In 2015, cataract was observed in juvenile Japanese dace Tribolodon hakonensis, reared in a hatchery in Nagano Prefecture, Japan. Metacercariae were found in the lens, with the prevalence and mean intensity of infection of dace (0.5–2.4 g in body weight) reaching 100% and 39.7, respectively, in the most heavily infected fish group. Histologically, the lens became partially fragmented around the metacercariae, which occupied a considerable space, surrounded by their own excreta. Among snails sampled from outdoor ponds and waterways, cercariae with a bifurcated tail were found in the lymnaeid, Radix auricularia japonica. ITS1-5.8S-ITS2 and cytochrome oxidase subunit I (COI) sequences of metacercariae from the lens of dace and cercariae from the lymnaeid were nearly identical to each other, indicating that the lymnaeid is the first and dace is the second intermediate host. However, since the obtained COI gene sequences were close but not identical to known Diplostomum spp., this species was not identified to the species level. The morphology of metacercaria was also described. This is the second reported case of worm cataract in fish caused by Diplostomum metacercariae in Japan.
In 2017, high mortalities were observed in cultured pond loach Paramisgurnus dabryanus in Tianjin, China. The symptoms observed in the moribund loach included hemorrhage on the body surface and head, exophthalmos, and erratic swimming around the water surface. Two different Gram-positive bacteria were isolated from the diseased loach in two different fish farms. They were identified as Streptococcus dysgalactiae and Streptococcus agalactiae, using 16S rRNA gene sequencing and biochemical characterization. The median lethal doses of the bacterial strains in the loach were 3.2 × 105 and 5.0 × 105 CFU/fish, respectively.
Outbreaks of suspected viral nervous necrosis emerged in cultured groupers in Guangxi, China in August 2018. Histopathology showed severe vacuolation in the brain tissue of the diseased juvenile hybrid grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂). A nervous necrosis virus (NNV) was isolated from pooled sample of the brain and eyes using a grouper cell line EAGB. Cytopathic effect with multiple cytoplasmic vacuolation cells developed in EAGB cells. The coat protein gene sequence of the isolate (GNNV-Gx) showed that it belonged to RGNNV genotype and was most closely related to that of a tiger grouper NNV registered in GenBank. The infectivity of the isolate decreased remarkably at pH 3 and pH 10 and was susceptible to 60°C.
Mortality of farmed coho salmon Oncorhynchus kisutch due to erythrocytic inclusion body syndrome (EIBS) has been posing severe economic losses in Japan since 1986. The etiologic agent of EIBS was reported to be piscine orthoreovirus 2 (PRV-2). We detected PRV-2 in the ovarian fluid samples of 10% of EIBS-experienced spawners (n = 30) by RT-PCR. When each of 30 individual ovarian fluid samples was inoculated into a naïve coho salmon, 37% of the blood samples from the fish tested positive for PRV-2 at 20 days post injection. These observations suggest that the ovarian fluid of EIBS-experienced fish can be contaminated with the infective viral particles of PRV-2.