Effect of water temperature on artificial infection of Rhabdovirus olivaceus (hirame rhabdovirus: HRV) to hirame (Japanese flounder, Paralichtys olivaceus)

Hirame (Japanese flounder; Paralichtys olivaceus) wihch had been artificially infected(I. P. injection, 105.30TCID50/fish)withRhabdovirus olivaceus(hirame rhabdovirus; HRV)were reared at 5, 10, 15 and 20°C, and the cumulative mortality was observed. Moribund fish were employed for histophathological and hematological study. Cumulative mortality of test groups reared at 5, 10, 15 and 20°C were 40, 60, 20, and 0%, respectively. Moribund fish showed typical signs of HRV infection. The highest virus titer was obtained from the fish of the 5°C test group, followed by the 10°C test group. Histopathologically, the kidney of all fish reared at 5°C and moribund fish reared at 10°C indicated necrotic changes and hemorrage in the hematopoietic tissue. Necrosis was most severe in the 5°C test group. The spleen of the 5°C test group showed necrosis and hemorrhage in the pulp. Similar necrosis was also observed in the spleen of the 10°C test group, but was less severe. Infiltration of macrophages into the spleen of the 15°C test group, and hyperplasia of meranomacrophage in the tissue of the 15 and 20°C test group were noted. Hematologically, the antibody neutralizing activity against HRV of the sera was generally not very high, but the activity was comparatively higher in high water temperature test groups than that of low temperature groups. The increase of immature erythrocytes at 5°C, small lymphocytelike cells at 10 and 15°C, neutrophiles at 15°C and lymphocyte-like cells at 20°C were observed in the blood smeares.