Since 1995 in Yamaguch Prefecture, Japan, mass mortalities of farm-raised kuruma prawn Penaeus japonicus due to unknown etiology have occurred from July to August when the pond water temperature reached around 30°C. A blue-green alga Chroococcus turgidus bloomed in the ponds, and the pH values of the pond water were found to be extremely high; frequently higher than 9.5. Many of the dead prawns displayed soft shells and whitish-opaque necrotic gills which were frequently covered with scar-like and/or granule-like foreign bodies. The intramuscular injection of the filtered homogenate of C. turgidus did not cause any significant damage to the prawn. The alga raised the pH of the medium to 9.73, much higher than any of the other phytoplanktons tested. Sea water with a pH of 9.48 or above prepared by adding sodium hydroxide solution killed the prawn with similar disease signs in the gills to those of the naturally diseased prawns. These results indicate that high pH of pond water by growth of C. turgidus is the main cause of the death.
The phylogenies of two enteric myxozoans, Myxidium fugu and Myxidium sp. TP, from cultured tiger puffer Takifugu rubripes were re-evaluated based on their small subunit ribosomal RNA (SSU rRNA) gene sequence and spore morphology. Phylogenetic analysis shows that M. fugu and Myxidium sp. TP are grouped with Enteromyxum species as a monophyletic clade, not with other Myxidium spp. It was also illustrated that spores of M. fugu satisfy the criteria for the genus Enteromyxum for spore morphology. SSU rRNA gene sequence of Myxidium sp. TP shared 99.6% identity to that of Enteromyxum leei. Experimental transmission of Myxidium sp. TP to red sea bream Pagrus major induced the formation of mature spores, which were indistinguishable from those of E. leei in morphology and morphometrics. Myxidium sp. TP is identified as E. leei, and M. fugu is reassigned as Enteromyxum fugu comb. n.
This study was conducted to investigate seasonal appearance of Edwardsiella tarda and E. tarda bacteriophages in cultured Japanese flounder Paralichthys olivaceus and the culture environment. The surveys were carried out twice a month at 4 commercial fish farms in Ehime Prefecture in 2002-2004. E. tarda was isolated from the environmental seawater and the organs (kidney and spleen) of apparently healthy fish from June to December when mortality due to edwardsiellosis was found among cultured populations. As expected, the outbreak of the disease preceded a rapid increase in the titers of serum agglutinins against E. tarda. In contrast, E. tarda phages were frequently identified in the environmental seawater at least one month before the disease outbreak and during the disease prevalence, but not detected after the outbreak terminated. The present study suggests that the appearance of E. tarda phages can be an indicator for existence of E. tarda in the culture environment.
Development of the monogenean Heterobothrium okamotoi on the gills of tiger puffer Takifugu rubripes was monitored, based on specimens collected from experimentally infected fish kept at 20°C for 20 days. The development of the parasite, indicated by both the body length and the number of clamp pairs, was faster in the second and third weeks of infection than in the first week. The number of hematin cells, arranged on the surface of the gut to digest host blood, which was measured as an indicator of the cumulative amount of host blood taken by the parasite, increased as the surface area of the gut increased. The increase was very low, moderate and rapid in parasites with 0-1 pair of clamps, 2-3 pairs and 4 pairs, respectively. These results indicate that the growth of H. okamotoi on the gills of tiger puffer is closely associated with the increase in the number of clamp pairs and the intake of the host blood.
Susceptibility of three different stocks of ayu Plecoglossus altivelis (domesticated, amphidromous and landlocked stocks maintained in Hiroshima Prefectural Fisheries Experiment Station) to bacterial coldwater disease (BCWD) were compared by field spontaneous infection, laboratory water-borne infection and intraperitoneal injection with Flavobacterium psychrophilum. Cumulative mortalities of the amphidromous stock were significantly lower than those of the other two stocks in any infection experiments. The mechanisms underlying the resistance of the amphidromous stock against BCWD were investigated with respect to non-specific defense parameters such as respiratory burst, WST-1 reduction and phagocytic activities of the blood leukocytes and the serum-killing or static activity against the bacterium. However, no significant differences were observed in any parameters tested among the stocks of fish. It is suggested that the amphidromous stock is useful to establish a BCWD-resistant stock.
ABSTRACT-Mass mortality among common carp Cyprinus carpio, especially in large-sized fish cultured in Lake Kasumigaura, has occurred since early October 2003. The affected fish swam lethargically near the surface. A histopathological examination revealed hyperplasia and necrosis of the epithelial cells in the gills. DNA of the koi herpesvirus (KHV) was detected in the gills and kidneys of moribund fish by polymerase chain reaction (PCR) assay. The virus could not be isolated from the tissues, but the inoculation of gill homogenate from affected fish into healthy carp reproduced the disease conditions and killed the fish. KHV was detected from the gills of the experimentally infected fish by PCR, and the virus was isolated from those fish with KF-1 cells.