Studies on pathogenicity and prevalence of white spot syndrome virus in mud crab, Scylla serrata (Forskal), in Zhejiang Province, China.

Mud crab, Scylla serrata (Forskal), is the most commercially important marine crab species in China. In recent years, serious diseases have occurred in major mud crab culture regions in SE China. PCR detection of white spot syndrome virus (WSSV) in diseased mud crabs collected from Zhejiang Province during 2006-2008 showed a prevalence of 34.82%. To study the pathogenicity of WSSV to mud crab, healthy mud crabs were injected intramuscularly with serial 10-fold dilutions of a WSSV inoculum. The cumulative mortalities in groups challenged with 10⁻¹, 10⁻², 10⁻³ and 10⁻⁴ dilutions were 100%, 100%, 66.7% and 38.9% at 10 days post-injection, respectively. All moribund and dead mud crabs except the control group were positive for WSSV by PCR. Based on the viral load of the WSSV inoculum by quantitative real-time PCR, the median lethal dose (LD50) of WSSV in S. serrata was calculated as 1.10 × 10⁶ virus copies/crab, or 7.34 × 10³ virus copies g⁻¹ crab weight. The phenoloxidase, peroxidase and superoxide dismutase activities in haemolymph of WSSV-infected moribund crabs, were significantly lower than the control group, whereas alkaline phosphatase, glutamate-pyruvate transaminase and glutamic-oxaloacetic transaminase were higher than in the control group. WSSV was mainly distributed in gills, subcuticular epithelia, heart, intestine and stomach as shown by immunohistochemical analysis with Mabs against WSSV. The epithelial cells of infected gill showed hypertrophied nuclei with basophilic inclusions. Numerous bacilliform virus particles were observed in nuclei of infected gill cells by transmission electron microscopy. It is concluded that WSSV is a major pathogen of mud crab with high pathogenicity.