Parasitology, toxicity and evermifugation with formalin and histopathology of juveniles of the mullet Mugil platanus

Author: Eduardo Pahor Filho (Currículo Lattes)
Supervisor: Dr Joaber Pereira Junior
Co-supervisor: Dr Kleber Campos Miranda-Filho

Abstract

Considering the importance of the mullet Mugil platanus Günther, 1880 as a source of food and alternative employment for local communities, this study aims to establish the parasitic fauna and the way of use of M. platanus by the parasites, to define the toxicity of formaldehyde and the effective concentration to eradicate or even minimize parasitosis and, finally, to verify which lesions are caused by formaldehyde and/or parasites in juveniles of the M. platanus mullet. Forty juveniles of M. platanus mullet weighing 1 ± 0.26 g and length 4.1 ± 0.4 cm were collected in a stream at Praia do Cassino, in the municipality of Rio Grande - RS - Brazil. The hosts were necropsied and the ectoparasites were fixed in 5% formaldehyde, stained, clarified and later mounted on permanent slides with Canadian balm for identification.Others were mounted on Gray-Wess to check sclerotized structures. Endoparasites were fixed in AFA and stained in Semichon carmine, clarified and then mounted on permanent slides with Canadian balm for identification. Ligophorus cf. uruguayense (Monogenoidea: Ancyrocephalidae) and Solostamenides cf. platyorchis (Monogenoidea: Microcotylidae) in the gills. Digenea and Nematoda in the intestines have been reported. The prevalence (P%) of L. cf. uruguayense was 100%, the infestation intensity (II) varied between 2 and 110, the average infestation intensity (IMI) was 23.4 and the average abundance (AX) was 23.4. The prevalence (P%) of S. cf. platyorchis was 10%, the infestation intensity (II) varied between 1 and 3, the average infestation intensity (IMI) was 1.5 and the average abundance (AX) was 0.15.The prevalence (P%) of Digenea was 92.5%, the intensity of infection (II) varied between 1 and 40, the average intensity of infection (IMI) was 2.67 and the average abundance (AX) was 2.47 . The prevalence (P%) of Nematoda was 6.7%, the intensity of infection (II) was 1, the average intensity of infection (IMI) was 1 and the average abundance (AX) was 0.07. The high rates of Monogenoid infestation and Digenea infection point to the potential damage that these parasites can represent in crops. In order to analyze the toxicity of formaldehyde to M. platanus and the effectiveness of this chemotherapy in controlling parasites, four experiments were carried out. Juveniles were kept in beakers of 1 L, density 8 fish/beaker, water temperature 21 ° C, photoperiod 12 h, water salinity 5,smooth and constant aeration and kept fasting and under observation for survival analysis. To determine the toxicity of formaldehyde to juveniles, a 96 h acute toxicity test was carried out, in which the treatments were a control (without the addition of formaldehyde in water) plus 5 concentrations: T1 (5), T2 (8), T3 (15 ), T4 (30) and T5 (50) mg/L of formaldehyde in triplicate. To check the effectiveness in parasitic control, a prophylactic bath of 1 h of formaldehyde was administered, in which the treatments were a control (without addition of formaldehyde in the water) plus 5 concentrations: T1 (25), T2 (50), T3 (100 ), T4 (150) and T5 (200) mg/L of formaldehyde in water in triplicate. In the lethal acute toxicity test, there was no significant difference (P> 0.05) between the replicates and temperature (° C), salinity, pH, O2 dissolved in water (mg/L),total ammonia (mg/L) and the median lethal concentration at 50% (LC50) in 96 h was estimated at 20.77 mg/L of formaldehyde. In the prophylactic bath with formalin for 1 h, all treatments were effective in eliminating 100% of the parasites, except in the treatment with 25 mg/L, in which L. cf. uruguayan in 13.33% of the fish. There was 100% survival of the hosts in all treatments, except for the 200 mg/L concentration where survival was 87.5%. Baths with 25 mg/L of formaldehyde for 1 h are indicated for the control of S. cf. platyorchis and 50 mg/L for the control of L. cf. uruguayense, for having less formaldehyde concentration and high fish survival. However, none of the tested concentrations is efficient in controlling endoparasites. Lastly,to determine the lesions caused in the gills of formaldehyde and/or by monogenoids, three fish from each treatment, coming from the prophylactic bath with formaldehyde to control parasites were referred for histological analysis. It was observed that the increase in formaldehyde concentration caused more severe lesions in the gills. Mild hyperplasia was observed in the animals kept as control, detachment of the respiratory epithelium at T2, moderate hyperplasia at T3, increased opercular activity at T4 and necrosis, erratic swimming and mortality at T5. The histopathological results of this study show that baths of 50 mg/L of formalin for 1 hour can be applied to juveniles of M. platanus for the control of Monogenoidea, since in this concentration, the histological lesions caused in the hosts are mild and the survival of the juveniles is tall.

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