Edwardsiellosis in the red seabream Pagrus major is one of the most significant bacterial infections in Japanese marine aquaculture. In this study, we investigated whether fish carcasses harboring Edwardsiella anguillarum (the causative agent of edwardsiellosis) serve as a source of infection using three experimental infection trials. In Experiment 1, healthy red seabream exhibited scavenging behavior on infected carcasses, resulting in increased E. anguillarum concentrations (viable counts and environmental DNA) in the rearing water and the bacterial detection in the intestines. This bacterium was subsequently isolated from the trunk kidney of fish in the same tank, and the fish developed edwardsiellosis. In Experiment 2, fish were infected with E. anguillarum through cohabitation with infected carcasses. Higher infection ratios were observed in the group with scavenging access to the carcasses than in the group without. In Experiment 3, oral administration of either an E. anguillarum suspension or a kidney homogenate from diseased fish successfully induced infection. In conclusion, infected fish carcasses may serve as a reservoir of E. anguillarum, with scavenging behavior playing a critical role in edwardsiellosis transmission in red seabream farms by contributing to both the release of the bacterium into surrounding water and the oral consumption of infected tissues.

Philometroides seriolae infection was monitored monthly over a 16-mo period in a single population of wild Japanese amberjack Seriola quinqueradiata, estimated to be 2–5 years old that maintained in a sea cage where no new infection could occur. The overall prevalence and mean intensity of infection was 65.0% (78/120) and 6.2 ± 5.5 worms per infected fish (1–26), respectively. Female nematodes, ranging from 2.0 to 41.3 cm in body length, were unevenly distributed within the skeletal muscle, with a higher proportion located in the dorsal (62.7%) and posterior (45.4%) regions. Some mature worms exited the host, while others remained in the muscle. Parasite remnants including dead worms, scars and empty cavities where nematode had resided, were observed in most of the fish examined. Live nematodes collected in July and August 2020 consisted of two size classes. In the following months, the larger worms disappeared, while smaller ones continued to grow, reaching lengths of up to 30 cm by June of the following year. These findings suggest that P. seriolae remains in Japanese amberjack for at least 1.5 year and that infection likely occurs through ingestion of small paratenic fish hosts.

We optimized a method for isolating Ichthyobacterium seriolicida, which is difficult to isolate even from freshly dead diseased fish. The bacterium was isolated from the blood of diseased yellowtail Seriola quinqueradiata up to 3 h after death, but not from the spleen, even when sampled immediately after death. The bacterium remained culturable from blood samples collected in medium-filled syringes and stored at 8°C for at least 72 h. Therefore, collecting blood within 3 h after death using a medium-filled syringe and storing it at low temperature is considered effective for preserving the viability of I. seriolicida.

One argulid specimen was collected from the caudal fin of a juvenile Japanese amberjack Seriola quinqueradiata cultured in coastal Pacific waters off Sukumo, Kochi Prefecture, western Japan. The specimen was identified as Argulus matuii based on morphological characters such as six streaks of yellow fringed with dark brown pigments on the dorsal surface of the carapace and foot-shaped natatory lobes on the fourth pair of legs. This species was previously reported from bastard halibut Paralichthys olivaceus farmed in Japan. Thus, the present collection of A. matuii represents its second record in the Japanese mariculture.

In this study, we investigated the virucidal activities of common disinfectants against the Japanese eel endothelial cells-infecting virus (JEECV) in vitro using JEE-2, a Japanese eel vascular endothelial cell line. After treatment with different concentrations of each disinfectant, virus suspension was added to the cells in 96-well plates and incubated for 10 days at 25°C. No cytopathic effects were observed after 1-min treatment with ≥50% ethanol, ≥20 ppm sodium hypochlorite, ≥50 ppm povidone-iodine, ≥500 ppm cresol, and ≥1,000 ppm benzalkonium chloride. These results suggest that appropriate disinfection controls the spread of JEECV infection on eel farms.

We aimed to develop DNA vaccines against Mycolicibacterium cyprinidarum that are commonly isolated from diseased koi carp Cyprinus carpio. Major antigenic genes, Ag85B, Ag85C, Ag85D and ESAT-6 were cloned into a mammalian expression vector and used as DNA vaccines. Common carp were injected intramuscularly with each DNA vaccine individually, multiple mixtures of each DNA vaccine, PBS, or empty vector DNA. Cumulative survival rate after a bacterial challenge was 65.5% in PBS-injected group, while all fish survived from the challenge in the group co-immunized with the plasmids encoding Ag85B, Ag85C, Ag85D, and ESAT-6. The DNA vaccine cocktail may be a promising prevention method against M. cypridarum infection in koi carp aquaculture.