Histological changes in Mugil liza mullet gills exposed to formalin therapeutic baths
Mugil liza mullets are valued in the consumer market and have characteristics that enhance them for cultivation. In fish farming, the impacts caused by the emergence of parasitic diseases are important and the use of chemotherapy is necessary to try to reduce the damage caused to fish. Among these, formalin is used to eliminate ectoparasites, such as Monogenoidea. Monogenoids preferentially parasitize gills and, in a culture system, their pathogenicity may be accentuated. The advantages of using formalin to eliminate this group of ectoparasites are still discussed, since its use can cause histological changes in the gills of fish. The objective of this work was to test different concentrations of formalin, effective in eliminating monogenoids in M. liza,to observe the occurrence of histological changes and establish the time necessary for a regeneration of the branchial epithelium. Juvenile mullets were collected and distributed in 15 tanks (3 per treatment) with 12L of water with salinity and controlled parameters and 2 fish per L. After a period of acclimatization, the fish were subjected to baths of 1 hour in duration with different concentrations formalin dissolved in water: T0 = without formalin; T60 = 60mg/L; T90 = 90mg/L; T120 = 120mg/L and T150 = 150mg/L formalin. Two experiments were carried out: experiment I, where samples (n = 5) were taken before the therapeutic bath, 24 hours after the bath and 1 and 2 weeks after the bath (n = 9 per treatment) and experiment II, with samples (n = 5 ) removed before the therapeutic bath and in the 3 and 4 weeks after the treatment (n = 9 per treatment).The gills were fixed in Bouin for 24 hours and then transferred to 70% ethyl alcohol. To prepare histological slides, the samples were embedded with paraffin and 5 μm histological sections were made, stained with hematoxylin and eosin. Fish sampled before bathing and T0 in both experiments showed lesions such as mild hyperplasia, moderate hyperplasia and severe hyperplasia, telangiectasia and detachment of the respiratory epithelium. Monogenoids have also been found. These injuries may be related to the low quality of the water in the collection environment and the presence of ectoparasites. At T60, the fish showed mild and moderate hyperplasia within 24 hours, 1 and 2 weeks after treatment and, after the third week,severe hyperplasia and shedding of the respiratory epithelium was observed, which persisted in the fourth week after treatment. Fish treated with 90mg/L formalin showed mild hyperplasia and detachment of the respiratory epithelium in all samples in experiment I. In experiment II, at T90, moderate hyperplasia, severe hyperplasia, detachment of the respiratory epithelium and the presence of monogenoids were observed. Mild and moderate hyperplasia were the lesions observed in samples from experiment I of T120. After 3 and 4 weeks, moderate and severe hyperplasia, necrosis and monogenoids were observed. The T150 fish showed moderate and severe hyperplasia within 24 hours after bathing with formalin and severe hyperplasia and telangiectasis after one week of treatment. No juvenile of mullet survived in the second week after this treatment.In experiment II, the fish sampled in the third and fourth week after treatment with 150mg/L formalin presented moderate to severe hyperplasia, shedding of the respiratory epithelium, necrosis and the presence of monogenoids. The histological changes found in the treated fish can be related to the use of formalin. Lesions worsened over time and with increased formalin concentration. In addition, the monogenoid re-infestation that occurred in the fish of experiment II, contributed to this fact. As no signs of regeneration of the branchial epithelium were found, further studies with concentrations between 60mg/L and 90mg/L should be tested so that there is the possibility of a second bath and for the integrity and survival of the mullets to be guaranteed.