High efficacy of white spot syndrome virus replication in tissues of freshwater rice-field crab, Paratelphusa hydrodomous (Herbst)

Species which may act as vectors or reservoirs of diseases covered by the Animal Health Law: Listed pathogens of crustaceans

Vector or reservoir species of three diseases of crustaceans listed in the Animal Health Law were identified based on evidence generated through an extensive literature review, to support a possible updating of Regulation (EU) 2018/1882. Crustacean species on or in which Taura syndrome virus (TSV), Yellow head virus (YHV) or White spot syndrome virus (WSSV) were identified, in the field or during experiments, were classified as reservoir species with different levels of certainty depending on the diagnostic tests used. Where experimental evidence indicated transmission of the pathogen from a studied species to another known susceptible species, the studied species was classified as vector species. Although the quantification of the risk of spread of the pathogens by the vectors or reservoir species was not part of the terms of reference, such risks do exist for the vector species, since transmission from infected vector species to susceptible species was proven. Where evidence for transmission from infected crustaceans was not found, these were defined as reservoirs. Nonetheless, the risk of the spread of the pathogens from infected reservoir species cannot be excluded. Evidence identifying conditions that may prevent transmission by vectors during transport was collected from scientific literature. It was concluded that it is very likely to almost certain (90-100%) that WSSV, TSV and YHV will remain infective at any possible transport condition. Therefore, vector or reservoir species that may have been exposed to these pathogens in an affected area in the wild or aquaculture establishments or by water supply can possibly transmit WSSV, TSV and YHV.

CqPP2A inhibits white spot syndrome virus infection by up-regulating antimicrobial substances expression in red claw crayfish Cherax quadricarinatus

Protein phosphatase 2A (PP2A) is an important serine/threonine phosphatase, a highly conserved enzyme widely expressed in eukaryotic cells, which accounts for a majority of the serine/threonine phosphatase activity in cells implicated in regulation of immune signaling pathways and antiviral response. However, most of studies about PP2A have been conducted in mammals but few in crustaceans. In this study, two subunits of PP2A (named as CqPP2Ab and CqPP2Ac) were characterized to be involved in white spot syndrome virus (WSSV) infection in the haematopoietic tissue (Hpt) cells from red claw crayfish Cherax quadricarinatus. The open reading frame (ORF) of CqPP2Ab was 1341 bp encoding 446 amino acids with seven WD40 domains, and the ORF of CqPP2Ac was 930 bp encoding 309 amino acids with a PP2Ac domain. Tissue distribution analysis showed that the mRNA transcript of CqPP2Ab and CqPP2Ac were both widely expressed in all the tested tissues with the highest expression in hemocyte, followed by high expression in Hpt. The gene expressions of CqPP2Ab and CqPP2Ac were both significantly down-regulated at 6 h post WSSV infection (6 hpi) in Hpt cells. Importantly, the expression of viral immediate early gene IE1 and late viral gene envelope protein VP28 were both significantly increased post WSSV infection after gene silencing of CqPP2Ab or CqPP2Ac in Hpt cells, suggesting that CqPP2Ab and CqPP2Ac could inhibit WSSV infection in Hpt cells, probably by increasing the antimicrobial substances expression in consideration to the significantly reduced expression of anti-lipopolysaccharide factor, crustin, and lysozyme after gene silencing of CqPP2Ab or CqPP2Ac, respectively. These findings provide a new light on the mechanism of WSSV infection and the antiviral response in crustaceans.

An Ns1abp-like gene promotes white spot syndrome virus infection by interacting with the viral envelope protein VP28 in red claw crayfish Cherax quadricarinatus

Influenza A virus non-structural-1A binding protein (named as Ns1abp) was originally identified as a host protein from human that bound to the viral NS-1 protein. In our previous study, the expression of an Ns1abp-like gene (denoted as CqNs1abp-like gene) was found to be up-regulated in a transcriptome library from the haematopoietic tissue (Hpt) cells of red claw crayfish Cherax quadricarinatus post white spot syndrome virus (WSSV) infection. To elucidate the role of CqNs1abp-like gene involved in WSSV infection, we cloned the CqNs1abp-like gene in which the open reading frame was 2232 bp, encoding 743 amino acids with two typical domains of one BTB (Broad-Complex, Tramtrack and Bric a brac) domain at N-terminal and six Kelch domains at C-terminal. The gene expression profile showed that the mRNA transcript of CqNs1abp-like gene was widely expressed in all the tested tissues with highest expression in nerve, relatively high expression in Hpt and lowest expression in eyestalk. Importantly, both the WSSV entry and the viral replication were significantly reduced in Hpt cells after gene silencing of CqNs1abp-like gene. By using protein pull-down assay, we found that the recombinant BTB domain, six Kelch domains and CqNs1abp-like intact protein were all bound to the WSSV envelope protein VP28, respectively, in which the BTB domain showed slightly less binding affinity than that of the six Kelch domains or the recombinant intact protein. Besides, the WSSV entry into Hpt cells was clearly decreased when the virus was pre-incubated with the recombinant BTB domain, six Kelch domains, or the recombinant CqNs1abp-like intact protein, respectively, suggesting that the CqNs1abp-like gene was likely to function as a putative recognition molecular towards WSSV infection in a crustacean C. quadricarinatus. Taken together, these data shed new light on the mechanism of WSSV infection and a putatively novel target on anti-WSSV infection in crustacean farming.

An Elegant Analysis of White Spot Syndrome Virus Using a Graphene Oxide/Methylene Blue based Electrochemical Immunosensor Platform

White spot syndrome virus (WSSV) is a major devastating virus in aquaculture industry. A sensitive and selective diagnostic method for WSSV is a pressing need for the early detection and protection of the aquaculture farms. Herein, we first report, a simple electrochemical immunosensor based on methylene blue dye (MB) immobilized graphene oxide modified glassy carbon electrode (GCE/GO@MB) for selective, quick (35 ± 5 mins) and raw sample analysis of WSSV. The immunosensor was prepared by sequential modification of primary antibody, blocking agent (bovine serum album), antigen (as vp28 protein), secondary antibody coupled with horseradish peroxidase (Ab2-HRP) on the GCE/GO@MB. The modified electrode showed a well-defined redox peak at an equilibrium potential (E_1/2), -0.4 V vs Ag/AgCl and mediated H₂O₂ reduction reaction without any false positive result and dissolved oxygen interferences in pH 7 phosphate buffer solution. Under an optimal condition, constructed calibration plot was linear in a range of 1.36 × 10^-3 to 1.36 × 10⁷ copies μL^-1 of vp28. It is about four orders higher sensitive than that of the values observed with polymerase chain reaction (PCR) and western blot based WSSV detection techniques. Direct electrochemical immunosensing of WSSV in raw tissue samples were successfully demonstrated as a real sample system.

The tidepool shrimp, Palaemon ritteri Holmes, constitutes a novel host to the white spot syndrome virus

The white spot syndrome virus (WSSV) is a lethal and contagious pathogen for penaeid shrimp and a growing number of other crustacean species. To date, there are no effective prophylactic or therapeutic treatments commercially available to interfere with the occurrence and spread of the disease. In addition, the significance of alternative vectors on the dispersal of this disease has been largely ignored and therefore the ecological dynamics of the WSSV is still poorly understood and difficult to ascertain. Thus, an important issue that should be considered in sanitary programmes and management strategies is the identification of species susceptible to infection by WSSV. The results obtained provide the first direct evidence of ongoing WSSV replication in experimentally infected specimens of the tidepool shrimp Palaemon ritteri. Viral replication was detected using a validated set of primers for the amplification by RT-PCR of a 141 bp fragment of the transcript encoding the viral protein VP28. It is therefore conceivable that this shrimp may play a significant role in the dispersal of WSSV.

Crayfish hematopoietic tissue cells but not hemocytes are permissive for white spot syndrome virus replication

Hemocytes are the major immune cells of crustaceans which are believed to be essential for the pathogenesis of white spot syndrome virus (WSSV) infection. Crayfish hemocytes and hematopoietic tissue (HPT) cells have been found to be susceptible to WSSV infection, but the procedure of WSSV infection to both cell types has not yet been carefully investigated. In this study, we analyzed the infection and proliferation of WSSV in crayfish hemocytes as well as HPT cells in detail through transmission electronic microscopy (TEM). The results showed that WSSV could enter both hemocytes and HPT cells through endocytosis, but the production of progeny virus was only achieved in HPT cells. Further investigation demonstrated that although WSSV could transcribe its genes in both cell types, viral genome replication and structural protein expression were unsuccessful in hemocytes, which may be responsible for the failure of progeny production. Therefore, we propose that both hemocytes and HPT cells are susceptible to WSSV infection but only HPT cells are permissive to WSSV replication. These findings will extend our knowledge of the interaction between WSSV and the host immune system.

Tissue distribution of hepatopancreatic parvo-like virus of shrimp in freshwater rice-field crab, Paratelphusa hydrodomous (Herbst)

An attempt was made to determine the replication efficiency of hepatopancreatic parvo-like virus (HPV) of shrimp in different organs of freshwater rice-field crab Paratelphusa hydrodomous (Herbst) using bioassay, PCR, RT-PCR, ELISA, Western blot and q-PCR analyses. Another attempt was made to use this crab as an alternative to penaeid shrimp for the large-scale production of HPV. This crab was found to be highly susceptible to HPV by intramuscular injection. The systemic HPV infection was confirmed by PCR and Western blot analyses in freshwater crab. The expression of capsid protein gene in different organs of infected crab was revealed by RT-PCR analysis. Indirect ELISA was used to quantify the capsid protein in different organs of the crab. The copy number of HPV in different organs of the infected crab was quantified by q-PCR. The results revealed a steady decrease in CT values in different organs of the infected crab during the course of infection. The viral inoculum that was prepared from different organs of the infected crab caused significant mortality in post-larvae of tiger prawn, Penaeus monodon (Fabricius). The results revealed that this rice-field crab could be used as an alternative host for HPV replication and also for large-scale production of HPV.