We analyzed the full-length glycoprotein gene sequences of 72 infectious hematopoietic necrosis virus (IHNV) isolates that had been obtained from diseased salmonid fish in the North Kanto region of Japan from 1981 through 2015. The analysis identified three genetical lineages in the genogroup J of IHNV; two known Shizuoka (JS) and Nagano (JN) lineages and a new lineage named as North Kanto lineage (Jnk). Most of the isolates from the 1980s were classified into JS lineage, whereas most collected after 2000 belonged to JN lineage. Jnk lineage was classified into North Kanto A clade [Jnk-A] and B clade [Jnk-B], and the number of isolates belonging to Jnk-B in the 2010s was higher than in the 2000s. These results suggest that the diversity of IHNV found in salmonid farms in the North Kanto region have been increasing with genetical lineage shifting from 1980 to the present, probably due to change of the fish trade.
Long-term parasitological surveys were conducted on the isopod Ceratothoa verrucosa infecting red seabream Pagrus major caught in Iyo-Nada, Ehime Prefecture during the period of 2003–2007. The isopods were almost always in a pair of a male and a female, attached to the buccal cavity wall. The earliest infection occurred when seabream was 0+ year-old and the isopods grew as host fish grew. Prevalence of infection ranged from 12.2% to 21.2% when the host was 0+ to 5-year-old and decreased to 3.4% at 6-year-old and no infection among 7-year-old and older fish. These results suggest that infection was established when the host was 0+ year-old, and their life span of C. verrucose was estimated to be up to 6 years. Infected fish sometimes showed deformation in the upper jaw and growth retardation, but there was no evidence of infection-related mortality. The main reproduction season of the isopod was in summer, estimated by the presence of manca larvae in the marsupium of the female. Sometimes the monogenean Choricotyle elongata attached to the pleotelson of C. verrucosa. Prevalence and abundance of the monogenean were higher in the hyperparasitized seabream than in the fish without isopod infection, suggesting that the isopod favored the monogenean infection.
Beko disease, caused by the Microsporidium seriolae infection, has been a problem in yellowtail aquaculture in western Japan. In recent years, severe cases of this disease have been confirmed, resulting in a significant decrease in product value due to mass mortality and residual cysts. Only polymerase chain reaction (PCR) assays have been reported so far as a detection method for the disease. In this study, quantitative real-time PCR (qPCR) and loop-mediated isothermal amplification (LAMP) methods have been optimized for M. seriolae detection in order to establish a more sensitive and rapid diagnosis. Target regions for each detection method were selected based on the nucleotide sequences obtained by the gene analysis of cysts in diseased fish. Primer sets for the qPCR and LAMP methods were designed, and the gene amplification efficiency of each method was evaluated. The results showed that the newly developed qPCR method could detect 1.4 copies of the target gene, and the LAMP method detected 100 copies within 15 minutes. In this study, the newly developed qPCR and LAMP assays were shown to be rapid and highly sensitive methods for quantitative detection of M. seriolae.
Beko disease, caused by infection of the microsporidian Microsporidium seriolae, results in significant damage to farmed amberjack, Seriola spp., in Japan. Little is known about the disease, including the general biology of the causative agent, and no therapeutic method has been established to treat it. We determine the therapeutic efficacies of various drugs by administering them in-feed to juvenile Seriola spp. naturally infected with M. seriolae. Candidate drugs include commercially available fishery anthelminthic, febantel, and eight other compounds. Fish were given various doses of drugs, and M. seriolae infection in their trunk muscle was then checked by quantitative PCR (qPCR) and macroscopic cyst detection. Fish treated with febantel at an early stage of infection had significantly lower rates of cyst formation and qPCR-positive. However, febantel administered to fish after cyst formation did not reduce cyst numbers. Other drugs showed no apparent efficacy to prevent cyst formation or to promote cyst reduction. Results indicate febantel is a promising therapeutic agent for beko disease when applied at an early stage of infection.
ブリ属魚類のべこ病に対するフェバンテル (FBT) の経口投与による効果を養殖場での野外試験により調べた。日間投与量 1～10 mg/kg bwのFBTをシスト形成前のブリ稚魚に5日間連続投与した結果，10 mg/kg bw投与区において，死亡率，シスト検出率，シスト数およびM. seriolae遺伝子量が対照区と比較して有意に減少した。また，カンパチへ同濃度を5日間投与した場合も，ブリと同様にシスト検出率等が有意に減少した。FBT 10 mg/kg bwを5日間投与，2日間休薬を1クールとして15回の反復投与を行った結果，シスト形成が完全に抑制された。以上のことから，FBT濃度10 mg/kg bwの5日間投与により，べこ病のシスト形成が大幅に軽減され，さらに反復投与により発症を完全に防げることが示された。
Recent studies have shown efficacy of febantel (FBT) against Microsporidium seriolae in Seriola fish. This study evaluated the toxicity of FBT in Seriola quinqueradiata. Fish were gavage fed with feed containing various doses of FBT (50 to 1,000 mg/kg/day (d)) for 14 or 28 consecutive days and monitored for growth and survival. Major mortality started at 7 days in fish given 200 and 1,000 mg/kg/d FBT, while the onset of mortality in the 50 and 100 mg/kg/d groups was delayed until 15 days. Doses of 50 mg/kg/d or higher affected fish growth. Histological analyses revealed changes in the distribution and density of Nissl bodies in the brain, and a reduction of vacuolies in hepatocytes. These histological changes were rarely observed after a 9-day drug washout period, suggesting that such changes were reversible. There was no drug-related mortality in fish that received 15 mg/kg/d for 20 days or 20 mg/kg/d for 14 days. These results suggest that, although FBT can have lethal toxicity in S. quinqueradiata when administrated at doses over 50 mg/kg/d for more than 14 days or 200 mg/kg/d for more than 7 days, the drug is considered safe at the normal antiparasitic dose, including the effective dose for M. seriolae treatment (10 mg/kg/d for 5 days).
海面生簀で飼育されているブリおよびカンパチの体側筋中におけるMicrosporidium seriolae遺伝子とシストの出現状況を調査し，べこ病の初期感染動態を観察した。また，飼育海域海水中のM. seriolae遺伝子量の変化もモニタリングした。その結果，過去にべこ病の発生が確認された海域では，生簀周辺の海水から102～103 copies/L程度のM. seriolae遺伝子が継続して検出される時に種苗を導入すると，ブリでは約2週間で体側筋中にM. seriolae遺伝子が検出され，その後約1週間でシスト形成に至ることが判明した。また，M. seriolaeの感染は，冬季（11月から1月）でもブリとカンパチで確認され，感染時期がこれまで考えられていたよりも長いことが明らかとなった。一方，カンパチはブリに比べ本症の感受性が高い傾向が窺われた。
Persistence of Febantel (FBT) and its metabolites (FBZ) were assessed in Seriola quinqueradiata after 5 days gavage administration of 20 mg/kg body weight. FBT was not detected in any organs at 1 day post administration (dpa), but traces of FBZ were detected in the liver and kidney at 7 dpa. Neither compound was detected at 14 dpa. While S. quinqueradiata received 20 mg/kg of FBT had higher bilirubin and lower triglyceride values in the blood at 1 dpa, and lower plasma creatinine level at 7 dpa, these were not likely associated with severe damages of organs and physiological functions.