In this study, the results of evaluating the acute toxicity of Bisphenol A on Danio rerio are presented, encompassing peripheral blood parameters, the composition of hematopoietic cells of erythroid and myeloid lines in the head kidney, and data from histological studies. The LC50 values of Bisphenol A for adult zebrafish individuals for 12, 24, and 48–96 h were determined, which were 18.04, 7.55, and 6.22 mg/L, respectively. The study includes data on the morphology and quantitative frequency of specific cells in the hematopoietic tissue of the head kidney, along with the consideration of adaptive mechanisms in hematopoiesis under BPA exposure. The application of polynomial regression analysis to reveal the concentration–effect relationship for some hematological and histological parameters was demonstrated. Significant increases in the frequency of erythrocyte nuclear abnormalities were observed at BPA concentrations of 6 and 8 mg/L, which indicates a genotoxic effect. BPA’s impact on fish peripheral blood parameters manifested as an increase in the number of erythrocytes (RBC) and immature erythrocytes, as well as a decrease in the number of lymphocytes. The most notable pathological changes in the head kidney’s hematopoietic tissue included circulatory disturbances and the formation of inflammation/degradation foci, as confirmed by histopathologic indices. At BPA concentrations of 2 and 4 mg/L, the observed changes were compensated for by hematopoietic adaptation mechanisms; however, at concentrations of 6 and 8 mg/L, acute systemic toxicity was evident.

Furunculosis caused by Aeromonas salmonicida is one of the most common diseases in aquaculture, leading to significant economic losses. This study comprehensively investigated the dynamics of pathophysiological and histopathological disorders in rainbow trout (Oncorhynchus mykiss) infected with the moderately virulent strain A. salmonicida SL0n. Whole-genome analysis showed that strain SL0n belongs to the A. salmonicida species complex, possessing a single circular chromosome. The genome encodes a wide range of virulence factors, including adhesion systems (type IV pili, fimbriae), toxins (aerolysin, hemolysins), and a type II secretion system (T2SS), but notably lacks plasmids and a type III secretion system (T3SS). This genomic profile likely dictates a pathogenic mechanism reliant on secreted exotoxins (via T2SS), explaining the observed systemic cytotoxic damage. In an acute experiment, the 4-day LD₅₀ was determined to be 1.63 × 10⁶ CFU/fish. In a prolonged experiment, fish were injected with a sublethal dose (1.22 × 10⁶ CFU/fish—75% of LD50). The disease progressed through three consecutive stages. The early stage (1–2 DPI) was characterized by maximal bacterial load and activation of nonspecific immunity. The acute stage (4 DPI) manifested as severe septicemia and anemia, associated with systemic organ damage, which correlated with peak AST and ALT enzyme activity. The recovery stage (6 DPI) was marked by partial regression of inflammation, key biochemical and histological parameters indicated persistent liver and kidney dysfunction, signifying an incomplete recovery. These results demonstrate the pathogenesis of acute furunculosis and reveal that the genomic profile of the SL0n strain causes a sequential, systemic infection characterized by severe organ dysfunction.

Total histological sections of laboratory fish Poecilia reticulata and Oryzias latipes with pronounced multiple organ failure caused by Mycobacterium infection (Mycobacterium sp.) were analyzed. The pathogenesis of the disease in question is expressed in the occurrence, in various tissues, of foci of inflammation, subsequently forming a connective tissue capsule — granuloma. These lesions were observed in almost all organs of the abdominal cavity of the fish. The largest and most common lesions were found in the pancreas, kidneys and liver (from 478.6 microns to 858.4 microns), significantly disrupting organ functions, which is a clinical picture characteristic of mycobacteriosis. The occurrence of granulomas and their average sizes in organs such as the intestine and gills were significantly lower and amounted to 328.3 and 468.9 microns, respectively. Data on the stages of granuloma development in different organs of fish are presented. The initial stage of granuloma development consists of clusters of macrophage-like cells, lymphocytes, granulocytes and a small number of cellular residues. At the final stage of development, it is a focus, including an eosinophilic-stained caseous-necrotic nucleus and a shell consisting of epithelioid macrophages. It is shown that there is a relationship between the size parameters of granulomas, the frequency of their occurrence and the degree of organ damage. The localization of granulomas and the degree of their development in guppies and medaka were similar, indicating the absence of species-specific virulence. It follows from this that any warm-water fish species cultivated under controlled conditions are at risk. This requires monitoring for signs of disease and regular preventive measures.

Морфологическая и функциональная характеристика лимфоидной ткани среднего отдела кишечника осетровых видов рыб на примере стерляди ( Acipenser ruthenus )

Describes cases of spontaneous occurrence of testicular neoplasms in four male koi carp (Cyprinus carpio var. koi). Macroscopic and microscopic examination of the tumours and adjacent tissues (liver and spleen) was performed. The size of the neoplasms varied from 8 to 12 cm with a weight of 30– 45 g. Two histological types of neoplastic structure were observed. The first type was characterised by a multilobular structure with dense and moderate cellular areas, pronounced pleomorphism of the cells and the presence of areas of necrosis. The second type was characterised by the presence of parallel strands of pleomorphic cells with large multifocal foci of necrosis and a thick connective tissue capsule. The average size of the nuclei in the first type of tumour was 6.9×5.9 μm (length × width), in the second type 6.8×5.9. In addition, in the tissue of the second type of tumour, clusters of basophilic cells with pronounced karyomegaly were detected, forming multifocal foci in the form of curved strands. In all cases, micro- and macroscopic examination showed no evidence of metastatic development. Based on the results obtained, the neoplasms studied can be classified as germ cell tumours with a predominance of seminoma (gonadoblastoma). No differences in the histogenesis or developmental stages of the studied tumours were observed. Histopathological changes were found in the liver (vacuolization of hepatocytes, fibrosis) and spleen (disruption of the pulp structure, adenomatous formations), probably related to tumour progression. Among the possible aetiological factors of carcinogenesis, the following have been identified: genetic predisposition, housing conditions and the use of gonadotropic drugs.