Temperature-dependent shedding dynamics of viral hemorrhagic septicemia virus in olive flounder (Paralichthys olivaceus)

Abstract

Although the viral hemorrhagic septicemia virus (VHSV) is a significant aquatic disease, its shedding dynamics at different water temperatures remain insufficiently studied. In this study, we investigated VHSV infection and shedding patterns in infected olive flounder (Paralichthys olivaceus) at different water temperatures. Briefly, fish infected with VHSV were divided into two groups: (1) maintained at 8 °C, 15 °C, and 22 °C, and (2) exposed to gradually increasing (from 15 °C to 22 °C in 1 °C increments per day) and decreasing (from 22 °C to 15 °C in 1 °C decrements per day) temperatures. The spleens of five randomly selected fish and 1 L of rearing water were sampled to quantify VHSV using a previously developed quantitative polymerase chain reaction (qPCR) assay. The VHSV decay rate was assessed at 8 °C, 15 °C, and 22 °C for 14 days, and 50% tissue culture infectious dose (TCID50) viral titers were measured. Viral loads in fish spleens increased at 8 °C and decreased at 22 °C over time. At low water temperatures, VHSV loads in spleen were higher and had a strong positive correlation with viral shedding. Notably, only fish infected with VHSV at >106 copies per spleen mg−1 seemed to shed enough virus for detection. Viral loads in fish were significantly higher in the increasing temperature group than in the decreasing temperature group, indicating that the temperature at the time of infection was an essential factor in the development of the disease. When the water temperature rose above 18 °C in the increasing temperature group, viral loads rapidly decreased, and VHSV was not detected in the rearing water. Extended virus survival, especially in colder temperatures, highlights the risk of environmental persistence and potential waterborne transmission. Our results suggest a higher likelihood of horizontal VHSV transmission in fish when the viral load exceeds 106 copies and water temperature is below 18 °C. © 2024