Evaluation of the Horizontal Transmission of White Spot Syndrome Virus for Whiteleg Shrimp ( Litopenaeus vannamei ) Based on the Disease Severity Grade and Viral Shedding Rate.

White spot syndrome virus (WSSV) is the most problematic pathogen in crustaceans. In this study, we investigated the horizontal transmission model of WSSV based on the correlation between the disease severity grade and viral shedding rate and determined the minimum infective dose of WSSV via the waterborne route. Intramuscular injection challenges at different doses and water temperatures revealed that the thresholds of viral shedding and mortality were G1 (3.1 × 10 copies/mg) and G2 (8.5 × 10 copies/mg), respectively. Furthermore, a positive linear correlation was observed between viral copies of pleopods and viral shedding rate (y = 0.7076x + 1.414; < 0.001). Minimum infective doses of WSSV were determined via an immersion challenge. Infection was observed within 1, 3, and 7 d in 10-, 10-, and 10 copies/mL of seawater, respectively. In the cohabitation challenge, infection was observed within six days with viral loads of 10 to 10 copies/mL of seawater, which further increased in the recipient group. Our results indicate a positive correlation between disease severity grade and viral shedding rate of infected shrimp and suggest that the waterborne transmission of WSSV depends on the viral load and exposure period.