WSSV infection in shrimp is still a major problem for shrimp farmers. There is no proper treatment to control the WSSV infection in shrimp. It is important to develop anti-viral drugs against WSSV. Several investigations have been carried out in search of seaweed extract with a substantial anti-WSSV activity that can protect cultured shrimp against this virus. Seaweed extracts possess antiviral as well as immunostimulant characteristics (Güroy et al. 2022). Fucoidans extracted from different seaweeds reported to inhibit the replication of several enveloped virus which includes HIV, HSV, and human cytomegalovirus(Thuy et al. 2015) (Garrido et al. 2017). Turbinaria ornata illustrated biological activity such as antibacterial, antioxidant anti-inflammatory activity, and wound healing properties (Shaibi et al., 2021).
Earlier, several attempts were done to find anti-WSSV properties from seaweeds and plants. In this present study, T. ornata acetone extract was tested for antiviral activity against WSSV in freshwater crab P. hydrodomous. In this experiment, we found that the acetone extract of T. ornata showed strong activity against WSSV with 100% survival. In the positive control showed 100% mortality. A similar study was done with methanolic extract of Hypane spinella that showed antiviral activity against WSSV in freshwater crabs (Sundaram et al. 2014) The WSSV inoculum incubated with T. ornata extract effectively inactivated the virus replication by the interaction between the extract and the envelope proteins of the WSSV virus. Furthermore, the effect of the extract on WSSV replication prevents virus multiplication in host organisms. It is assumed that the immune mechanism is triggered by T. ornata extract which tries to combat against WSSV in the crab.
Sudheer et al.(2011) reported that aqueous extract of Ceriops tagal when administered as feed to P. monodon showed 74% survival rate against WSSV. The shrimp feed with ethanolic leaf extract of Pongamia pinnata showed 80% survival rate against WSSV (Rameshthangam and Ramasamy 2007). Fucoidan isolated from Sargassum wightii z fed to P. monodon enhanced the immunity and resistance against WSSV(Immanuel et al. 2012).Similarly sulfated galactans isolated from Gracilaria fisheri exhibited antiviral activity against WSSV in P.monodon and the mortality rate was lower (Wongprasert et al. 2014). It was reported that the hot water extract of Ulva instestinalis when supplemented as feed to Litopenaeus vannamei upregulated the immune response and inhibited WSSV (Klongklaew et al. 2021).
Dupuy et al., (2004) reported that pre incubation of WSSV with Mytillin for 3hours at room temperature showed anti-WSSV activity by decreasing the mortality in shrimp. In this study, the acetone extract of T. ornata administered along with WSSV was incubated for 3hours at room temperature showed virucidal activity suggesting the presence of molecules in extract that could render the virus inactive.
GC-MS analysis revealed that T. ornata acetone extract contains bioactive compounds that are considered to have anti-WSSV activity in P. hydrodomous. In this experiment pregnane-3,20-dione, 17,21-[(methylborylene)bis(oxy)]-, (5.beta)- was recorded highest in the acetone extract of T. ornata and this compound may be responsible for the suppression of WSSV. It has been reported that pregnane-3,20-dione, 17,21-[(methylborylene)bis(oxy)]-, (5.beta)- has anticancer activity (Parthasarathi et al. 2021).
A chemically synthesised compound methyl 1-chloro-7-methyl-2-propyl-1h-benzo[d] imidazole-5-carboxylate was tested against WSSV target protein VP28 by in-silico docking analysis (Karthikeyan et al. 2022). They also reported that the compound interacted with METB139 of VP28 envelope protein. According to pervious reports VP28 binds to shrimp cells as an attachment protein, allowing the virus to enter the cytoplasm (Yi et al. 2004).As a result blocking of this protein will prevent the entry of WSSV in host. Similarly docking of VP28 with A. marina derived phytochemicals exhibited the potential to block VP28 protein (Sahu et al. 2012). In this study in-silico analysis revealed that pregnane-3,20-dione, 17,21-[(methylborylene)bis(oxy)]-, (5.beta)- interacts with PROD:145 LYSD:147,ALAE:107 and VALE:172. of VP28 protein and it also showed highest binding affinity to the protein VP28 which can inhibit the replication of WSSV. It is proposed that the interaction between pregnane-3,20-dione, 17,21-[(methylborylene)bis(oxy)]-, (5. beta)- with VP28 can prevent the formation of PmRab-VP28 complex. There have been very few efforts to purify components responsible for anti-WSSV activity and all these studies focused primarily on crude extract from single seaweed or a combination of both. Hence, pregnane-3,20-dione, 17,21-[(methylborylene)bis(oxy)]-, (5. beta)- showed minimum docking score and could be considered for further in-vitro and in-vivo studies. The in-silico results further support the anti-WSSV activity of T. ornata acetone extract.