FIELD EFFICACY EVALUATION OF A FORMALIN-INACTIVATED WHITE SPOT SYNDROME VIRUS (WSSV)VACCINE FOR THE PREVENTIVE MANAGEMENT OF WSSV INFECTION IN BRACKISHWATER SHRIMP PONDS
A shrimp grow-out trial was conducted in six 500 m2 earthen ponds in Dumangas Brackishwater Station, in Iloilo, Philippines to evaluate the efficacy of an orally-administered formalin inactivated vaccine (FIV) as an antiviral strategy against WSSV. Ponds were prepared thoroughly by sun drying, lime application, and chlorine treatment. Purified virus prepared by sucrose gradient ultracentrifugation was inactivated by overnight incubation in 4% buffered formalin. PCR-screened Penaeus monodon postlarvae (PL14) obtained from a commercial hatchery were divided into two groups: 1) immersed in FIV solution for 2 h before stocking in ponds (VAC); and 2) unexposed control (CON). Both groups were stocked at a density of 10/m2 and grown for 115 days following recommended good practices. Pond water was aerated using two 20-HP electric blowers
Shrimp were given commercial feed following the manufacturer's protocol. They were booster-vaccinated by administering FIV-mixed feed every 15 days until the termination of the trial. Shrimp samples were regularly collected to obtain estimates of growth, survival, and feed conversion ratio. In addition, shrimp and extraneous crustacean species were analyzed for the presence of WSSV and IHHNV by PCR. Water quality and bacterial load were monitored regularly.
After 115 d of culture (DOC), shrimps grew to 12.9±3.02 g in VAC and 8.5±0.5 g in CON (Fig. 1A). Likewise, survival was higher in VAC (71.2±1.29%) compared to CON (52.7±9.6%). PCR analysis showed both shrimp and extraneous crustaceans to be lightly infected with WSSV and IHHNV. A few dead shrimp were noted during routine pond inspection as shrimp biomass increased towards the end of the trial. However, no mass mortalities occurred and shrimp were successfully harvested on 115 DOC. VAC and CON shrimp were also collected on days 70, 90, and 105 and experimentally infected with WSSV in tanks. Significant differences in survival rates (p<0.05) between the groups were obtained 15 d post-challenge (Fig. 1B). Further studies on prophylactic oral delivery of antiviral treatments are recommended.