Estimation of white spot syndrome virus severity grades dynamics in whiteleg shrimp (Penaeus vannamei) during the disease transition process.

White spot syndrome virus (WSSV), the causative agent of white spot disease (WSD), is a major threat to global shrimp aquaculture. Regular WSSV monitoring is essential for disease management, and severity grades may serve as indicators for evaluating shrimp health and anticipating the risk of outbreaks. This study aimed to elucidate the relationships among WSSV replication properties, severity grades, and disease transition dynamics in whiteleg shrimp using an intramuscular challenge (Exp. 1) to establish a controlled infection and immersion challenge (Exp. 2) to simulate the natural transmission. In Exp. 1, severity grades were associated with distinct viral replication phases, with the eclipse, logarithmic, and plateau phases corresponding to G0-G1, G2-G3, and G4 grades. In Exp. 2, the transition rate from asymptomatic to ill (λai) peaked at 40-48 h post-infection (hpi, 0.24), approximately 24 h earlier than the transition from ill to deceased (λid) at 64-72 hpi (0.28). Collectively, shrimp remained asymptomatic but shed virus during the eclipse phase (G0-G1; 0-48 hpi), whereas anorexia and moribundity appeared during the logarithmic phase (G2-G3; 48-72 hpi), followed by extensive mortality in the plateau phase (G4; 72-120 hpi). Accordingly, the results of the present study suggest that shrimp exhibiting G0-G1 grades are likely to represent chronic infection, whereas grades ≥ G2 indicate ongoing disease progression. The correlations identified between severity grades and disease transition provide a practical framework for interpreting longitudinal WSSV monitoring data and improving risk assessment and management in shrimp aquaculture.