Fate of viral hemorrhagic septicemia virus in Japanese flounder Paralichthys olivaceus challenged by immersion

Fate of viral hemorrhagic septicemia virus (VHSV) in tissues of Japanese flounder Paralichthys olivaceus (20.7 g in average body weight) challenged by immersion for 1 h with a flounder isolate (Obama 25, an American genotype) of VHSV at 10(4.0) TCID50/mL was monitored for 7 weeks by virus isolation method with FHM cell line. The mortality of Japanese flounder challenged by the same method in a companion experiment reached 68% at 13degreesC at week 6 post-immersion challenge (PIC). VHSV was detected in the gills just after the immersion, in the kidney, spleen and skin at day 1 PIC, and in all the tissues tested at day 3 PIC. The infective titers of VHSV higher than 10(9.0) TCID50/g were measured in the heart and blood as well as the foregoing tissues except skin at week 1 or 2 PIC. VHSV was not detected from most tissues at week 4 PIC except from the gills, heart, brain and blood, and at week 6 it was isolated only from the heart and brain. When the virus was not isolated from any tissues at week 7 PIC, the surviving fish were given stresses by 3 different methods, i.e. shaking of the fish container, or abrupt raise (13degreesC to 18degreesC) or fall (13degreesC to 8degreesC) in water temperature. As a result, VHSV was detected from the heart of some fish after the temperature stresses, indicating that the heart is the most probable hiding site of the host for VHSV.