Ichthyobodo (Euglenozoa) and Trichodina (Ciliophora) are important ectoparasites in aquaculture, causing substantial disease burdens in salmonids and flounder. With formalin use increasingly restricted, effective alternatives are required. This study assessed the antiparasitic efficacy and acute toxicity of glutaraldehyde (GA) against I. salmonis and T. truttae on chum salmon Oncorhynchus keta and T. hokkaidoensis on barfin flounder Verasper moseri. Infected fish were treated with GA (28–44 mg/L) for 15, 30, and 60 min, with 93 mg/L formaldehyde (FA) serving as a positive control. Mortality was recorded at 24 h post-initiation (hpi), and parasite counts were quantified microscopically. Acute toxicity tests involved 1-h exposures to GA (chum salmon: 93–3,682 mg/L; flounder: 35–866 mg/L) and FA (chum salmon: 93–2,327 mg/L; flounder: 93–2,270 mg/L), with mortality monitored for 96 h. GA achieved >90.0% clearance of I. salmonis and T. truttae on chum salmon. In flounder, GA eradicated >99.0% of T. hokkaidoensis, with 28 mg/L for 30 min achieving 99.7% clearance without fish mortality. The 1% lethal concentration values of GA (salmon: 63 mg/L; flounder: 102 mg/L) exceeded the treatment concentrations effective for parasite clearance. These findings highlight GA as a promising alternative for managing Ichthyobodo and Trichodina infections in aquaculture.

Red sea bream iridovirus (RSIV) causes substantial economic losses in net-pen mariculture. In this study, we evaluated the inactivation characteristics of RSIV under various physical conditions and disinfectant treatments to inform the implementation of appropriate hygiene management practices on fish farms. More than 90% of virus infectivity declined under the following physical conditions: exposure to natural seawater for 7 days, exposure to sunlight for 1 h, drying for 60 days, heating at 50°C for 30 min, or heating at 60°C for 30 s. These results indicate that appropriate hygiene management using effective disinfectants is important for preventing disease transmission. The present study demonstrated that the following disinfectant treatments achieved complete RSIV inactivation (>99.9%): 40% ethanol for 30 s, 0.5 ppm sodium hypochlorite for 30 s, 0.14 ppm free chlorine in electrolyzed seawater for 60 min, and 0.1% benzalkonium chloride for 30 s. When disinfectant solutions were prepared with seawater containing 10% organic matter (cell culture medium), 100 ppm sodium hypochlorite was required to ensure the rapid and complete inactivation of RSIV. Our findings provide fundamental information for the establishment of effective disinfection protocols to prevent RSIV transmission in semi-open system aquaculture.

Minimum inhibitory concentrations (MICs) of nine antibacterial agents were determined for Mycobacterium paragordonae pathogenic to koi carp Cyprinus carpio. MICs of ampicillin and florfenicol against 10 isolates were in the range of < 0.06–0.25 and < 0.06–0.5 μg/mL, respectively. Ampicillin (20 mg/kg/day) and florfenicol (10 mg/kg/day) were orally administered to koi carp challenged with M. paragordonae for 5 days (FF-5 and ABPC-5) or two 5-day periods separated by a 10-day withdrawal period (FF-10 and ABPC-10). Cumulative survival rates in florfenicol-treated groups were significantly higher than those in the control group, whereas those in the ampicillin-treated groups were not. Oral administration of florfenicol is a promising countermeasure against M. paragordonae in koi carp.

The susceptibility to two genogroup II strains of carp edema virus (CEV) was examined in various koi carp strains via experimental infection. The test groups included common carp, common-carp derived straines (Goshiki and Shusui), and strains distant from common carp (Kohaku, Taisho Sanshoku, Ogon, and Shiroutsuri). Among these, Shusui and Ogon were developed with the introduction of German carp during their breeding process. Fish were infected using the immersion method in rearing water containing the virus solution (final concentration 3.0 × 10⁶ copies/mL) for 1 h at 20°C, and then maintained at 20°C for 14 days. As the results, Kohaku, Taisho Sanshoku and Shiroutsuri exhibited higher cumulative mortality rates, suggesting that susceptibility to CEV tends to be lower in strains with a closer lineage to common carp and in those crossbred with German carp during their development.

Lectins are key components of innate immunity, acting as pattern-recognition molecules and mediators of complement activation and pathogen elimination. In teleosts, two homologous collectins—a mannan-binding lectin (MBL) and a galactose-binding lectin (GalBL)—function in the lectin complement pathway. This study compared their binding specificities toward diverse microbes. Recombinant carbohydrate-recognition domains of MBL and GalBL were expressed and used to generate specific, non-cross-reactive antibodies. MBL exhibited a broader pathogen-binding spectrum, whereas GalBL showed distinct specificity. These results suggest complementary roles of the two collectins, contributing to functional diversification of innate immune responses in teleosts.