Application of carriers and RNAi to enhance the antiviral immune response of shrimp to WSSV

English. We are indebted to the Government of Japan-Trust Fund for the grant funds (study code: FH-03-C2015T) and SEAFDEC/AQD management for the financial and moral support. We are also thankful to the Fish Health Section for assistance in various analyses.

English. In aquaculture, vaccination is one of the approaches for disease prevention and control. The aim of the present study was to determine the efficacy of a VP28 double stranded RNA (VP28 dsRNA) and recombinant VP28 protein (rVP28) administered together as an antiviral treatment against WSSV. Double-stranded RNA was produced in RNAsedeficient Escherichia coli HT115 following published methods. To determine the appropriate dose, different concentrations of dsRNA ranging between 0.2 μg and 20 μg, were either injected intramuscularly or delivered orally to the shrimp via the feed ration. Thereafter, the shrimp were challenged with WSSV either by injection (LD₅₀=10^-7 dilution of the gill tissue filtrate) or bath immersion (LD₅₀=10^-4 dilution of the filtrate) in glass aquaria and transferred to fiberglass tanks for daily monitoring and recording of mortalities. Results showed significant differences in survival between PBS and the 0.2, 10, and 20 μg dsRNA/shrimp doses. Time to 100 % mortality significantly differed among the treatments with the control reaching mortality earlier (day 4) while shrimp injected with 0.2 and 10 μg dsRNA succumbed to WSSV much later on days 9–12. Different frequencies of dsRNA administration were also tested. The best result obtained was a dose of 20 μg/shrimp administered 4 times over 28 days (2 times before and 2 times during challenge for a total 80 μg/shrimp). Finally, VP28 dsRNA was combined with rVP28 at ratios of 1:1, 1:2, 1:3, 3:1, and 2:1, entrapped in chitosan microparticles and delivered per os via the feed according to the dose and frequency as previously determined. Following bath exposure challenge with WSSV, the best survival obtained in trials 1 and 2 was 40 % and 43 % at 1:3 VP28 dsRNA to rVP28 ratio.