Quantitative Analysis of Individual‐ and Group‐Level Viral Shedding Reveals Transmission Dynamics of Viral Hemorrhagic Septicemia Virus ( <scp>VHSV</scp> ) in Olive Flounder ( <scp> <i>Paralichthys olivaceus</i> </scp> )

ABSTRACT Viral hemorrhagic septicemia virus (VHSV) is a highly pathogenic rhabdovirus that primarily spreads through horizontal transmission. Characterizing viral shedding dynamics is therefore critical for understanding its epidemiology and improving disease control strategies in aquaculture. In this study, the median lethal dose (LD₅₀) of VHSV in olive flounder ( Paralichthys olivaceus ) was determined to be 10 3.7 TCID 50 /100 μL. We subsequently investigated viral shedding patterns at both individual and group levels under different initial infection ratios. Two distinct peaks in viral shedding were consistently observed at 6 to 8 and 10 to 12 days post‐challenge (dpc). In group tanks, the first peak was more pronounced, whereas in individual tanks, the second peak predominated, suggesting that the timing of mortality strongly influences shedding dynamics. Notably, moribund or deceased fish released substantially higher viral loads, with some individuals reaching 10 6 copies/400 mL on day 10, whereas surviving fish maintained relatively stable and much lower levels (&lt; 10 4 copies/400 mL) throughout the study. Among different infection ratios, cumulative mortality was highest in the 50% infection group (approximately 60% ~ 70% by 20 dpc). However, the 30% infection group exhibited the greatest viral shedding, indicating that viral release does not necessarily scale with infection prevalence. Instead, transmission dynamics appear to be disproportionately driven by a limited number of high‐shedding individuals capable of sustaining infection at the population level. Collectively, these findings demonstrate that viral shedding is concentrated in severely affected individuals and that individual shedding capacity, rather than infection prevalence alone, plays a dominant role in VHSV transmission. Early intervention targeting high‐shedding individuals may therefore be critical for effective disease control in aquaculture systems.