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

SIMPLE SUMMARY: White spot syndrome virus, which causes white spot disease, is the most prevalent crustacean pathogen. However, its waterborne transmission based on the correlation between disease severity grade and viral shedding rate has not yet been explored. In this study, we investigated the horizontal transmission model of white spot syndrome virus based on the correlation between disease severity grade and viral shedding rate. We also determined the minimum infective doses of white spot syndrome virus via the waterborne route. We found a positive correlation between the disease severity grade and viral shedding rate of infected shrimp, suggesting that waterborne transmission of white spot syndrome virus is influenced by the viral load and exposure period. Our results can aid in understanding the dynamics of white spot syndrome virus in pond culture systems and facilitate further studies on the interactions between species during outbreaks and the spread of white spot disease in pond culture systems. ABSTRACT: 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(3) copies/mg) and G2 (8.5 × 10(4) copies/mg), respectively. Furthermore, a positive linear correlation was observed between viral copies of pleopods and viral shedding rate (y = 0.7076x + 1.414; p < 0.001). Minimum infective doses of WSSV were determined via an immersion challenge. Infection was observed within 1, 3, and 7 d in 10(5)-, 10(3)-, and 10(1) copies/mL of seawater, respectively. In the cohabitation challenge, infection was observed within six days with viral loads of 10(1) to 10(2) 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.