For shrimp with low-level white spot syndrome virus (WSSV, = PRDV) infection, environmental or physiological stresses can trigger replication of the virus and lead to a full-blown outbreak of white spot disease. This review will describe how pathogenomic studies have revealed some of the mechanisms that underlie this aspect of WSSV pathology. In the model that emerges, stressors activate shrimp STAT (signal transducer and activator of transcription), which is then annexed by the virus and used to activate the promoter of the immediate early gene WSSV ie1. This in turn leads to rapid replication of the virus.
Three regions of koi herpesvirus (KHV) genomic DNA were compared for 34 samples from Japan, six from Indonesia, two from Taiwan, one from the Philippines, 13 from the Netherlands, one from the UK, one from the USA and one from Israel. The analyzed genomic regions included known PCR-detection targets (SphI-5, 9/5 and the thymidine kinase gene). The KHVs from Asian countries were very homogeneous, although two variants were noted based on a single nucleotide polymorphism. In contrast, seven variants were found in KHVs from outside of Asia, and although closely related to one another, they were clearly distinct from those from Asian. The results suggest that a clear genetic distinction exists between Asian and European (including each single isolate from the USA and Israel) types of KHV, and that unique types of KHV were independently introduced or emerged in the respective geographic locations.
We conducted six field trials on the efficacy of febantel against the monogenean Heterobothrium okamotoi infecting cultured tiger puffer Takifugu rubripes at four sites in Kumamoto and Ehime, Japan. From 2,500 to 4,500 one-year-old tiger puffers per experiment group were used in these trials, in which tiger puffer were orally administrated febantel at 12.5 or 25 mg/kg fish/day for 5 days. The maximum rate of worm eradication was 80.9%. However, a large difference was observed in the eradication rate between the experiment groups. We observed no adverse effect of febantel such as decreased appetite or increased mortality on the experimental fish.
Myxosporean sleeping disease is known to cause severe mortality of masu salmon Oncorhynchus masou masou and amago salmon O. masou ishikawae farmed in Hiroshima, western Japan. Diseased fish become unable to swim, and shortly afterwards die on the bottom of the pond. The causative agent is Myxobolus sp. which infects the peripheral nerves of salmon, but its definitive identity and taxonomic status have been unresolved for years. We investigated interspecific relationships among Myxobolus spp. infecting the nerve tissues of salmonid fishes from Japan, Europe and North America. Our morphological comparisons revealed that myxospores varied in spore shape, and separated into two types: spherical (typically M. neurobius, M. kisutchi and M. neurotropus) or pyriform (typically M. arcticus and M. fryeri). Molecular analysis based on small subunit ribosomal DNA (SSU rDNA) demonstrated that Myxobolus sp. from the lateral line nerve of amago salmon with sleeping disease in Hiroshima was genetically distinct from other myxosporean species, but had 99.5% sequence similarity to Myxobolus sp. from the lateral line nerve of subclinical masu salmon in the Mena River, Hokkaido. Both myxosporeans had similar spherical-type spores. Consequently, Myxobolus sp. from the lateral line nerve of both amago and masu salmon in Hiroshima and Hokkaido are considered to be a new species, and described as M. murakamii n. sp.
This study was carried out to determine pathogenicity of anamorphic fungi Plectosporium oratosquillae NJM 0662 and Acremonium sp. NJM 0672, which were isolated from gills of mantis shrimp Oratosquilla oratoria caught in Yamaguchi and Aichi Prefectures in Japan. Cumulative mortality of the mantis shrimp injected with a high dose (5.0 × 106 conidia/mL) and a low dose (5.0 × 104 conidia/mL) of the isolate NJM 0662 reached 100% and 60% at day 25, respectively. Cumulative mortality of the shrimp injected with the high dose and the low dose of the isolate NJM 0672 reached 100% and 80% at day 25, respectively. The gill lesions in the shrimp experimentally infected with the fungi were similar to those of naturally infected shrimp. Histopathologically, the hyphae and conidia were found in the gill filaments and heart, and the hyphae were encapsulated by hemocytes in the gill filaments and the base of gills. The result confirmed that these two anamorphic fungi were pathogenic to mantis shrimp.
Experiments with crustacean viruses are hampered by lack of susceptible continuous cell lines. To overcome this problem, immortal mosquito and lepidopteran cell lines were both separately challenged with a shrimp DNA virus (white spot syndrome virus: WSSV, = PRDV) and RNA virus (yellow head virus: YHV) followed by serial, split-passage with immunohistochemical monitoring by confocal laser microscopy using labeled monoclonal antibodies to shrimp viral antigens. Stable, immortal cultures with 100% of the cells expressing shrimp-virus antigens were obtained, although the infected cells appeared grossly normal by phase contrast microscopy. Nor did they show any ultrastructural modifications characteristic of the challenge viruses. These persistently-expressing insect cell cultures were stable and could be continuously passaged, stored and revived as required. Since disparate viruses and insect cells were used, this appears to be a generic process that may be applicable to other shrimp viruses as well.
In this study, we have developed a DNA vaccine encoding viral envelope protein VP28 of penaeid rod-shaped DNA virus (PRDV, = WSSV). Protective efficacy of the DNA vaccine against PRDV was confirmed at 7 days post vaccination in kuruma shrimp Penaeus japonicus. The VP28 transcript derived from the vaccine was detected in various tissues at 1, 3 and 7 days post vaccination. Moreover, notable up-regulated expression of Rab7, penaeidin, lysozyme and crustin genes was observed upon DNA vaccination. These results suggest that the DNA vaccine significantly increased the protection and innate immune responses in kuruma shrimp against PRDV.