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

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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.