Studies on the epizootiology of MBV infection in P. monodon showed that the incidence of MBV in postlarval and juvenile P. monodon cultured in Taiwan increased from 15-17% in 1984-1986 to 80-85% in 1988. The infection rate of MBV in P. monodon broodstock populations investigated in 1988 was approximately 50%. A survey on cultured P. penicillatus in 1987 revealed 20-25% MBV infection rate. Studies of hatchery-reared postlarval P. monodon showed that MBV infection initiated mortality. Results from these pathogenic studies showed that environmental stressors significantly affect the survival of MBV-infected postlarvae, juvenile (s) and adult P. monodon and P. penicillatus. Histopathological observations performed in the present study revealed that mortality of P. monodon broodstocks may be caused by secondary infection of pathogenic bacteria.
Infectious hematopoietic necrosis virus (IHNV) was detected in mucus collected from the external surface of naturally infected juvenile and adult salmonids. Virus concentrations ranged from 101.0 to 106.0 plaque forming units (PFU) /ml. Juvenile rainbow trout Oncorhynchus mykiss artificially infected with IHNV by the waterborne route had a 100% prevalence of the virus in mucus and 50% in gill tissues 24 h post-exposure. Mucus examined from individual rainbow trout had increasing IHNV concentrations, 101.5 to 103.9 PFU/ml, from 24 to 84 h postinfection. This suggested IHNV detected in mucus resulted from the normal progression of the disease and the integument may be a site of virus replication and a possible portal of entry.
A severe anemia occurring in sea-cultured coho salmon (Oncorhynchus kisutch) has been associated with serious losses in the northern coastal water of Miyagi Prefecture. The death due to the disease has been recorded from February to May, since 1985. Clinical signs of the disease were pale gills, yellowish liver, blood retain in the atrium and watery blood. The hematological parameter of the diseased fish exhibited lower hematocrits, erythrocyte counts, hemoglobin concentrations, mean corpuscular hemoglobin concentration and greater numbers of immature erythrocytes in comparison to healthy fish. Blood smears showed a few bodies of inclusion like structure ranging in size from 1 to 2 μm and a large number of rods in the cytoplasm. Electron microscopy revealed virus particles scattered randomly in the cytoplasm, and marked swelling in mitochondria with occasional loss of mitochondrial cristae. It was thought that bodies of inclusion like structure were formed by deposited electron dense materials in the swollen mitochondria. Histopathological examination showed necrosis of muscle fibers in the ventricle and atrium, and deposits of hemosiderin and ceroid like materials were also found within macrophages in the spleen, kindney and liver. As a result, the cause of the disease was thought to be as follows : The virus first infected erythrocytes to make hemolysis, leading to hypoxemia, and then produced phospholipid peroxidation; finally heart lesions and hemolysis were progressed by hypoxemia and phospholipid peroxidation.
Cultured yellowtail with symptoms of prominent yellow discoloration of the body were histopathologically investigated. Material fish were obtained from two different areas (area A and area B) in Nagasaki Prefecture in 1987. In gross pathology, the most obvious change of the affected fish from both areas was the prominent yellow discoloration of mouth, orbit and abdominal region. The liver and spleen from the affected fish were enlarged and more breakable than normal. Pale discoloration of the gills was found in the diseased fish. Histologically, systemic deposition of a ceroid-like pigment and advanced hemolysis were observed in the diseased fish from area A. On the other hand, in the diseased fish from area B, focal necrosis and crystalloid substnces in the liver parenchyma were remarkable in addition to the similar findings observed in the diseased fish from area A. From the results of histological observations, it was suggested that there would exist larger amount of free fat peroxides in diseased fish showing yellow discoloration than in healthy fish.
Hemolysin production by β-Streptococcus sp. isolated from yellowtail, Seriola quinqueradiata was investigated. The isolated strain was inoculated into Todd-Hewitt broth (THB) at pH 7.0 or 7.8, or into brain heart infusion broth (BHIB) at pH 7.0 or 7.4. After inoculation, the inocula were incubated at 25°C with shaking. Also the bacterium was cultured with THB and BHIB at pH 7.0 supplemented with 0.1, 1 or 10% (v/v) fetal calf serum (FCS), or with 0.001, 0.01 or 0.1% (w/v) bovine serum albumin (BSA) with the same conditions mentioned above. The growth of the bacterium was estimated by measurement of absorbance at 620 nm. The hemolytic activity against sheep erythrocytes of culture supernatant was assayed by measurement of hemoglobin absorption at 541 nm. Hemolysin production was high, when the strain was cultured with THB at pH 7.0, and the maximum activity was found at 15 h after incubation, at late-logarithmic phase. In case of BHIB at pH 7.0, little hemolysin production was found. No hemolysin production was observed either with culture in THB at pH 7.8 or BHIB at pH 7.4. Using THB at pH 7.0, growth and hemolysin production was greatly increased by supplementation with 10% FCS, and was slightly increased by supplementation with 0.1% BSA.
Pronepheric leucocytes of Japanese eel, Anguilla japonica were classified on the basis of morphorogical, cytochemical and phagocytosing characteristics. Neutrophil, macrophage, lymphocyte and thrombocyte were discriminated by observing imprints of the pronephron. They consisted of 57.3 percent neutrophils, 19.3 percent macrophage, 21.8 percent lymphocyte and 1.6 percent other cells including thrombocyte. It is concluded that the pronephron is the major site of neutrophil and macrophage production in Japanese eel.
To reveal the relationship between the peripheral and pronephric neutrophils, their changes in number and PAS reaction were investigated in eel, Anguilla japonica. The eel were intraperitoneally injected with formalin-killed Escherichia coli or physiological saline. Twelve hours after injection of E. coli, the neutrophils in both pronephron and opisthonephron decreased in number to a minimum while peripheral neutrophils increased to a maximum. In the fish injected with physiological saline, the neutrophils reduced in the kidney, but the peripheral neutrophils did not show any significant change in number. Some changes in PAS reaction were observed in neutrophils of fish which had been injected with E. coli. PAS positive fine or coarse granules appeared in the cytoplasm of both kidney and peripheral neutrophils 12 hours after injection and became more conspicuous after 24 hours. In the fish injected with physiological saline, such neutrophils were not found for 24 hours but there appeared a few neutrophils possessing PAS positive granules after 48 or 72 hours. In conclusion, neutrophils in the blood were supposed to be supplied from the kidney in response to the bacterial stimulus.