Studies on infection cycles of newly emerging and re-emerging diseases of fishes

1.对野生海水鱼类中的比目鱼病毒和VHSV进行了现场调查，结果表明，从马鱼、牙牙鲆和黑带石斑鱼中分离到水生病毒，从牙鲆和沙枪鱼中检测到VHSV。除1株外，其余分离株均属于基因组3(美国基因组)。VHSV在感染鱼和未感染鱼之间很容易通过水路传播，表明野生比目鱼和养殖比目鱼之间完成了感染循环。2.调查了岩手县宫古湾和福井县奥巴马湾0岁和1岁牙鲆新异博氏菌的发生情况。在这两个地区，野生和放生的比目鱼冬季感染率较高，夏季感染率较低，奥巴马的感染率是宫古县的数倍。3.在野生淡水鱼(鲫鱼、银鱼和阿鱼)中发现23株冷水病的致病菌--嗜冷黄杆菌。我们尝试将这5株来自阿育养殖场的溶菌噬菌体菌株应用于冷水病的生物防治。4.测定了刺激性隐核体原生孢子形成包囊所需的沉积速度和时间。测定值表明，寄生虫包囊是在海底形成的，寄生虫的感染循环发生在海底和网箱之间。此外，该寄生虫的暴发与潮汐有关。5.金龟子的发育受到低温的抑制，但随着温度的升高而复发，表明潜伏感染的鱼的释放导致了该病的传播。未知的感染阶段，大小约40μm，被证明起源于几丁糖孵化场上游的大坝，并经口入侵鱼类。

1.Field investigation of the flounder birnavirus and VHSV among wild marine fishes showed that aquabirnavirus was isolated from horse mackerel, Japanese flounder and dark banded rockfish, and that VHSV was detected from Japanese flounder and sand lance. All isolates of VHSV except one belonged to Genogroup 3 (American genogroup). VHSV was easily transmitted by water-borne route from infected to uninfected fish, suggesting that the infection cycle completed between wild and cultured flounder.2.Occurrences of Neoheterobothrium hirame in 0-and 1-year-old Japanese flounder in Miyako Bay, Iwate Prefecture and Obama Bay, Fukui Prefecture were surveyed. In both areas, infection levels in both wild and released flounders were high in winter and low in summer and were several fold higher in Obama than in Miyako.3.Twenty three isolates of Flavobacterium psychrophilum, the causative bacterium of cold-water disease, were found in wild freshwater fishes (crucian carp, dace and ayu). We tried to apply the 5 isolates of bacteriolytic phages from ayu farms as biological control of cold-water disease.4.Sedimentation velocity and time required for the cyst formation of the protomont of Cryptocaryon irritans was determined. The determined values indicate that the parasite cyst is formed on the sea bed and the infection cycle of the parasite occurs between the sea bottom and the net-cage. Furthermore, it is suggested that the outbreak of the parasite is related to tide.5.Development of Kabatana takedai was suppressed by low temperature, but recurred by increase of temperature, suggesting that a release of latently infected fish results in spread of the disease. The unknown infective stage, ca. 40μm in size, was shown to originate in the dam located in an upper stream of the Chitose hatchery and invade fish perorally.