Differences of Viral Hemorrhagic Septicemia Virus Loads among Organs of Dead and Surviving Olive Flounder Infected by Intramuscular Injection and Immersion Challenge

Viral hemorrhagic septicemia virus (VHSV) is an important viral pathogen in the culture of Olive Flounder Paralichthys olivaceus . Based on cumulative mortality, the virulence of VHSV was found to be highly different depending on challenge routes and exposure doses (using tissue culture infectious dose with 50% endpoint [TCID50]). Olive Flounder were injected with VHSV at 10 2.5 , 10 4.5 , 10 6.5 , and 10 8.5 TCID50/100 μL/fish. A second group of fish was immersed at 10 3.5 , 10 5.5 , and 10 7.5 TCID50/mL at 10°C for 1 h in this study. The cumulative mortality was observed at 15 d postinfection. Immersion challenge at 10 3.5 TCID50/mL caused no mortality, while intramuscular injection challenge resulted in high levels of mortality with all VHSV exposure doses. Overall, Olive Flounder was susceptible to VHSV, with cumulative mortality of 90% or 100% in fish intramuscularly injected with high or low doses of VHSV. The cumulative mortality was 40% and 70% at 10 5.5 and 10 7.5 TCID50/mL, respectively, in the immersion challenge group. The VHSV titration and copy numbers were estimated by TCID50 and quantitative reverse transcription PCR methods. From dead Olive Flounder, VHSV titration was consistently detected in all tested organs, ranging from 10 5 to 10 9 TCID50/mL. The VHSV titration was under the detection limit from surviving Olive Flounder, but the VHSV N gene was detected.