Two species of trichodinid ciliates were found on the gills of cultured tiger puffer, Takifugu rubripes; one was a new species, described as Trichodina fugu n.sp., and the other was identified as Trichodina jadranica (Raabe, 1958). This new species was discriminated from the known species by its slender blade of denticle in the adhesive disc. A scanning electron microscopy of T. fugu revealed an aboral ciliary complex consisting of sparsely arranged marginal cilia, a basal ciliary ring with short cilia, and strong locomotory wreath. Basal septum consisting of two layers was also observed between the locomotor ciliary wreath and the basal ciliary ring. The border membrane was narrow. Denticles and radial pins were also clearly observed by scanning electron microscopy.
Parasites of cultured tiger puffer, Takifugu rubripes, produced in a hatchery, introduced to a culture site in Nagasaki Prefecture in summer and cultured for one year and a half in net cages, were examined bimonthly. Thirteen species of parasites were recorded : ciliates, Trichodina fugu and T. jadranica on the gills, Cryptocaryon irritans in the fins and body surface; myxosporeans, Kudoa shiomitsui in the heart and Multivalvulida gen. sp. (syn. : Septemcapsula yasunagai Hsieh et Chen, 1984) in the brain; monogeneans, Gyrodactylus rubripedis sp. n.on the fins and body surface and Heterobothrium okamotoi on the gills and branchial cavity wall; trematodes, Tetrochetus sp. and Maculifer pacificus in the rectum; acanthocephalan, Longicollum pagrosomi in the rectum; copepods, Caligus fugu on the buccal cavity wall, Pseudocaligus fugu on the body surface and Neobrachiella hugu (syn : Clavellopsis hugu Yamaguti, 1939) on the buccal cavity wall. Gyrodactylus rubripedis sp. n. is different from the most closely related G. pardalidis sp. n. collected from the fins of panther puffer, Takifugu pardalis and finespotted puffer, T. poecilonotus in smaller size of hamuli and shape of the marginal hook proper;the two new gyrodactylids are similar to ones from freshwater fish of the genus Noemacheilus in having inwardly bent root of hamuli, but clearly separated from them by the shape of the ventral bar. In some selected species of parasites, seasonal occurrences are described and possible sources of infection are discussed.
The infection of cultured tiger puffer with the monogenean Heterobothrium okamotoi was monitored bimonthly from juvenile to marketable size for one year and a half. The infection was first detected in November, about five months after the introduction of host fish juvenils to the culture site. The parasite first attached to the gill filaments. They did not become matured until they moved to the branchial cavity wall. The life span was estimated to be less than 6 months in summer. The prevalence and intensity of infection varied with seasons and among host individuals. They sharply increased with an increase in water temperature in summer. As the intensity of infection increased, the parasite showed a higher variation in the number. The parasite sometimes showed strikingly biased distribution on the gills;it was found in significantly larger numbers on one side of gills and branchial cavity wall than on the other side, or present almost exclusively on one side. The infection did not severely affect blood parameters of host, probably because the infection level was not very high in the present case.
One-year old tiger puffer, Takifugu rubripes, were infected with the monogenean, Heterobothrium okamotoi, by exposing oncomiracidia of the parasite. Maximal intensity of infection on the gills and branchial cavity wall was 79 and 76, respectively. Immature parasites were found on the gill filaments of the host for the first 5-7 weeks until they grew up to 4.3 mm in average body length. Then, they moved to the branchial cavity wall and became matured. Approximatly 40% of immature parasites were estimated to fail to move successfully from the gills to the branchial cavity wall. Adult body size ranged from 7.0 mm to 24.3 mm long. Copulation and egg laying were first observed 49 days post-exposure. Estimated life span was within 4 months post-exposure. Parasites sometimes showed biased distribution either on the right or on the left side of the gills or the branchial cavity wall. A significant difference was also noticed in the growth between parasites on one side of the gills or branchial cavity wall and on the other. Infected fish showed anemia, as indicated by the decreased hematocrit value and hemoglobin content with the growth of parasites.
Escherichia vulneris was isolated from naturally infected rainbow trout Oncorhynchus mykiss, balloon moly Poecilia sp., silver moly Poecilia sp., and Caucasian carp Carassius carassius, and its pathogenicity was tested by intramuscular injection using healthy 1.5-year-old rainbow trout. The experimental infection caused mortalities with gross clinical and histopathological abnormalities in tissues of epidermis, gills, liver, kidney, spleen and intestine of the fish. Leucocytes, lymphocytes and granulocytes in blood of the infected fish were significantly increased as compared to healthy fish and/or recovered fish. Other blood parameters such as haemoglobin content haematocrit value, thrombocyte and erythrocyte numbers were not significantly different between the experimentally infected and healthy. In a therapeutic experiment injection with oxytetracycline controlled the infection.
The present study attempted to investigate the white spot disease virus (WSDV) infection in the wild-captured greasy back shrimp, Metapenaeus ensis. The shrimp showed no symptom of white spots in the carapace that usually occur in the other cultured shrimps. In histopathological studies, the hypertrophied nuclei of the necrotic cells were found in the various tissues originated from ectoderm and mesoderm which were similar to those observed in the giant tiger shrimp (Penaeus monodon) and kuruma shrimp (P. japonicus). Neither occlusion bodies nor cytoplasmic inclusions were found in the infected greasy back shrimp. Transmission electron micrographs revealed rod-shaped and enveloped viruses in the hypertrophied nuclei. The virions measured 221±6nm by 107±7nm in size, and were comparatively smaller than those found in WSDV infected P. monodon and P. japonicus When in situ hybridization using DIG-labelled WSDV probe was used, positive reaction was found in the hypertrophied nuclei. DNA amplification by polymerase chain reaction (PCR) with two oligonucleotide primers developed in our laboratory demonstrated that the virus found in M. ensis is geneticallysimilar to WSDV in the other penaeid shrimp.
Yellowtail Seriola quinqueradiata were experimentally infected with Enterococcus seriolicida in order to investigate the effect of dissolved oxygen (DO) on horizontal transmission. Forty-five fish orally inoculated with E. seriolicida (2.9×109 CFU/fish) (donor fish) and an equal number of non-inoculated fish (recipient fish) were reared in separated net cages kept in a tank with high oxygen concentration (HDO, 112∼161% oxygen saturation) for 20 days. The same set of fish was kept in another tank with low oxygen concentration (LDO, 59∼82% oxygen saturation). Average mortality of the donor fish in LDO group was higher than that of HDO group. Although there was no mortality among recipient fish in HDO tank, 5 recipient fish died in LDO tank. E. seriolicida was isolated from the recipient fish in LDO tank, but not from those in HDO tank. The venous blood oxygen tensions of the recipient fish in LDO tank were significantly lower than a control value measured prior to the experiment. These results indicated that horizontal transmission occurs more easily among yellowtail kept in hypoxic than in hyperoxic conditions.
Very high mortalities of juvenile Metapenaeus ensis occurred in the seedling production at public sea-farming centers in west Japan during 1994 and 1995. Moribund shrimp exhibited no apparent external signs except small obscure white spots on the body, and were proved to be free from any causative bacteria, fungi and parasites. Infection trials reproduced the disease in both healthy juvenile M. ensis and Penaeus japonicus. The disease was characterized by nuclear hypertrophy of the necrotic cells in various organs and tissues originated from meso- and ectoderm, such as cuticular epidermis and hematopoietic tissue. Enveloped ovoid-shaped virus particles were observed in such hypertrophied nuclei in sectioned preparations and in the hemolymph in negatively stained preparations. Penaeid rod-shaped DNA virus (PRDV) specific DNA fragments were detected by PCR method. These results indicate that the mass mortalities were caused by penaeid acute viremia (PAV).
A taxonomical re-examination of two pathogenic bacteria belonging to the genus Vibrio, one from an eye disease of milkfish (Chanos chanos) in the Philippines and the other from vibriosis of larval swimming crab (Portunus trituberculatus) in Japan, was made by examining their phenotypic features and measuring DNA-DNA relatedness. As a result, the bacterium from milkfish was identified as V. harveyi based on the high DNA homology value of above 84% between the two milkfish isolates and the type strain of V. harveyi and coincidence in phenotypic characteristics. Strain 85Z-1, the representative strain of the causative bacterium (Vibrio sp. Zoea) of vibriosis in swimming crab, showed higher DNA relatedness than 75% to other 3 strains of Vibrio sp. Zoea, about 50% relatedness to strains of V. harveyi, and lower relatedness than 46% to the other species examined. These results indicate that Vibrio sp. Zoea is a new genomic species most closely to V. harveyi. However, it is difficult to distinguish phenotypically Vibrio sp. Zoea from V. harveyi isolated from milkfish, thus species name for Vibrio sp. Zoea could not be proposed.
Bacteria were isolated from diseased marine fishes (greenling Hexagrammos otakii, Japanese flounder Paralichthys olivaceus and Schlegel's black rockfish Sebastes schlegeli) cultured in Oita Prefecture. Irrespective of sources, these bacteria showed the same characteristics. They were Gram negative, facultatively anaerobic and non-motile rod. Cytochrome oxidase, catalase, brown pigment and auto-aggregation were positive. Growth at 37°C and O/129 sensitivity were negative. The bacteria exhibited positive reaction against an anti-Aeromonas salmonicida subsp. salmonicida serum. The isolates were identified as atypical A. salmonicida on the basis of these characteristics. Intraperitoneal injection confirmed the virulence of the bacterium to Japanese flounder. This atypical A. salmonicida is thought to be the causative agent of the disease.
In 1994, a mass mortality occurred among 0 and 1-year-old Schlegel's black rockfish (Sebastes schlegeli) cultured in the eastcoast of Okayama Prefecture. The affected fish were characterized by ulcer formed on the trunk. A bacterium was isolated from the kidney and brain of diseased fish and identified as atypical Aeromonas salmonicida by its biochemical and serological properties. Infection experiments showed that the present isolate was highly pathogenic to black rockfish, the LD50 being 1.1×10°CFU/fish by intramuscular injection or 4.4×105CFU/ml by immersion method.