The climate crisis and growing petroleum demand have put the health of aquatic animals in jeopardy. Fish are sensitive to chemical pollutants in aquatic environments, such as polycyclic aromatic hydrocarbons, dioxins, and dibenzofurans. This study investigated the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and β-naphthoflavone (β-NF) exposure on histopathological and immunohistochemical features and expression patterns of cytochrome P450 1 (CYP1) family genes in black rockfish, Sebastes schlegelii. Histopathological alterations in the liver included congested central vein, sinusoidal dilatation, lymphocyte infiltration, and severe vacuolation within hepatocytes. The most prevalent alterations in TCDD-exposed kidneys were glomerular enlargement, narrowing of tubular lumen, melanomacrophage centers (MMCs), and necrosis. Moreover, CYP1A immunostaining was strong in renal tubules following TCDD exposure. All CYP1 family genes (CYP1A, CYP1B, CYP1C1, and CYP1C2) were significantly increased in the gills, liver, and kidney exposed to TCDD. Similarly, a significant increase of CYP1A mRNA expression in the kidney was observed upon exposure to TCDD (30.9-folds) and β-NF (25.5-folds) compared with that of the control group (p < 0.05). TCDD and β-NF exposure exerted more adverse effects on the kidney than the liver, and TCDD had a greater in vivo toxic effect than β-NF. The combined histopathological, immunohistochemical, and molecular alternations may be helpful for diagnosing chemical contaminant exposure in S. schlegelii.

Interferon regulatory factors (IRFs) play crucial roles in antiviral processes, such as in the transcriptional induction of interferon (IFN) and IFN-stimulated genes (ISGs). In this study, the genes encoding IRF5 and IRF6 were identified in Pagrus major, and their expression in various organs after pathogen infection was analyzed. In the coding sequences of P. major (Pm)IRF5 and PmIRF6, the DNA binding domain, IRF association domain, and viral-activated domain were found to be highly conserved. Phylogenetic analysis revealed that PmIRF5 and PmIRF6 were most closely related to IRF5 and IRF6 of large yellow croakers. The mRNAs for PmIRF5 and PmIRF6 were constitutively expressed in all organs analyzed but were highly expressed in the liver and gills. As a result of an infection with red sea bream iridovirus, a major pathogen of red sea bream, PmIRF5 and PmIRF6 expression was significantly upregulated in the spleen and kidney. On the basis of these results, it can be concluded that IRF5 and IRF6 expression play an influential role in the immune system of red sea bream infected with viruses.

Aeromonas salmonicida is a major pathogen in aquaculture, and its ability to form biofilms contributes significantly to antibiotic resistance and chronic infections. This study investigated the effects of four antibiotics—ampicillin, amoxicillin, oxytetracycline, and doxycycline—at various concentrations on bacterial growth, biofilm formation, and gene expression related to antibiotic resistance and quorum sensing (QS) in two subspecies: A. salmonicida subsp. masoucida (ASM) and A. salmonicida subsp. salmonicida (ASS). Bacterial isolates from Atlantic salmon were identified using 16S rRNA and vapA gene sequencing. Growth inhibition was more pronounced in ASS than ASM under high antibiotic concentrations. Conversely, sub-inhibitory concentrations (sub-MICs) enhanced biofilm formation in both subspecies, particularly in ASM. PCR results showed that tetA and tetE resistance genes were present only in ASM. qRT-PCR analysis revealed that expression of QS-related genes (ahyI and ahyR) was generally downregulated under tetracycline treatment, while litR expression varied across antibiotic conditions and strains. Some isolates showed increased litR expression alongside elevated biofilm formation, suggesting involvement of additional regulatory mechanisms. These results highlight the potential for sub-MIC antibiotic exposure to promote biofilm development and modulate gene expression, emphasizing the need for careful antibiotic use in aquaculture and providing insight into alternative pathogen control strategies.

This study is the first report of parasite and fungal disease separated from domestically reared Atlantic salmon in Korea; the characteristics of the pathogens were identified, and histopathological analysis was conducted. Fungal and parasitic diseases were detected in Atlantic salmon and were isolated as Saprolegnia parasitica and Neoparamoeba perurans based on morphological and genetic analysis. External symptoms observed in Atlantic salmon infected with S. parasitica include fin ulcers and hemorrhage, abdominal hemorrhage, and necrosis of the gills and gill covers. The histopathological analysis results showed necrosis, hemorrhaging, and inflammatory cell infiltration in the abdominal muscles, while only inflammatory cell infiltration was observed in the gill covers. The clinical symptoms observed in Atlantic salmon infected with N. perurans included excessive mucus secretion in the gills, a dense amoebic presence on the gill filaments, respiratory distress, and opening of the mouth and gill covers after death. Through histopathological analysis, we observed lesions in epithelial cells, characterized by the proliferation of epithelial cells and the fusion of secondary lamellae. Numerous lamellae were observed to be attached or fused with each other. To ensure the successful establishment of the Atlantic salmon aquaculture industry in Korea, it is essential to swiftly quarantine infected fish based on the morphological characteristics of S. parasitica and N. perurans revealed in this study, along with the external symptoms of Atlantic salmon infected with these pathogens. Developing disease control strategies based on the findings of this research is imperative.