Redmouth disease is caused by the infection of the enteric bacterium Yersinia ruckeri. ​The disease was first noticed in rainbow trout Oncorhynchus mykiss in the USA in 1950s and has since been observed in salmonids and some other fishes in many countries. ​Y. ruckeri can be classified into different biotypes, serotypes, or genotypes, according to biological properties such as motility or enzyme activity, antigenicity, or gene sequences. ​In particular, O-antigen, which is one of the markers for serotyping, is an important antigen for the vaccination for of Y. ruckeri. ​In Japan, redmouth disease is listed as one of the notifiable diseases of aquatic animals by the government to prevent occurrences or spreading of the disease. ​This review describes topics necessary important for the diagnosis and control of the disease.

Kuchijirosho is a lethal infectious disease of fugu Takifugu rubripes, and the causative pathogen has been predicted to be an RNA virus. ​Although the homogenate of kuchijirosho-affected brain is pathogenic to fugu, the suspected viral particles have not been found in the brain and the viral genome has not been isolated. ​We attempted to clone the cDNA of the kuchijirosho virus genome using the Rapid Determination System for Viral RNA Sequence method. ​Three cDNA segments of ca. 1,000 nt each, which could be parts of the viral genome, were obtained from total RNA extracted from the brains of fugu artificially infected with kuchijirosho. ​According to RT-qPCR, the brain had more of these three kuchijirosho-associated RNAs (KARs) than any other tissues. ​KARs in the brain were detected 1–2 days after injecting the homogenate of kuchijirosho-affected brain and KARs expression levels were increased rapidly until death. ​These results show that the detection of KARs can be sufficiently effective for the molecular diagnosis of kuchijirosho. ​Even if KARs are parts of the viral genome, it is unclear to which taxonomic family the kuchijirosho virus belongs, because the nucleotide sequences of KARs did not correspond to those of any other organisms including viruses.

In August 2017, a mass mortality of cultured ayu Plecoglossus altivelis occurred in Hiroshima Prefecture. ​Most of the diseased fish showed hemorrhagic and swelling lesions on the body surface. ​Bacterial isolates from diseased ayu were identified as Aeromonas hydrophila by conventional characterization tests and genetic analysis. ​However, the clinical signs differed from previously reported A. hydrophila infection in ayu. ​The present isolate caused higher mortality in ayu without clinical signs at 24°C than at 19°C. ​These results indicate that the present mass mortality was caused by A. hydrophila and the pathogenicity of the isolates was related to water temperature.

We measured gene quantities of the abalone asfa-like virus (AbALV), which is presumed to be the pathogen of abalone amyotrophia, in various tissues of affected Japanese abalone (Haliotis discus discus) with a quantitative PCR. ​Large quantities of the gene were detected in any tissues or swabs from the body surface. ​No significant difference in the gene quantity was found among tissues, although the gene quantity in the hemolymph was lower. ​The virus was also detected by a conventional PCR from affected animals. ​These results indicate that both PCRs are capable of the detection of AbALV, in any tissues by the destructive test and in body surface mucus by the non-destructive test.

Multiple cell lines permissive to nervous necrosis virus (NNV) infection may be necessary for identifying receptor binding sites of viral surface protrusions. ​We have established new cell lines, SeGE-12, SeGE-22, SeGF and SeGB, derived from eyed-egg embryos, fin and brain of sevenband grouper. ​The optimum growth temperatures of these cell lines were all around 30°C. ​The growth temperature range for SeGE-12 and SeGE-22 was 25°C–32°C, whereas for SeGF and SeGB it was 15°C–32°C. ​These cell lines exhibited no cytopathic effect (CPE) due to NNV infection, but released extracellular NNV at 10^2.8 to 10^4.7 TCID₅₀/mL after 14 days incubation.