Toxicity and parasiticidal in vivo and in vitro evaluation of acetic acid on metazoan ectoparasites in juvenile Mugil liza

In this study, the anti-parasitic activity of acetic acid was evaluated in vivo and in vitro in juveniles of Mugil liza. The parasites studied were the copepods Ergasilus lizae and E. versicolor and the monogenoideans Solostamenides cf. platyorchis and Ligophorus spp. To determine concentrations for use in the in vivo evaluation, and to determine the therapeutic index, a toxicity test (LC50-1h) was carried out using juveniles of M. liza. The LC50-1h was estimated at 1402.62 mg L-1. Glacial acetic acid concentrations were determined for in vivo evaluation from the values of 0%, 25%, 50% and 75% of LC50-1h, which corresponded to concentrations of 0 mg L-1 (control), 350.65 mg L-1, 701.31 mg L-1 and 1051.96 mg L-1, respectively. Due to high mortality, fish exposed to 701.31 mg L-1 and 1051.96 mg L-1 concentrations were not examined in the in vivo evaluation. The results of the in vivo evaluation were presented in parasitic indexes of prevalence (P%), mean infestation intensity (MII) and mean abundance (MA). The MA values were used to determine the efficacy of the 350.65 mg L-1 concentration for each parasite found, which was 100% for S. cf. platyorchis, 91.47% for Ligophorus spp. and 73.57% for E. lizae. No specimens of E. versicolor were found in the in vivo evaluation. The in vitro evaluation comprised toxicity tests (EC50-1h) for each parasite taxa. The EC50-1h values were 83.38 mg L-1 for E. lizae, 89.97 mg L-1 for E. versicolor, 144.79 mg L-1 for S. cf. platyorchis and 289.58 mg L-1 for Ligophorus spp. With the values of LC50-1h of M. liza and EC50-1h values of each parasite taxa, the therapeutic indices were calculated, ranging from 16.82 for E. lizae, 15.59 for E. versicolor, 9.69 for S. cf. platyorchis and 5.53 for Ligophorus spp. It is possible that the presence of parasites affected the survival of the fish during in vivo evaluation. Although E. lizae presented the highest therapeutic index, it was the taxon that presented the lowest efficacy, probably due to the strong adhesion in the gills by its modified antennas, even after the death of the parasite. It has been found that glacial acetic acid at a concentration of 350.65 mg L-1 is an effective drug against M. liza parasites.