Virulence Variation of White Spot Syndrome Virus in Pacific White Shrimp Litopenaeus vannamei

The virulence of seven geographic isolates of white spot syndrome virus (WSSV; genus Whispovirus; China (strain CH1995), Nicaragua (strain N2000), Honduras (strain H2000), Ecuador (strains E‐L1999 and E‐LT2002), and Mexico (strains M‐M2001 and M‐LP2001)) was compared using a series of challenge experiments, each lasting 10 d. For each isolate, four quantified dilutions (10−6, 10−7, 10−8, and 10−9) of a viral inoculum were prepared from WSSV‐infected shrimp tissue. Each viral inoculum was injected into 10 specific pathogen‐free juvenile Pacific white shrimp Litopenaeus vannamei (0.25–1.50 g); controls received injections of marine crustacean physiological saline (3.2%). The minimum dose of viral inoculum that killed 50% of injected shrimp (LD50) was calculated for dilution, tissue concentration, and viral DNA amount. The CH1995 and M‐M2001 isolates were the least virulent, with LD50 values of 10−6 to 10−7 of viral inoculum. The isolates could be grouped into three virulence clusters (CH1995 and M‐M2001; N2000 and E‐LT2002; and H2000, E‐L1999, and M‐LP2001). Virulence clusters were not altered by LD50 values based on viral DNA concentration, although a slight shifting of order in regards to virulence was seen among the three most virulent isolates (E‐L1999, H2000, and M‐LP2001). Overall, results indicate that there is a measurable virulence difference among WSSV isolates, which may correspond to geographical region.