Rey J. dela Calzada1 and Jeffrey M. Lotz2
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 (LD50), survival 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 of F. duorarum. Log LD50 in L. vannamei was 4.20 WSSV genome copies µg-1 total DNA. Log LD50 in F. duorarum was 5.32 WSSV genome copies µg-1 total DNA. Median survival times of L. vannamei and F. duorarum injected with 104 and 105 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. Mean log of the WSSV lethal load for L. vannamei was 9.34 (SE+9.09) copies µg-1 of tatal DNA and for F. duorarum was 11.80 (SE+11.55). No significant differences was noted in lethal load for the shrimp species using Student’s t-test. Overall mean WSSV lethal load was 2.86 x 1011 (SE+1.63 x 1011) genome copies µg-1 of total DNA. In conclusion, WSSV was found to be less virulent in F. duorarum than in L. vannamei by LD50 and mean survival time but not in mean lethal load. This suggests that shrimp resistance is imparted by controlling WSSV loads rather than by tolerating higher loads.
Key Words: WSSV-China isolate, median lethal dose (LD50), median survival times, lethal loads