The innate antiviral response through the RIG-1-like receptor pathway is oriented towards a key regulatory molecule, mitochondrial antiviral signaling protein (MAVS). ​Localized to the mitochondria, MAVS coordinates the interferons (IFN) production through regulated signaling cascades. ​Protein-protein interaction, post-translational modifications, and mitochondrial dynamics considerably regulate MAVS signaling on the mitochondrial membrane. ​This review gives insight into the inevitable role of MAVS in the innate antiviral immune response of fish.

The mandarin fish ranavirus (MRV), classified within the genus Ranavirus of the Iridoviridae family, is a lethal pathogen to Mandarin fish (Siniperca chuatsi). ​Currently, no approved treatment exists, underscoring the urgent need for therapeutic strategies. ​In this study, we aimed to investigate the antiviral activity of epigoitrin against MRV. ​An in vitro antiviral activity assay revealed that the CC₅₀ of epigoitrin is 43.16 μM, as determined by the CCK-8 assay. ​Epigoitrin effectively reduced the number of MRV copies from 8.63 × 10⁸ copies/mL to 1.82 × 10⁸ copies/mL, exhibiting dose-dependent inhibition of MRV replication. ​The effective concentration (EC₅₀) of epigoitrin was further determined to be 13.72 μM. ​Moreover, epigoitrin diminished the MRV-induced cytopathic effect as observed under a light microscope and suppressed the expression of major caspid protein of MRV (MRV-MCP), as visualized by immunofluorescence assay. ​It was revealed that epigoitrin primarily exerts its antiviral effects during the stage of viral replication. ​Further investigation into the immune response revealed that epigoitrin significantly upregulated the gene expression of Mx1 (5-fold), IFITM (2.5-fold), viperin (10-fold), ISG15 (15-fold), and IRF5 (2-fold). ​This suggests that epigoitrin indirectly unleashes its antiviral function by activating the host's interferon (IFN) immune response. ​In conclusion, our findings indicate that epigoitrin holds great potential as a therapeutic agent against MRV infection by modulating the IFN pathway.

Pathogenic bacteria that cause lactococcal infections in marine aquaculture in Japan are currently classified into three types based on serotype and genetic characterization: Lactococcus garvieae type I, L. formosensis (formerly, L. garvieae type II), and L. garvieae type III. ​Although a slide agglutination test with rabbit antiserum is used to discriminate these three types, its accuracy is insufficient for clinical diagnosis owing to the existence of isolates that show agglutination with more than one diagnostic antiserum. ​Furthermore, a previously developed PCR assay was not able to distinguish L. garvieae type I from L. garvieae type III, a newly emerging pathogen. ​In this study, primers specific to each type were designed and applied in a newly developed multiplex PCR assay to discriminate these three types of lactococcal pathogens isolated from diseased fish. ​Amplification products with the expected sizes of 262, 1,007, and 500 bp could be obtained for L. garvieae type I, L. formosensis, and L. garvieae type III, respectively. ​Amplification products could not be obtained from isolates of Lactococcus spp. from non-marine fish species. ​These results show that the multiplex PCR assay can discriminate these three pathogens that cause lactococcal infections in fish farms.

Aquaculture of the red seabream Pagrus major predominantly utilizes artificial seedlings of high-growth strains. ​Although effective broodstock management is crucial for sustainable production, internal parasitism in brood fish, particularly by the gonadal nematode Philometra madai, is often neglected. ​Despite its common occurrence in adult red seabream gonads, infection status of broodstocks and the effects on the brood fish health remain unexplored. ​This study conducted a 14-month survey on 3- to 4-year-old brood red seabream. ​Worm carcasses were consistently found in both male and female gonads throughout the year, with overall average prevalence of 86.4%. ​In some cases, these carcasses filled the entire gonad, including ovarian cavity and sperm ducts, extending to the area near the cloaca, potentially impeding ovulation and sperm release. ​However, no significant effects on gonadal development were observed and there was no correlation between worm weight and host weight, condition factor, or gonadosomatic index. ​Live P. madai were detected only during the red seabream spawning season, from March to May, with their maturation synchronized with host reproduction. ​This study represents the first survey of P. madai infections in high-growth strain red seabream broodstock.

In coho salmon Oncorhynchus kisutch challenged with piscine orthoreovirus 2 (PRV-2), hematocrit values showed an immediate decline, reaching their lowest point at 40 days post-challenge (dpc), followed by a gradual recovery. ​Erythrocytic inclusion bodies were observed only between 20 and 40 dpc. ​The viral genome copy numbers peaked at 30 dpc. ​These findings suggest that the disease progressed until 40 dpc, after which recovery took place. ​The presence of ELISA antibodies against the virus was detected after 60 dpc, with a significant increase noted at 80 dpc.