White spot syndrome virus strains of different virulence induce distinct immune response in Cherax quadricarinatus

• Three WSSV strains differ in virulence were isolated.
• The high-virulent strain did not have a replication advantage over the low- and moderate-virulent strains.
• WSSV stains differ in virulence induced distinct immune response in crayfish.

In this study, we identified three white spot syndrome virus (WSSV) strains (WSSV-CN01, WSSV-CN02 and WSSV-CN03) with significant differences in virulence. Among them, WSSV-CN01 caused significant higher and earlier mortality in redclaw crayfish Cherax quadricarinatus, thus was determined as high-virulent, while WSSV-CN02 and WSSV-CN03 were moderate-virulent and low-virulent. By investigating the total number of the circulating haemocytes and the activity of immune relative enzymes, we demonstrated that the different virulent WSSV strains induced distinct immune response in the host. Notably, a dramatic reduction of circulating haemocytes was observed in the crayfish infected with WSSV-CN01 and WSSV-CN02 but not WSSV-CN03. Further analysis revealed that cell death induced by WSSV-CN01 and WSSV-CN02 might be responsible for the decrease of circulating haemocytes.