Anti-viral RNAi nanoparticles protect shrimp against white spot disease

Nearly 20% of cultured shrimp die every year due to viral diseases. In this study, we evaluated the capacity of nanoparticulate RNA interference (RNAi) to down-regulate genes in Penaeus vannamei shrimp and protect shrimp against white spot syndrome virus (WSSV, i.e. white spot disease). Using a reporter target gene, Rab7, we show that the length of the administered dsRNA correlates with gene knockdown. We found that 250 bp-long dsRNA strands knocked down gene expression most effectively, followed by 125 and 70 bp-long strands. The 21 bp long strands did not downregulate the gene. We also show gene downregulation to be concentration dependent. Even at low RNA concentrations of 0.01 μg per gram-body-weight gene knockdown exceeded 80%. Knockdown levels were similar in multiple organs, including the gills, gut, hepato-pancreas, pleopods and muscle. Gene knockdown lasted for one month, after which gene expression recuperated. To protect the RNA molecules from enzymatic degradation, we complexed the RNA with the cationic polysaccharide chitosan, forming 90-200 nm particles that facilitated efficient RNAi in vivo. In a double-blinded viral challenge test, RNAi targeting viral-protein 28 (VP28) protected the shrimp against WSSV infection. Survival of animals treated with RNAi nanoparticles exceeded 95% compared to no survival in the untreated controls. Nanoparticulate RNAi is an effective modality for protecting against viral diseases.