A new species of microsporidian parasite of the ayu, Plecoglossus altivelis T. & S., is described and the name Glugea plecoglossi n. sp. is suggested. In a heavy infection of the parasite xenomas develop in large number in various organs, viz., peritoneum, visceral cavity, ovaries, testis, fat body, pyloric caeca, spleen, muscle, liver, heart, gills, iris, etc., and cause serious damage to fish. The xenomas are variable in size, being 0.5-3 mm, rarely as large as 5 mm. Fresh spores of the parasite are elongate or ellipsoidal and are 5.1-6.2×2.0-2.5μ, the average of 200 spores being 5.8×2.1μ Spores fixed in Bouin's solution and stained with Heidenhein's iron hematoxylin are 3.8-4.8×1.5-2.0μ, the average of 50 spores being 4.0×1.8μ. Everted filaments are 180μ at maximum, being 100-150μ in the majority. The processes of schizogony and sporogony of the parasite studied with artificially infected ayu are in substance the same as those reported by WEISSENBERG (1968) on Glugea anomala and by SPRAGUE and VERNICK (1968) on G. weissenbergi. A very small xenoma of 4×8μ with a single schizont and a single host cell nucleus was found in the lamina propria of the villi of the gut of an ayu killed 5 days after infection. Sporonts in smear preparations of parasite are 3×4μ and each sporont produce two spores. Therefore the parasite is classified into the genus Glugea. Inoculation experiments have shown that Salmo gairdneri is susceptible to the parasite, but Gasterosteus acelatus microcephalus is unsusceptible. Of about 15 species of Glugea in fish, G. hertwigi has spores similar in size to those of the present Glugea. But the former differs from the latter in the following points. The hosts of G. hertwigi are Osmerus mordax and O. eperlanus and it has been reported by WEISSENBERG (1968) that Gasterosteus aceleatus is susceptible to G. hertwigi. G. anomala and G. weissenbergi are distinguishable in having smaller spores, 3-6×1.5-2μ, and larger spores, 6.0-7.0×2.5-3.6μ, respectively, and in having different hosts, the sticklebacks. G. takedai which was found in the rainbow trout in Japan is distinguishable in having smaller spores, 2.8-4.9×1.7-2.3μ, and in not causing hypertrophy of the infected host cells.
Histopathological observation was made on 20 diseased rainbow trout fry and 5 amago salmon fry (O. rhodurus f. macrostomus). 1) Saprolegnia diclina infection: Fungal hyphae grew in the lumen of the pyloric region of the stomach. Hyphae extended from the gastric epithelium into the muscle layers in which they grew markedly. Further hyphae extended out from the stomach wall into the pancreas, pyloric caecae, spleen, abdominal cavity, abdominal musculature and visceral blood vessels in which hyphal emboli were formed frequently. 2) Infection with an unidentified fungus which had thin septate hyphae: Fungal hyphae markedly grew in the swim bladder. They invaded the wall of the anterior region of the swim bladder and extended into the liver, gall bladder, kidney, wall of the esophagus and the cardiac region of the stomach, body musculature and into the visceral blood vessels.
An epizootic of visceral mycosis occurred among hatchery-reared fry of Gifu Prefecture (TASHIRO et al., 1977). Two kinds of fungi were isolated from diseased amago salmon fry by inoculating materials from the abdominal cavity onto Sabouraud dextrose agar and incubating at 20°C. One kind of fungus, strain GFA 7501 was identified as Saprolegnia diclina according to SEYMOUR (1970) from its characteristics of the asexual reproductive stages and the sexual reproductive structures. Histopathological observations suggested that the primary site of infection of this fungus might be the pars pylorica of stomach. Another strain GFA 7502 could not be identified because evident sporulation was observed neither on Sabouraud dextrose agar nor on agar block in water culture. It had filamentous mycelia with septa. The hyphae was 4 to 12μ in diameter (Fig. 7). Spherical structures formed on hyphae after a prolonged period of time were usually 14 to 20μ in diameter. The character of the structures was unknown. Histopathological observations suggested that the primary site of infection of strain GFA 7502 which was named tentatively as an unidentified filamentous fungus with septa might be the pars cardiaca of stomach or the air bladder. Histopathological observations were made on 7 diseased amago salmon fry. Of the 7 fry, 5 were found to be infected with only S. diclina, while 2 fry showed a complicated infection of S. diclina and an unidentified filamentous fungus with septa. Effects of temperature, pH, malachite green and methylene blue on the growth of hyphae of the strain GFA 7501 and the strain GFA 7502 were also described in this paper (Table 2).
To know the effect of water temperature on the development of xenoma of Glugea plecoglossi inoculation experiments were conducted by utilizing two sources of water of different temperatures. Experimental animals were ayu fingerlings weighing an average of 0.68g. Experimental animals were infected with G. plecoglossi regardless of water temperature(12.8°Cand 18°C). The progress of schizogony and sporogony, however, was significantly affected by differences in temperature. In fish kept at temperatures below 16°C xenomas remained without showing active schizognoy in the submucosa and muscle layers of the intestinal wall.When the fish were transferred to 18deg;C and above schizogony became immediately active and xenomas started to hypertrophy and move to the abdominal cavity. While host responces became active and “glugea cysts”were formed. The progress of schizogony and sporogony were retarded when fish were transferred from higher temperature to lower temperatures below 18°C. It was suggested from these results that glugeasis of the ayu may be prevented by keeping the temperature of culturing water at 17°C and below.
A saprolegniasis occurred in a group of rainbow trout fingerlings of about 1.2 g in body weight at the Koide branch of the Niigata Prefectural Freshwater Fisheries Experimental Station. The initial sigh of the disease was an appearance of fin rot. Saprolegnia sp. was thought to infect secondarily on the area of fin rot. Six strains of the fungus were isolated from the base of the dorsal fin of 6 diseased fish. Inoculation experiments using moon fish showed that these six strains differed to some extent in pathogenicity from each other (Table 2.). Among the 6 strains strain N7619 was most virulent. All the strains were classified into Saprolegnia sp. according to SPARROW (1960) and DICK (1973) from the characteristics of the asexual organs on hemp seeds. Of the 6 strains, only strain N7619 formed sex organs. In this strain diclinous antheridial branches were predominantly produced, and the oogonial wall were pitted. The type of internal structure of oospores was subcentric. From these structures the strain was identified as S. australis according to SEYMOUR (1970). The characteristics of the sex organs of the strain also agreed with those of S. australis described originally by ELLIOTT (Table 3.).
It was found that yellowtails were readily infected with Pasteurella piscicida through the digestive tracts under an experimental condition. Healthy jevenile cultured yellowtails were used for experimentation. Newly isolated P.piscicida culture was stocked in a liquid nitrogen. container and was subcultivated just befboe use. A given quantity of the cells was put in a gelatin capsule together with some diets. The capsules were then administered directly to the stomack of the fish by means of a vinyle tube and piston rod. Groups of two or three fish were held in each 280 liter concrete tank with recircling and filtrating water system. All of the experimental fish were infected with the bacteria and died by the third to seventh day after administering the cells. Formation of white spots in the internal organs was observed in some victimes (Table 1 and 2). Recovery of P. piscicida from the blood of infected fish was investigated. Blood samples were serially taken from the fish by means of a syringe, introduced in Cuvier's tube, and living bacteria in the samples were counted by the plate method(Table 3). Recovery of P. piscicida from the digestive tracts of infected fish also was examined. The data obtained indicated that orally introduced bacteria were maintained in the intestine for long time(Table 4). Challenge of vaccinated yellowtail with a gastral administration of P. piscicida was tried. Experimental fish used were orally vaccinated with formalin-killed bacterin prepared from virulent P.piscicida broth-culture.The bacterin was given in a dose of 50 mg per fish per day for 21 days.The result suggested that oral vaccination with the killed bacterin was ineffective in the control of Pasteurelosis (Table 5).
Observations were made on eight outbreaks of a new fungus disease among fry of Salmo gairdneri, Onchorhynchus rhodurus and O. masou in several trout farms located in Giru prefecture from December, 1974 to May, 1975. Moribund fry were characterized with a distended abdomen and a heavy growth of mycelia in the abdominal cavity and in visceral organs. No growth of mycelium on the surface of the body was observed in most moribund fish. Mortalities due to the disease were estimated to be 10-20%.