The characterization of rainbow trout peripheral blood leucocytes (PBL) was carried out by flow cytometry (FCM) analysis and by transmission electron microscopy. Two major cell populations were identified in FCM analysis. One population showed small values in both forward scatter (FS) and side scatter (SS), and the other population showed large values in both parameters. These major populations were divided into two sub-regions each (A/B and C/D). There was no significant difference in cell populations between sub-regions A and B. However, a difference was observed in the small and large lymphocyte percentages between sub-regions C and D. The majority of regions (A and B) were constituted by neutrophils (56% and 66%, respectively), and monocytes accounted for 9% and 7% of the cells, respectively. Large and small lymphocytes were also present in these regions, accounting for 26% and 6% of the cells in sub-region A and 14% and 7% in sub-region B, respectively. In sub-regions C and D, cells were mainly constituted by large and small lymphocytes (33% and 49% in C, and 7% and 70% in D, respectively) and thrombocytes (14% and 23%).
Mass mortalities of juvenile abalones belonging to 4 Haliotis species, H. discus discus, H. discus hannai, H. madaka and H. gigantea, often occur in hatcheries from spring to early summer in western Japan. Some mortalities in H. discus discus have been attributed to the disease named amyotrophia. Diseased and healthy juvenile abalones were collected from 8 different public sea-farming centers from 1988 to 1996, as well as experimentally infected juvenile H. discus discus, and were histopathologically examined to determine if the mortalities were associated with amyotrophia. Moribund H. discus discus showed atrophy of foot muscle, reduced activity to adhere to the substrate, and some of them exhibited a few incisions on the front margin of the shells. In sections abnormal cell masses originated from the nerve tissue, which is the most characteristic histopathological change in amyotrophia, were observed in various organs of H. discus discus, H. discus hannai and H. madaka, but not in H. gigantea. In moribund abalones such abnormal cell masses were extensively produced mainly in nerve trunk and peripheral nerve in the foot muscle. No possible etiological agent was detected by light or electron microscopy even in experimentally infected H. discus discus. These histological findings suggest that most of mass mortalities of H. discus discus, H. discus hannai and H. madaka are attributed to amyotrophia.
Defense activities of neutrophils in tilapia, Oreochromis niloticus were investigated under the conditions of social stress. Two fish, one from the large fish group (mean weight; 143 g) and the other from the small fish group (mean weight; 91 g) were transferred into an aquarium. As a control, a large fish or a small fish was held individually in aquaria of the same size. Five such pairs were observed in this study. Within a few hours post-pairing, the large fish became dominant in all pairs tested, and charged, chased and rammed the subordinate small fish. The cortisol and glucose concentrations in the plasma of the subordinates were significantly higher than those of the dominants and the controls, indicating that the subordinates were stressed. The swim bladders of both fish were injected with formalin-killed Escherichia coli cells just before pairing, and after 24 h, the neutrophils exuded into the swim bladder were collected and their defense activities measured. The number of neutrophils that migrated to the swim bladders of the subordinates was significantly decreased, and their phagocytic ability and respiratory. burst activity were both reduced. These results indicate that social stress suppresses the defense activities of neutrophils. Interestingly, the dominant fish showed high respiratory burst activity compared with the controls, indicating that slight stress may stimulate the defense activities of neutrophils.
Penaeid acute viremia (PAV) caused by penaeid rod-shaped DNA virus (PRDV : one of white spot baculovirus complex) is the most serious disease affecting kuruma prawn (Penaeus japonicus) culture in Japan. In this study an experimental challenge was carried out in different larval (nauplius, zoea and mysis) and postlarval (PL 1, 6, 9 and 11-12) stages of kuruma prawn in order to elucidate the difference in susceptibility to PRDV among different developmental stages of the host. The PRDV challenge was done by immersion in all the tested groups. No infections were recorded in the larval stages and PL1. The PL6 first showed mortality due to PAV on 10th day post-inoculation. The PL9 exhibited the onset of PAV on 5th day post-inoculation. In PL11-12, experimental PAV was produced more rapidly and resulted in higher mortality (78.6%). These results demonstrate that the PRDV-susceptibility of P. japonicus increases with the progress in developmental stage until PL12 and that PRDV may not display its pathogenicity in the larval and early postlarval stages younger than PL6.
Anguillicolosis is a parasitic disease of eel caused by infection with swimbladder nematode, Anguillicola crassus. This study aims to establish a serodiagnosis method for anguillicolosis. A competitive ELISA with an anti-Onchocerca volvulus mouse monoclonal antibody (No. 107) was applied to detect the specific antibodies against A. crassus in eel sera, using various capture antigens for ELISA. Infected eel sera showed a higher competitive reaction to the cuticle antigen of A. crassus than non-infected eel sera. The positive detection rate of infected eel sera and the false positive rate of non-infected eel sera were calculated as 95% and 0%, respectively. When an indirect ELISA with polyclonal rabbit anti-eel lgM antibodies was performed, the positive detection rate and the false positive rate were 80% and 20%, respectively. The competitive ELISA technique provided a more sensitive and specific serodiagnostic tool than the indirect ELISA for detecting A. crassus infection in eels.
In 1996, a hatchery of Japan Sea-Farming Association obtained kuruma prawn (Penaeus japonicus) eggs for seedling culture from wild broodstocks, among which some were found to be rod-shaped DNA virus (PRDV) infected by PCR test. Eggs were washed once withfiltrated seawater and reared in the hatchery. During the culture, those post-larval groups in which PRDV were detected were excluded. Although no PRDV was found in the seedlings prior to the transportation to the nursery by PCR test, PAV occurred among them during the culture in the nursery facilities. In 1997, the hatchery again obtained prawn eggs from wild broodstocks. However, in this year, PCR-check was applied to select non-PRDV infected spawners. Furthermore, eggs were disinfected with iodine before being served for rearing. No PAV infection occurred throughout the culture in the hatchery and the nursery facility in this year. These results strongly suggest that the infection. source of PAV occurred in 1996 originated from spawners.
Between 1994 and 1998, epizootiological investigations were conducted on kudoosis amami caused by Kudoa amamiensis (Multivalvulida : Myxozoa) in yellowtail Seriola quinqueradiata and amberjack Seriola dumerili at 14 coastal areas in Okinawa Prefecture, Japan. Surveys indicated that the geographical distribution of K. amamiensis was limited specifically within a local area; the prevalences of infection were 70-100% in yellowtail and 30-90% in amberjack in the Motobu area, whereas they were 0-9% in yellowtail and 0-1% in amberjack in the other areas examined. Amberjack were less susceptible to K. amamiensis than yellowtail; the intensities of infection (number of pseudocysts per one gram of muscle tissue) in amberjack were one order of magnitude lower than those in yellowtail. Infections with K. iwatai forming similar pseudocysts in the musculature were rarely found, and the intensities of infection were remarkably lower than those of K. amamiensis.