The arylhydrocarbon receptor (AhR) is an important signaling pathway in the immune system of mammals. In addition to its physiological functions, the receptor mediates the immunotoxic actions of a diverse range of environmental contaminants that bind to and activate the AhR, including planar halogenated aromatic hydrocarbons (PHAHs or dioxin-like compounds) and polynuclear aromatic hydrocarbons (PAHs). AhR-binding xenobiotics are immunotoxic not only to mammals but to teleost fish as well. To date, however, it is unknown if the AhR pathway is active in the immune system of fish and thus may act as molecular initiating event in the immunotoxicity of AhR-binding xenobiotics to fish. The present study aims to examine the presence of functional AhR signaling in immune cells of rainbow trout (Oncorhynchus mykiss). Focus is given to the toxicologically relevant AhR2 clade. By means of RT-qPCR and in situ hybdridization, we show that immune cells of rainbow trout express ahr 2α and ahr 2β mRNA; this applies for immune cells isolated from the head kidney and from the peripheral blood. Furthermore, we show that in vivo as well as in vitro exposure to the AhR ligand, benzo(a)pyrene (BaP), causes upregulation of the AhR-regulated gene, cytochrome p4501a, in rainbow trout immune cells, and that this induction is inhibited by co-treatment with an AhR antagonist. Taken together, these findings provide evidence that functional AhR signaling exists in the immune cells of the teleost species, rainbow trout.

Microcotyle sebastis is a gill monogenean ectoparasite that causes serious problems in the mariculture of the Korean rockfish, Sebastes schlegelii. In this study, we isolated the parasite from fish farms along the coasts of Tongyeong, South Korea in 2016, and characterized its infection, morphology and molecular phylogeny. The prevalence of M. sebastis infection during the study period ranged from 46.7% to 96.7%, and the mean intensity was 2.3 to 31.4 ind./fish, indicating that the fish was constantly exposed to parasitic infections throughout the year. Morphological observations under light and scanning electron microscopes of the M. sebastis isolates in this study showed the typical characteristics of the anterior prohaptor and posterior opisthaptor of monogenean parasites. In phylogenetic trees reconstructed using the nuclear 28S ribosomal RNA gene and the mitochondrial cytochrome c oxidase I gene (cox1), they consistently clustered together with their congeneric species, and showed the closest phylogenetic relationships to M. caudata and M. kasago in the cox1 tree.

Japanese eel endothelial cells-infecting virus (JEECV) and Anguillid herpesvirus 1 (AnHV) are major pathogens in farmed eels. JEECV causes eel viral endothelial cell necrosis (VECNE), while AnHV leads to symptoms such as head erythema and gill necrosis. Both viruses cause severe mortality alone or in combination, necessitating rapid and early detection of their presences. In this study, we developed a highly efficient duplex quantitative PCR method (r² = 0.999) using hydrolysis probes for the rapid and simultaneous detections of AnHV and JEECV. This new diagnostic method demonstrated a 1.7-fold higher detection rate for AnHV and a 2.5-fold higher detection rate for JEECV than conventional PCR. Quantitative analysis of water and eel tissue samples from aquaculture facilities revealed that the two viruses could be detected in water 1–3 months prior to mortality, enabling their early identification of infections through water testing alone. Notably, the method reliably detected low viral loads (< 1 copy) in both water and tissue samples, facilitating preclinical detection and proactive disease management. This approach reduces the risk of mass mortality and economic losses in eel farming. This study underscores the critical role of advanced molecular diagnostic technologies in enhancing health management in aquaculture.

A liver fluke, Clonorchis sinensis is a representative fish-borne parasite infecting humans, and sensitive detection in fish hosts or aquatic environments is important for monitoring infection sources in endemic areas. Conventional diagnostic methods based on microscopy or conventional PCR often show limited sensitivity, particularly under low-parasite conditions. In this study, we developed a high-sensitive and species-specific molecular marker and established a real-time PCR (qPCR)-based diagnostic method targeting metacercariae isolated from freshwater fish, representing the transmission stage of C. sinensis. Primers and a hydrolysis probe targeting the mitochondrially encoded cytochrome c oxidase 1 (COI) gene were designed, and all primer combinations produced stable amplifications with single melt curves in C. sinensis-positive samples. Among them, one combination was finally selected as the optimal marker due to its high specificity, including validation against mixed trematode samples to confirm species-specific detection. The qPCR assay showed excellent linearity (R² = 0.998), with a detection limit of 10¹ copies per reaction and a quantification limit of 10² copies per reaction. In addition, the assay successfully detected C. sinensis DNA in environmental water samples spiked with metacercariae, demonstrating its applicability to aquatic samples for environmental surveillance purposes. Compared with conventional PCR, the developed qPCR method in this study exhibited markedly improved sensitivity in fish-derived samples. Overall, this qPCR assay provides a robust diagnostic tool for laboratory analysis and has potential utility for environmental DNA-based monitoring of clonorchiasis risk areas within a One Health framework.

The Japanese eel endothelial cell-infecting virus (JEECV) and Anguilla herpesvirus (AnHV) are major pathogens in farmed eels. JEECV causes eel viral endothelial cell necrosis (VECNE), while AnHV leads to symptoms such as head erythema and gill necrosis, respectively. Both viruses can result in severe mortality, either independently or in combination, necessitating rapid and early detection of their presence. In this study, we developed a highly efficient duplex quantitative PCR method (R² = 0.999) using TaqMan probes for the rapid and simultaneous detection of AnHV and JEECV. This new diagnostic method demonstrated a 1.7-fold higher detection rate for AnHV and a 2.5-fold higher detection rate for JEECV than those of conventional PCR. Quantitative analysis of water and eel tissue samples from aquaculture facilities revealed that viruses could be detected in water 1–3 months prior to mortality, enabling early identification of infections through water testing alone. Moreover, the method reliably detected low viral loads (<1 copy) in both water and tissue samples, facilitating preclinical detection and proactive disease management. This approach reduces the risk of mass mortality and economic losses in eel farming. These findings underscore the critical role of advanced molecular diagnostic technologies in enhancing health management in aquaculture.