Quantification of the Relative Virulence of White Spot Syndrome Virus(WSSV) in the Penaeid Shrimps Litopenaeus vannamei (Boone, 1931) and Farfantepenaeus duorarum (Burkenroad, 1939) by Quantitative Real Time PCR

The relative virulence of the China isolate of white spot syndrome virus (WSSV-CN) in the penaeid shrimps Litopenaeus vannamei and Farfantepenaeus duorarum, was assessed by a comparison of 7-d median lethal dose (LD ), survival 50 curve, and mean lethal load after exposure by injection. Shrimps were injected intramuscularly with known WSSV dose. Median lethal dose of L. vannamei was lower than that of F. duorarum. Log LD in L.vannamei was 4.20 WSSV genome 50 -1 copies μg total DNA. Log LD in F.duorarum was 5.32 WSSV genome copies μg 50 1 4 total DNA. Median survival times of L. vannamei and F. duorarum injected with 10 5 and 10 WSSV genome copies were 54.17 h and 38.91 h, respectively for L. vannamei whereas they were 119.58 h and 82.67 h, respectively for F. duorarum. -1 Mean log of the WSSV lethal load for L. vannamei was 9.34(SE ± 9.09) copies μg of total DNA and for F. duorarum was 11.80 (SE ± 11.55). No significant difference was noted in lethal load for the shrimp species using Student's t-test. Overall mean 11 11 -1 WSSV lethal load was 2.86 x 10 (SE±1.63 x 10 ) genome copies μg of total DNA. In conclusion, WSSV was found to be less virulent in F.duorarum than in L. vannamei by LD and mean survival time but not in mean lethal load. This suggests that 50 shrimp resistance is imparted by controlling WSSV loads rather than by tolerating higher loads.