The use of acetic acid as an antiparasitic in aquaculture: effectiveness and histopathology caused by Mugil liza Valenciennes, 1836
Author: Mario Roberto Castro Meira Filho (Currículo Lattes)
Supervisor: Dr Joaber Pereira Júnior
Co-supervisor: Dr Rogério Tubino Vianna
This thesis studied the antiparasitic activity of glacial acetic acid in metazoan and protozoan ectoparasites from the Mugil liza mullet, the toxicity (LC50-1h) of glacial acetic acid and the histological changes in the liver and gills of juveniles of M. liza, caused by the bath therapeutic this acid. In the first chapter, the LC50-1 h for juveniles of M. liza was estimated at 1402.62 mg L-1. The parasite species studied were the copepods Ergasilus lizae and E. versicolor, and the monogenoids Solostamenides cf. platyorchis and Ligophorus spp. The median effective concentration (EC50-1h) of acetic acid was determined using in vitro toxicity tests for each species of parasite. With the LC50-1 h and EC50-1 h data, therapeutic indexes for each species of parasite in relation to the host were established, which were 16,82 for E. lizae, 15.59 for E. versicolor, 9.69 for S. cf platyorchis and 5.53 for Ligophorus spp. For in vivo evaluation, concentrations equivalent to 0%, 25%, 50% and 75% of the LC50-1 h were used, which corresponded to the concentrations of 0 mg L-1 (control), 350.65 mg L-1, 701 , 31 mg L-1 and 1051.96 mg L-1, respectively. Part of the fish used in the in vivo evaluation were analyzed for parasitology and part for histopathology, whose data are presented in the third chapter. Due to the high mortality of hosts at the highest concentrations, fish surviving exposure to acetic acid at concentrations of 701.31 mg L-1 and 1051.96 mg L-1 were examined only for histopathology. Parasitological data were presented using the parasitic prevalence rates (P%),average infestation intensity (IMI) and average abundance (AM). The AM values were used to determine the effectiveness of the concentration of 350.65 mg L-1 for each parasite rate found, which were 100% for S. cf platyorchis, 91.47% for Ligophorus spp. and 73.57% for E. lizae. In the in vivo evaluation, no specimens of E. versicolor were found. Although E. lizae was the taxon in which glacial acetic acid had the highest therapeutic index, it was also the taxon in which this drug showed the least effectiveness, probably due to the strong adhesion on the gills by its modified antennae, even after the death of the parasite. With the first chapter it was found that acetic acid at a concentration of 350.65 mg L-1 was effective against the metazoan ectoparasites of M. liza.In the second chapter, the effectiveness of acetic acid in controlling ciliate Trichodina sp. and Apiosoma sp. in juveniles of M. liza. For this, the fish were subjected to concentrations of 0, 238, 476, and 715 mg L-1 of acetic acid, which are equivalent to 0, 33.3, 66.6 and 100% of the value of CL01- 1h, obtained from the toxicity test data in the first chapter. The fish after the treatment had two hours of recovery. After this period, the tegument and gills on the right and left sides of the host were scraped. After preparing the slides, the ciliates were counted. It was observed that acetic acid at a concentration of 238 mg L-1 was effective against Apiosoma sp. and at a concentration of 476 mg L-1 it was effective against Trichodina sp.The third chapter consists of the assessment of histological changes in the liver and gills of juveniles of M. liza exposed to glacial acetic acid. Baths were performed for 1 h with acetic acid at concentrations of 0, 350.65, 701.31 and 1051.96 mg L-1. After the bath, the fish were kept in clean water for 24 h for recovery. Due to mortality in the highest concentrations, eight fish from the concentrations of 0 and 350.65 mg L-1, and four from the concentrations of 701.31 and 1051.96 mg L-1 were analyzed. Liver analysis was performed by quantifying the total area of the histological section and the area with each type of lesion. To ensure a uniform sampling of the distribution of pathological changes in the gills, a branchial arch from each host was examined. Each arc was divided into three fields: the first five filaments,the middle five and the last five were examined. For each filament, each type of histological alteration found was counted (presence / absence). In a second analysis, a new sampling was performed. In this evaluation, the central primary lamella in each of the fields was examined to determine the number of secondary lamellae with each type of pathology. With these data, the occurrence of injury was determined for each type of injury. There was a strong positive correlation between the percentage of liver area with focal necrosis and inflammatory infiltrates, with the concentrations used. In all treatments, including the control, lamellar lesions were observed characterized by hyperplasia, fusion of the secondary lamellae, shedding of the lamellar epithelium and loss of the lamellar epithelium.There was no difference in the frequency of hyperplasia, lamellar fusion and shedding of the lamellar epithelium between treatments. However, the use of acetic acid, even in the lowest tested concentration (350.65 mg L-1), caused a significant increase in the percentage of epithelial loss of the secondary lamellae.