Induced resistance to white spot syndrome virus infection in Penaeus stylirostris through pre-infection with infectious hypodermal and hematopoietic necrosis virus—a preliminary study

Abstract

White spot syndrome virus (WSSV) is highly virulent to penaeid shrimp and has been responsible for serious economic losses on shrimp farms throughout the world. Infectious hypodermal and hematopoietic necrosis virus (IHHNV) is a small DNA virus that also infects penaeid shrimp; and, although once virulent to culture stocks of Penaeus stylirostris, it has not been associated with mass mortalities in recent years. Through three laboratory challenge studies, we discovered an interference interaction between these two viruses in P. stylirostris. In the first bioassay, juvenile P. stylirostris were infected with IHHNV by feeding them with IHHNV-infected tissue. These shrimp, along with a group of non-infected control shrimp, were then fed WSSV-infected tissue. Two days after the WSSV challenge, mortalities began to occur. All of the control shrimp, which were not exposed to IHHNV, died within 5 days. In contrast, replicated challenge groups of the IHHNV pre-infected shrimp had 31% and 44% survival. Quantitation by real-time PCR determined that the surviving shrimp had high levels (10 9 copies per μg DNA) of IHHNV and very low levels (50–400 copies per μg DNA) of WSSV DNA. Moribund shrimp had high levels (10 6–10 7 copies per μg DNA) of WSSV and low levels (10 4–10 6 copies per μg DNA) of IHHNV DNA. A second challenge study with P. stylirostris that were heavily infected with IHHNV showed 21 of 23 (91%) shrimp survived 19 days after exposure to WSSV. Ten of these surviving shrimp were re-exposed to WSSV-infected tissue, and six were still alive after 3 weeks. The third challenge study showed that 28% of the IHHNV pre-infected P. stylirostris survived 22 days after exposure to WSSV. In situ hybridization analysis in all the challenge studies confirmed that the surviving shrimp were strongly infected with IHHNV and had, at most, only low levels of WSSV infection. Laboratory WSSV challenges showed that there were no survivals in either IHHNV pre-infected Penaeus vannamei or in IHHNV-resistant P. stylirostris (SuperShrimp™). In conclusion, juvenile P. stylirostris that are highly infected with IHHNV show resistance to WSSV infection.