A microsporidian species which invades the trunk muscle of yellowtail juveniles (Seriola quinqueradiata) and causes “Beko” disease was studied. The parasites appeared as multiform masses in cross sections of the muscle and usually elongated masses in longitudinal sections. The masses were bounded externally by a host-produced fibrous membrane. Almost all the parasites clearly represented stages in the sporulation sequence. Sporogony was believed to occur by multiple fission of sporogonial plasmodia. Sporocyst, sporogony vacuole and pansporoblast membrane were absent. The diplocaryon was not confirmed in any sporulation stage. Fresh spores were ovoid and measured 2.9-3.7×1.9-2.4 μm and had a coiled polar tube within the spore membrane. Extruded polar tubes were 44-52 μm in length. This species could not be classified in any established genus and therefore it seemed appropriate to place it provisionally in the collecting group Microsporidium. The name M. seriolae was proposed. The specific name refers to the genus name of the host.
P. anguillarum胞子を配合飼料に混合してウナギ稚魚に経口投与するか,あるいは胞子液に魚体を浸漬した結果,両者ともに感染が成立し実験的にべこ病を作出することができた。感染魚は胞子投与後20日頃から顕微鏡観察により躯幹筋にシストが認められ,25日頃には自然感染魚に見られる白斑病変が体表に形成された。その白斑病変は経口感染魚では腹部躯幹筋に限られ形成されたが,浸漬感染魚では躯幹筋全体に不規則に散在した。感染率は水温の影響をうけ,低水温下ではP. anguillarumの感染あるいは発育が抑制されることがわかった。これら経口または浸漬感染魚に感染直後よりフマジリン250mg力価/kg魚体重/日を30日間連続経口投与,もしくは浸漬感染魚を感染5日後まで60.5ppm水溶液中で薬浴したところ,シストの形成は何れにおいても認められず,フマジリンがP. anguillarum感染の予防に極めて有効であることがわかった。
Efficacy of oral vaccination for control of streptococcal disease in cultured yellowtail was studied. The bacterin, the formalin-killed cells of Streptococcus sp., as the etiological agent of this disease, was used. This bacterin was administrated 1, 2, 4, 8, 16 times at a level of 5 mg dry weight per fish per day, and 8, 12 times at a level of 20 mg wet weight per fish per day. About a week after the last vaccination, fish were challenged for the estimate of efficacy of oral vaccination. Duration of oral vaccination effect was also tested. Dry bacterin was fed 2, 4, 8 times, and about 1, 2, 3, 4 weeks after the first vaccination, fish were challenged for the estimate of oral vaccination effect. Both dry and wet bacterin were effective, and the efficacy became more active with an increase in frequency of oral vaccination. But antibody in the blood was not detected. Duration of oral vaccination effect was very short. That effect was prolonged for only 2 weeks after the last vaccination. In order to compare with oral vaccination, efficacies of hyperosmotic infiltration and intraperitoneal injection for control of this disease were also studied. Hyperosmotic infiltration was more effective than oral vaccination. Intraperitoneal injection was the best method of three experimented.
At the middle of July, 1979, a bacterial infection occurred in a young population of black seabream (Acanthopagrus schlegeli) cultured in three tanks. These fish had been reared in sea water tanks since they hatched out at the end of May. The epizootic broke out in all tanks, lasted about one month, and killed 3300 fish out of 8000 (41% mortality). The causative agent isolated from liver or kidney materials of all the fish examined was identified as Pasteurella piscicida by comparing the characteristics of the present isolates with that of the isolates from black seabream by MUROGA et al. (1977) The source of the infection was thought to be the sea water supplied to the tanks because of the frequent occurrence of Pseudotuberculosis (P. piscicida infection) in yellowtail (Seriola quinqueradiata) in adjacent area.
Although vibrioses caused by Vibrio parahaemolyticus, V. alginolyticus and some other unnamed vibrios have been reported in reared juvenile or farmed red sea-bream (Pagrus major), V. anguillarum infection of the fish species has never been reported so far in Japan. In June 1981, V. anguillarum was isolated from reared juvenile red sea-bream (average body length 17 mm) às a cause of an epidemic outbreak in a hatchery. At the same time, other two strains which had been isolated from the same fish species at another hatchery in 1978 were also identified as V. anguillarum. The infection caused by the organism seems to be one of the significant diseases in artificial seed production of red sea-bream.