One hundred sixty-four Edwardsiella specimens isolated from cultured fishes, red seabream Pagrus major, Japanese flounder Paralichthys olivaceus, thread-sail filefish Stephanolepis cirrhifer, greater amberjack Seriola dumerili and Japanese eel Anguilla japonica were analyzed by PCR for identification, and 124 of these isolates were examined using biochemical tests for characterization. Hundred and four strains from red seabream, three from Japanese flounder, and six from other fish species were identified as E. anguillarum, whereas one strain from seabream and 50 strains from flounder were identified as E. piscicida. Among the E. anguillarum strains, all strains from Japanese eel were motile and ornithine decarboxylase (ODC)-positive, whereas all strains from other fish species were non-motile and ODC-negative. The findings of this study reveal the existence of two distinct types of E. anguillarum differing in motility and ODC production. Susceptibility testing using nine antimicrobials (tetracycline, erythromycin, lincomycin, ampicillin, sulfamonomethoxine, florfenicol, thiamphenicol, oxolinic acid, and fosfomycin) showed that all strains were sensitive to fosfomycin, which had a relatively narrow range of minimum inhibitory concentrations compared to other drugs.

Saprolegniasis causes significant mortality of fish in aquaculture. Historically, malachite green, formaldehyde, and bronopol have been used to control saprolegniasis. However, their use has either been banned or faced issues with unstable supply chains. Given these limitations, there is a pressing need to develop an alternative chemical solution to prevent this disease effectively. This study focused on glutaraldehyde (GA) as a potential chemical and aimed to assess its effectiveness against various Saprolegnia strains. Seven Saprolegnia strains, including S. parasitica, S. diclina, and S. hypogyna were exposed to GA, malachite green, formaldehyde, or bronopol to evaluate their fungicidal effects. The growth of all tested strains was successfully inhibited by GA at concentrations ranging from 53 to 212 mg/L and temperatures between 10 and 20°C, with a minimum exposure duration of 30 min. The high efficacy of GA indicates its potential for controlling saprolegniasis in fish eggs. Additionally, one strain of S. parasitica was exposed to other aldehyde compounds (formaldehyde, glyoxal, acetaldehyde, isobutyraldehyde, malonaldehyde, or valeraldehyde) to assess the effectiveness of GA based on the presence of the aldehyde group. The fungicidal effects of these compounds were observed at higher concentrations (exceeding 405 mg/L) compared to GA, suggesting that the mere presence of aldehyde groups does not necessarily indicate fungicidal action.

In October 2022, a disease associated with a sudden increase in morbidity and mortality occurred among one-year-old red-spotted grouper Epinephelus akaara cultured in land-based tanks in Tottori Prefecture, Japan. Daily monitoring of the disease showed that fish losing equilibrium at the bottom of the tanks appeared one after another, and chronic mortality continued over a period of three months. All moribund fish examined had a bloated swimbladder and were heavily infected with monogeneans of the genus Pseudorhabdosynochus on the gills (P. epinepheli and P. lantauensis), with infection intensities up to over one thousand worms per fish. In vitro and in vivo experiments showed that hydrogen peroxide was effective, removing over 98% of the worms through bathing in 350 ppm for 60 min or 700 ppm for 30 min at around 20°C. Histopathological examination revealed that the monogenean infection did not induce significant host reactions in the gills. In contrast, lesions were found in the brain and retina, and morbid fish tested positive for nervous necrosis virus (NNV) by RT-PCR. These results suggested that the morbidity and mortality associated with loss of equilibrium were caused by infection with NNV rather than that with the monogeneans.

Inodosporus fujiokai is a lethal microsporidian that forms cysts in the muscle of salmonids. Fish are infected by ingesting the invertebrate host, the common prawn Palaemon paucidens. However, the precise transmission routes of I. fujiokai remain uncertain. Three trials were carried out to infect amago salmon Oncorhynchus masou ishikawae and rainbow trout O. mykiss through oral intubation or immersion using I. fujiokai spores from infected prawns. Both methods effectively induced I. fujiokai infections and high mortalities in challenged fish. Oncorhynchus masou ishikawae appeared more vulnerable to I. fujiokai infection, with up to 100% mortality. The fish cohabited with spore-intubated fish also exhibited high infection prevalences, suggesting that spores excreted from fish which consumed infected prawns cause waterborne transmission to other fish. Spore immersion at 2.6 × 10⁴ spores/mL and above led to 67% mortality and significant cyst formation in O. mykiss, while 2.6 × 10² spores/mL induced neither mortality nor cyst formation. Besides, fish fed with infected fish muscle tissue or cohabiting with infected fish did not show signs of illness or death, indicating that fish-to-fish transmission does not occur. This study provides significant insights into the transmission biology of I. fujiokai and establishes an experimental infection model for future research.

Inodosporus fujiokai is a pathogenic microsporidian causing cyst formation within the muscles of salmonids. Confirmed infections exist in Oncorhynchus mykiss, O. masou ishikawae, and O. masou subsp. (Biwa trout), yet its full host range and differences in susceptibility among salmonid species remain undefined. This study assessed the susceptibility of various freshwater fish species to I. fujiokai, providing a risk assessment for local freshwater aquaculture in Shiga Prefecture, Japan. Salmonids (O. mykiss, O. masou ishikawae, O. masou subsp., and Salvelinus leucomaenis) and the cyprinid honmoroko Gnathopogon caerulescens were collectively exposed to infective spores of I. fujiokai from common prawn Palaemon paucidens, and the mortality and the infection levels were compared. Additionally, the susceptibility of ayu Plecoglossus altivelis (Plecoglossidae) was investigated using spore immersion and intubation methods. All tested salmonids exhibited high susceptibility, with S. leucomaenis being particularly vulnerable. Some fish died from the infection without evident cyst formation, suggesting the involvement of multiple pathogenic factors. Infection levels in G. caerulescens and P. altivelis were low, suggesting that I. fujiokai does not pose a significant risk to their aquaculture. These findings expand the understanding of the host range of I. fujiokai and offer valuable insights for risk assessment in freshwater aquaculture.

The histopathological features of yellowtail Seriola quinqueradiata experimentally infected with Lactococcus garvieae (strain Lg-Aq-33) and L. formosensis (Lg-Aq-55 and Lg-Aq-103) were compared to elucidate the histopathology of lactococcosis caused by L. formosensis. Experimental infections were performed in duplicate for each bacterial strain using injection (1.0 × 10⁴ cfu/100 g body weight) and immersion (1 h, 1.0 × 10⁶ cfu/mL) methods. The cumulative mortality trend after experimental infection in each experimental group did not differ between the two experiments. All infected fish showed lesions typical of lactococcosis, such as inflammation of the inner operculum, ulceration of the caudal peduncle, epicarditis, and encephalitis. Histopathological observations confirmed previously reported lesions of lactococcosis, and revealed new lesions of the disease and those that are characteristic of L. formosensis infections. New histopathological features of lactococcosis in yellowtail included bacterial invasion and granulomas in the parenchyma of the heart, as well as bacterial invasion in the spleen and kidney. Lactococcosis caused by L. formosensis had more frequent granulomatous inflammation and verrucos endocarditis than L. garvieae infection.

This study evaluated the protective efficacy of two vaccines against lactococcosis in chub mackerel Scomber japonicus: a commercial vaccine for Lactococcus garvieae serotype I approved for use in yellowtail Seriola quinqueradiata, and an inactivated vaccine prepared from L. garvieae serotype I strain L17-2, isolated from S. japonicus. Following experimental infection via intraperitoneal inoculation with strain L17-2, the relative percentage of survival for both vaccines was 55.6%. The serum agglutination antibody titers against L. garvieae were significantly higher in vaccinated fish than in the negative control, with no significant differences between the two vaccines. These results indicate that the commercial vaccine against L. garvieae serotype I is effective in chub mackerel.