparasitic agents within experimental protocols targeting fish handling, welfare, and parasite management.