After the establishment of a large scale production method for penaeid shrimp larvae in the middle of 1960s, the culture production of kuruma shrimp Penaeus japonicus rapidly increased and attained the peak of 3, 020 metric tons in 1988 in Japan. Development of formulated diet and introduction of a double-harvest system were the main factors for the expansion. In addition to aquaculture, 300 million juveniles of P. japonicus have been produced at public sea farming centers and stocked in coastal waters to enhance the natural resources every year since the late 1970s. The number of shrimp pathogens reported in Japan is smaller than that reported from overseas, mainly because almost one shrimp species, P. japonicus has been farmed in Japan. Baculoviral mid-gut gland necrosis (BMN), penaeid acute viremia (PAV) (= white spot disease : WSD) and vibriosis (Vibrio penaeicida infection) have been known as major infectious diseases of cultured shrimp in Japan. BMN was a serious problem in larval production at hatcheries from the early 1970s to the mid 1980s, but the disease has not occurred since 1993, due to the dissemination of the practical countermeasure, egg washing. Vibriosis was prevalent especially from the late 1980s to the early 1990s when the culture system became intensive, and the losses by this disease were estimated to be 20-30% of annual shrimp production. PAV, which was introduced into Japan with live stock of young P. japonicus from China in 1993, has been not only giving serious economic losses to the shrimp aquaculture industry, but also causing various troubles in sea farming operation for shrimp. In this paper eight infectious and three non-infectious diseases of P. japonicus reported in Japan are reviewed.
The efficacy of inactivated betanodavirus as a vaccine against viral nervous necrosis (VNN) was evaluated using juvenile sevenband grouper Epinephelus septemfasciatus. Fish were intraperitoneally injected once with formalin-inactivated redspotted grouper nervous necrosis virus (RGNNV). Virus-neutralizing antibodies were detected in the vaccinated fish from Day 10 to the end of the experimental period (Day 160), showing 1 : 2, 000 or higher mean antibody titers from Day 21 to Day 77. The vaccinated and unvaccinated control fish were challenged by intramuscular injection with the homologous virus at 14, 35 and 74 days post-vaccination. The vaccinated fish showed significantly lower mortalities at any challenges than the control fish, with the RPS (relative percent survival) values 67 or higher. A field trial, in which fish were exposed to natural infection in net pens, also resulted in higher survival rates in the vaccinated fish (RPS = 85) during the experimental period of 9 weeks. This high induction of neutralizing antibodies and protection indicates the potential of the inactivated virus vaccine against VNN.
A polymerase chain reaction (PCR) -based method was developed to detect enteric myxozoans which cause the emaciation disease of cultured tiger puffer Takifugu rubripes. Three primer sets were designed based on the sequences of small subunit ribosomal DNA of Enteromyxum leei, Enteromyxum fugu and Leptotheca fugu. All of the primer sets specifically amplified the target DNA of each species. The PCR successfully detected parasite DNA not only from the intestinal mucosa of killed fish but also from gut contents collected from live fish using a swab inserted into the anus. These PCR analyses were more sensitive to detection of parasites than microscopic observation on the intestinal imprints. The present PCR method is useful for rapid diagnosis of the myxosporean emaciation disease.
A total 645 wild marine fish of 40 species were collected at the north (Wakkanai and Haboro), south (Hakodate) and east (Akkeshi) coasts of Hokkaido from 2000 to 2004 to conduct surveillance of fish viruses. Four fish cell lines (RTG-2, EPC, CHSE-214, FHM) were used for virus isolation from the brain, kidney and spleen of fish. Aquabirnavirus, which was identified as yellowtail ascites virus by a neutralization test, was isolated only from ten fish of four species caught at Akkeshi coastal area in 2002. These species were saffron cod Eleginus gracilis, snowy sculpin Myoxocephalus blandti, Japanese dace Tribolodon hakonensis and rainbow smelt Osmerus eperlanus mordax. Other viruses including viral hemorrhagic septicemia virus were not isolated.
Effects of low water temperature (below 10°C) on the oncomiracidial attachment and its subsequent development of Neoheterobothrium hirame were investigated. The cumulative attached larvae to the gill pieces from olive flounder Paralichthys olivaceus was reduced by 30% at 5°C when compared with 20°C. At 8°C the parasite development on flounder was significantly retarded compared at 20°C, and considerable number of worms disappeared from the host before reaching maturation. These results suggest low water temperature is a factor limiting the population growth of N.hirame in cold temperature regions.
Improvement of a PCR method using the Sph I-5 primer set, which has been used for detection of KHV in Japan, was carried out to reduce negative factors, such as the appearance of non-specific reactions and the comparatively long reaction time. Moreover, there was a wrong sequence in the original reverse primer. The modified PCR protocol with the corrected primer set is as follows : initial denaturation at 94°C for 30s, 40 cycles of amplification (denaturation at 94°C for 30s, annealing at 63°C for 30s and elongation at 72°C for 30s) and final elongation at 72°C for 7min. The improved PCR reduced non-specific reactions and the total reaction time to almost half reguired by the original one.