Natural and experimental infection of white spot syndrome virus (WSSV) in benthic larvae of mud crab Scylla serrata

Incidence of mud crab reovirus (MCRV) outbreak in polyculture ponds of Andhra Pradesh, south east coast of India

Reovirus designated as Mud crab reovirus (MCRV) is associated with the mass mortalities of mud crabs resulting in significant economic loss to crab and shrimp-mud crab polyculture farmers in the Nagayalanka, Krishna district, Andhra Pradesh. The 100 % chronic mass mortalities have been attributed to the outbreak of Mud crab reovirus (MCRV) in the polyculture farms. The moribund crabs showed autotomy, discoloration of carapace, loss of appetite, slow movement and loose gills. Histopathological observations of the infected mud crabs showed an atrophied hepatopancreas, complete degeneration of tissues along with viral inclusions in hepatopancreas, gills and muscles. Further analysis using Transmission electron microscopy (TEM), showed that the viral particles had a diameter of 70 nm and exhibited a non-enveloped, icosahedral shape arranged in a crystalline manner. The virus mainly infects the connective tissue of hepatopancreas, gills, muscle and develops in the cytoplasm. RT-PCR reconfirmed the presence of reovirus in the hepatopancreas of spontaneously infected mud crab Scylla serrata. The current study shows the importance of monitoring the MCRV prevalence in polyculture farms to minimize its spread and precautionary measures can be taken by screening the brooders from the crab hatchery and stocking of wild crabs without screening should be avoided in order to prevent MCRV outbreak.

Experimental Infection Models and Their Usefulness for White Spot Syndrome Virus (WSSV) Research in Shrimp

White spot syndrome virus (WSSV) is marked as one of the most economically devastating pathogens in shrimp aquaculture worldwide. Infection of cultured shrimp can lead to mass mortality (up to 100%). Although progress has been made, our understanding of WSSV's infection process and the virus-host-environment interaction is far from complete. This in turn hinders the development of effective mitigation strategies against WSSV. Infection models occupy a crucial first step in the research flow that tries to elucidate the infectious disease process to develop new antiviral treatments. Moreover, since the establishment of continuous shrimp cell lines is a work in progress, the development and use of standardized in vivo infection models that reflect the host-pathogen interaction in shrimp is a necessity. This review critically examines key aspects of in vivo WSSV infection model development that are often overlooked, such as standardization, (post)larval quality, inoculum type and choice of inoculation procedure, housing conditions, and shrimp welfare considerations. Furthermore, the usefulness of experimental infection models for different lines of WSSV research will be discussed with the aim to aid researchers when choosing a suitable model for their research needs.

Identification and characterization of mitogen-activated protein kinase kinase 4 (MKK4) from the mud crab Scylla paramamosain in response to Vibrio alginolyticus and White Spot Syndrome Virus (WSSV)

Mitogen-activated protein kinase kinase 4 (MKK4), serves as a critical component of the mitogen-activated protein kinase signaling pathway, facilitating the direct phosphorylation and activation of the c-Jun N-terminal kinase (JNK) and p38 families of MAP kinases in response to environmental stresses. In the current research, we identified two MKK4 subtypes, namely SpMKK4-1 and SpMKK4-2, from Scylla paramamosain, followed by the analysis of their molecular characteristics and tissue distributions. The expression of SpMKK4s was induced upon WSSV and Vibrio alginolyticus challenges, and the bacteria clearance capacity and antimicrobial peptide (AMP) genes' expression upon bacterial infection were significantly decreased after knocking down SpMKK4s. Additionally, the overexpression of both SpMKK4s remarkably activated NF-κB reporter plasmid in HEK293T cells, suggesting the activation of the NF-κB signaling pathway. These results indicated the participation of SpMKK4s in the innate immunity of crabs, which shed light on a better understanding of the mechanisms through which MKK4s regulate innate immunity.

Identification of three novel Spätzle genes in Eriocheir sinensis and their roles during white spot syndrome virus infection

Proteins of Spätzle family play an essential role in innate immunity in invertebrates by activating the Toll pathway to induce the expression of antimicrobial peptides. However, little is known about the function of Spätzle in in the immune response of the Chinese mitten crab. In the present study, three novel Spätzle genes (named as EsSpz1, EsSpz2, and EsSpz3) were identified from Eriocheir sinensis. The genome structure of EsSpz1 contains two exons and an intron. Three Spätzle proteins all contain a Pfam Spaetzle domain. In the evolution, EsSpz1-3 cluster with other Spätzle proteins from crustaceans. EsSpz1-3 were widely distributed in multiple immune tissues. The expression levels of EsSpz1-3 in the intestine were remarkably upregulated after white spot syndrome virus (WSSV) challenge. The knockdown of EsSpz1-3 remarkably decreased the expressions of crustins and anti-lipopolysaccharide factors during WSSV infection. Moreover, EsSpz1-3 silencing remarkably increased the expression of WSSV envelope protein VP28. These findings suggest that new-found EsSpz1-3 in E. sinensis could promote the synthesis of antimicrobial peptides and inhibit the expression of VP28 during WSSV infection. Our study indicates that EsSpz1-3 in E. sinensis may participate in the innate immune defenses against WSSV by inducing the expression of antimicrobial peptides. This study provides new knowledge for the function of Spätzle in the antiviral immune defense in crustacean.

Full-Length Transcriptome Analysis Provides New Insights Into the Diversity of Immune-Related Genes in Portunus trituberculatus

Generally, invertebrates were thought to solely rely on their non-specific innate immune system to fight against invading microorganisms. However, increasing studies have implied that the innate immune response of invertebrates displayed diversity and specificity owing to the hyper-variable immune molecules in organisms. In order to get an insight into the diversity of immune-related genes in Portunus trituberculatus, a full-length transcriptome analysis of several immune-related tissues (hemocytes, hepatopancreas and gills) in P. trituberculatus was performed and the diversity of several immune-related genes was analyzed. The full-length transcriptome analysis of P. trituberculatus was conducted using a combination of SMRT long-read sequencing and Illumina short-read sequencing. A total of 17,433 nonredundant full-length transcripts with average length of 2,271 bp and N50 length of 2,841 bp were obtained, among which 13,978 (80.18%) transcripts were annotated. Moreover, numerous transcript variants of various immune-related genes were identified, including pattern recognition receptors, antimicrobial peptides, heat shock proteins (HSPs), antioxidant enzymes and vital molecules in prophenoloxidase (proPO)-activating system. Based on the full-length transcriptome analysis, open reading frames (ORFs) of four C-type lectins (CTLs) were cloned, and tissue distributions showed that the four CTLs were ubiquitously expressed in all the tested tissues, and mainly expressed in hepatopancreas and gills. The transcription of the four CTLs significantly increased in several immune-related tissues (hemocytes, hepatopancreas and gills) of P. trituberculatus challenged with Vibrio alginolyticus and displayed different profiles. Moreover, the four CTLs displayed distinct bacterial binding and antibacterial activities. The recombinant protein PtCTL-1 (rPtCTL-1) and rPtCTL-3 displayed bacterial binding and antibacterial activities against all tested bacteria. rPtCTL-2 only showed bacterial binding and antibacterial activities against V. alginolyticus. No obvious bacterial binding or antibacterial activities for PtCTL-4 was observed against the tested bacteria. This study enriches the transcriptomic information on P. trituberculatus and provides new insights into the innate immune system of crustaceans. Additionally, our study provided candidates of antibiotic agents for the prevention and treatment of bacteriosis.

Circulating Phylotypes of White Spot Syndrome Virus in Bangladesh and Their Virulence

White Spot Syndrome Virus (WSSV) has emerged as one of the most prevalent and lethal viruses globally and infects both shrimps and crabs in the aquatic environment. This study aimed to investigate the occurrence of WSSV in different ghers of Bangladesh and the virulence of the circulating phylotypes. We collected 360 shrimp (Penaeus monodon) and 120 crab (Scylla sp.) samples from the south-east (Cox’s Bazar) and south-west (Satkhira) coastal regions of Bangladesh. The VP28 gene-specific PCR assays and sequencing revealed statistically significant (p < 0.05, Kruskal–Wallis test) differences in the prevalence of WSSV in shrimps and crabs between the study areas (Cox’s Bazar and Satkhira) and over the study periods (2017–2019). The mean Log load of WSSV varied from 8.40 (Cox’s Bazar) to 10.48 (Satkhira) per gram of tissue. The mean values for salinity, dissolved oxygen, temperature and pH were 14.71 ± 0.76 ppt, 3.7 ± 0.1 ppm, 34.11 ± 0.38 °C and 8.23 ± 0.38, respectively, in the WSSV-positive ghers. The VP28 gene-based phylogenetic analysis showed an amino-acid substitution (E→G) at the 167th position in the isolates from Cox’s Bazar (referred to as phylotype BD2) compared to the globally circulating one (BD1). Shrimp PL artificially challenged with BD1 and BD2 phylotypes with filtrates of tissue containing 0.423 × 109 copies of WSSV per mL resulted in a median LT50 value of 73 h and 75 h, respectively. The in vivo trial showed higher mean Log WSSV copies (6.47 ± 2.07 per mg tissue) in BD1-challenged shrimp PL compared to BD2 (4.75 ± 0.35 per mg tissue). Crabs infected with BD1 and BD2 showed 100% mortality within 48 h and 62 h of challenge, respectively, with mean Log WSSV copies of 12.06 ± 0.48 and 9.95 ± 0.37 per gram tissue, respectively. Moreover, shrimp antimicrobial peptides (AMPs), penaeidin and lysozyme expression were lower in the BD1-challenged group compared to BD2 challenged shrimps. These results collectively demonstrated that relative virulence properties of WSSV based on mortality rate, viral load and expression of host immune genes in artificially infected shrimp PL could be affected by single aa substitution in VP28.

Co-Infection of Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) and White Spot Syndrome Virus (WSSV) in the Wild Crustaceans of Andaman and Nicobar Archipelago, India

The present study was intended to screen the wild crustaceans for co-infection with Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) and White Spot Syndrome Virus (WSSV) in Andaman and Nicobar Archipelago, India. We screened a total of 607 shrimp and 110 crab samples using a specific polymerase chain reaction, and out of them, 82 shrimps (13.5%) and 5 (4.5%) crabs were found positive for co-infection of IHHNV and WSSV. A higher rate of co-infection was observed in Penaeus monodon and Scylla serrata than other shrimp and crab species. The nucleotide sequences of IHHNV and WSSV obtained from crab in this present study exhibited very high sequence identity with their counterparts retrieved from various countries. Histopathological analysis of the infected shrimp gill sections further confirmed the eosinophilic intra-nuclear cowdry type A inclusion bodies and basophilic intra-nuclear inclusion bodies characteristics of IHHNV and WSSV infections, respectively. The present study serves as the first report on co-infection of WSSV and IHHNV in Andaman and Nicobar Archipelago, India and accentuates the critical need for continuous monitoring of wild crustaceans and appropriate biosecurity measures for brackishwater aquaculture.

Recombinant viral proteins delivered orally through inactivated bacterial cells induce protection in Macrobrachium rosenbergii (de Man) against White Tail Disease

White tail disease (WTD) is a disease of Macrobrachium rosenbergii caused by Macrobrachium rosenbergii nodavirus (MrNV) and extra small virus (XSV) with the potential to devastate the aquaculture industry. The present study aimed to explore the possible protection of M. rosenbergii against the disease by oral administration of bacterially expressed recombinant capsid proteins of MrNV and XSV. Juvenile M. rosenbergii were fed with the feed coated with inactivated bacteria encapsulated expressed recombinant viral proteins either individually or in combination for 7 days. Challenge studies using WTD causing agents were carried out after 3 (group I), 10 (group II) and 20 (group III) days post-feeding of viral proteins. Recombinant capsid protein of MrNV showed better protection when compared to other treatments with relative per cent survival of 62.5% (group I), 57.9% (group II) and 39.5% (group III). Treatment controls of groups I, II and III showed 100%, 95% and 95% mortality, respectively. The study demonstrates that oral administration of recombinant capsid proteins of MrNV and XSV provides effective protection against WTD in freshwater prawn.

Genomic and antibody-based assays for the detection of Indian strains of Macrobrachium rosenbergii nodavirus and extra small virus associated with white tail disease of Macrobrachium rosenbergii

White tail disease (WTD) of cultured Macrobrachium rosenbergii is caused by Macrobrachium rosenbergii nodavirus (MrNV) and extra small virus (XSV). Since both the viruses have small single strand RNA as genetic material with short generation time, they are more prone to mutations. Hence detection methods developed for one strain may be suboptimal for the detection of isolates from the different geographical locations. In the present study two new genomic based methods (RT-PCR and dot-blot hybridization) along with one immunological method (polyclonal antibodies based detection) were developed for the detection of Indian isolates of MrNV and XSV. Among genomic based methods, RT-PCR assay developed was most sensitive. Sensitivity of detection of RT-PCR was 1 fg (both MrNV and XSV) of total RNA extracted from purified viral inoculum preparation. In case of WTD positive whole tissue total RNA, the limit of detection was 10 fg for both MrNV and XSV. Dot-blot hybridization had a detection limit of 10 pg and 0.1 ng for MrNV and XSV respectively when RNA extracted from viral inoculum preparation was used; 0.1 ng and 1 ng when WTD positive whole tissue total RNA was used. Polyclonal antibodies against recombinant proteins (MrNV and XSV capsid) were synthesised. Western blotting and indirect ELISA revealed that the antibodies produced to be specific and highly sensitive. Recombinant protein (antigen) of MrNV and XSV capsid were detected at the dilution of 1:8000. However in case of infected prawn tissue sample, MrNV and XSV were detected at the dilution of 1:32,000 and 1:64,000 respectively. All methods developed are field applicable.

Elucidation of metabolic responses in mud crab Scylla paramamosain challenged to WSSV infection by integration of metabolomics and transcriptomics

White spot syndrome virus (WSSV) is a severe pathogen of mud crab Scylla paramamosain (S. paramamosain). Hemolymph, containing three types of hemocytes, is the key immunoregulatory tool of mud crab in response to pathogens. Herein, the metabonomics and transcriptomics analysis of hemocytes were adopted to investigate the immune response of S. paramamosain challenged to WSSV. We established the metabolic and transcriptional profiles of mud crab hemocytes with different treatments, including the control group (WT), WSSV early infected group (WSSV-6 h) and WSSV later infected group (WSSV-72 h). The results showed that 68 metabolites were dysregulated both in WSSV-infected mud crab of early stage and later stage, while 4452 genes were up-regulated and 9746 genes were down-regulated in WSSV-6 h, and 2016 genes were up-regulated and 6229 genes were down-regulated compared in WSSV-72 h. We found that several pathways were dysregulated at both metabolic and transcriptional levels, including ABC transporters, purine metabolism, taurine and hypotaurine metabolism in the WSSV early infected group, cysteine metabolism, methionine metabolism and biosynthesis of unsaturated fatty acids in the WSSV later infected group. In this context, through the integration of metabolomics and transcriptomics, our study provided a more comprehensive understanding of the biological process in mud crab against viral invasion.

Differential expression of microRNAs in mud crab Scylla paramamosain in response to white spot syndrome virus (WSSV) infection

Till date numerous microRNAs (miRNAs) have been discovered from various organisms, including fish, shellfish and crustaceans. The miRNAs are known to regulate immune functions in crustaceans, but little is known about the role of miRNAs against viral infection in crab. We performed small RNA sequencing to characterize the differentially expressed miRNAs in WSSV infected Scylla paramamosain, in comparison to that in uninfected crab, at 2 h and 12 h post infection. In total, 24 host miRNAs were up-regulated and 25 host miRNAs were down-regulated in response to WSSV infection at 2 h post infection. And 27 host miRNAs were up-regulated and 30 host miRNAs were down-regulated in response to WSSV infection at 12 h post infection. Further, the gene ontology analysis revealed that many signaling pathways were mediated by these miRNAs. The integral component of membrane is the most important biological process and endocytosis pathway is the most important pathway, which indicates that endocytosis is very important for WSSV infection. This study is one important attempt at characterizing crab miRNAs that response to WSSV infection, and will help unravel the miRNA pathways involved in antiviral immunity of crab.

The miRNAs profiling revealed by high-throughput sequencing upon WSSV infection in mud crab Scylla paramamosain

microRNAs (miRNAs) are known to regulate various immune functions by silencing the target genes in both vertebrates and invertebrates. However, in mud crab Scylla paramamosain, the role of miRNAs during the response to virus invasion remains unclear. To investigate the roles of miRNAs in S. paramamosain during virus infection, the mud crab was challenged with white spot syndrome virus (WSSV) and then subjected to the transcriptional analysis at different conditions. The results of high-throughput sequencing revealed that 940,379 and 1,306,023 high-quality mappable reads were detected in the hemocyte of normal and WSSV-infected mud crabs, respectively. Besides, the total number of 261 unique miRNAs were identified. Among them, 131 miRNAs were specifically expressed in the hemocytes of normal mud crabs, 46 miRNAs were specifically transcribed in those of WSSV-infected individuals, the other 84 miRNAs were expressed in both normal and WSSV-infected individuals. Furthermore, a number of 152 (89 down-regulated and 63 up-regulated) miRNAs were found to be differentially expressed in the WSSV-infected hemocytes, normalized to the controls. The identified miRNAs were subjected to GO analysis and target gene prediction and the results suggested that the differentially regulated miRNAs were mainly correlated with the changes of the immune responses of the hemocytes, including phagocytosis, melanism, and apoptosis as well. Taken together, the results demonstrated that the expressed miRNAs during the virus infection were mainly involved in the regulation of immunological pathways in mud crabs. Our findings not only enrich the understanding of the functions of miRNAs in the innate immune system but also provide some novel potential targets for the prevention of WSSV infection in crustaceans.

The role of Astakine in Scylla paramamosain against Vibrio alginolyticus and white spot syndrome virus infection

Astakine is a crucial factor in the proliferation and differentiation of hematopoietic stem cells and is directly involved in hematopoiesis in crustaceans. To assess the role of Astakine in the innate immune system of Scylla paramamosain, the immune responses in healthy and Astakine-inhibited S. paramamosain were investigated in the present study. The RNA transcripts of Astakine were widely distributed in all examined tissues, with significantly higher levels of expression in hemocytes of both healthy and challenged S. paramamosain with Vibrio alginolyticus and WSSV. When Astakine was knocked down by RNA interference technology, immune-related genes, including Janus kinase, prophenoloxidase, hemocyanin, β-actin, myosin II essential light chain-like protein, signal transducer and activator of transcription, Relish, and C-type-lectin, were significantly down-regulated in hemocytes. The levels of phenoloxidaseactivity (PO), total hemocyte counts (THC) and hemocyte proliferation decreased significantly in hemocytes of Astakine-dsRNA treated S. paramamosain. After being challenged with V. alginolyticus and WSSV, the THC decreased significantly and the levels of hemocyte apoptosis increased significantly in Astakine-dsRNA treated S. paramamosain in comparison with those in infected groups without Astakine-dsRNA treatment. After being challenged with WSSV, the WSSV copies were significantly lower in Astakine-dsRNA treated groups than those in the WSSV infection group, which suggested that knockdown of Astakine was not conductive to WSSV replication and this might be associated with the decreasing THC. The results of survival analysis showed that the survival rate of V. alginolyticus or WSSV infected S. paramamosain decreased significantly following Astakine knockdown. These results suggested that RNA interference of Astakine might weaken the resistance of S. paramamosain to V. alginolyticus or WSSV infection. The weaken resistivity after knockdown Astakine might be related to the changes of important immune-related gene expression, THC, PO activity, proliferation and apoptosis of hemocytes.

First detection of white spot syndrome virus (WSSV) in the mud shrimp Austinogebia edulis in Taiwan

The white spot syndrome virus (WSSV) causes mass mortalities in the aquaculture of shrimps worldwide. The mud shrimp Austinogebia edulis (Ngoc-Ho & Chan, 1992) is an economically important sea food item occurring along the west coast of Taiwan. While the population of A. edulis began to decrease with some fluctuations in the last decade, the current study aims to discover the causes for such sporadic population decline. This study explores the effects of microbial pathogens and innate immunity on the populations of A. edulis. Here, we report firstly about WSSV infection of A. edulis from the coastal zone of western Taiwan which is one of the possible causes of population decrease of A. edulis in Shengang. However, WSSV infection is not the only reason for its population decrease because a similar infection rate of WSSV was found in Wangong. Population changes may be related to both environmental pollution stress and WSSV. Both factors likely caused a massive reduction of hemocytes and an abnormal increase of phenoloxidase and superoxide dismutase activity, which were spectrophotometrically measured. Since there is no effective way to treat WSSV infection, improving the coastal environment appears the most effective way to increase the population size of feral shrimps.

Characterization of the innate immunity in the mud crab Scylla paramamosain

Mud crabs, Scylla paramamosain, are one of the most economical and nutritious crab species in China and South Asia. Inconsistent with the high development of commercial mud crab aquaculture, effective immunological methods to prevent frequently-occurring diseases have not yet been developed. Thus, high mortalities often occur throughout the different developmental stages of this species resulting in large economic losses. In recent years, numerous attempts have been made to use various advanced biological technologies to understand the innate immunity of S. paramamosain as well as to characterize specific immune components. This review summarizes these research advances regarding cellular and humoral responses of the mud crab during pathogen infection, highlighting hemocytes and gills defense, pattern recognition, immune-related signaling pathways (Toll, IMD, JAK/STAT, and prophenoloxidase (proPO) cascades), immune effectors (antimicrobial peptides), production of reactive oxygen species and the antioxidant system. Diseases affecting the development of mud crab aquaculture and potential disease control strategies are discussed.

Comparative transcriptomic analysis of crab hemocytes in response to white spot syndrome virus or Vibrio alginolyticus infection

To assess the immune response of hemocytes to white spot syndrome virus (WSSV) or Vibrio alginolyticus infection in the mud crab Scylla paramamosain, a transcriptome analysis was performed. We report the analysis of 45131 transcripts from S. paramamosain hemocytes by de novo assembly. A comparison with GenBank protein and nucleotide sequences identified 33699 genes as previously known. The length distribution of the genes was 8147 genes ≥200 bp, 4714 genes ≥300 bp, and 3517 genes ≥2000 bp. A total of 21579 simple sequence repeats (SSRs) were found in the transcriptomic dataset, including 9% monomers, 53.34% dimers and 32.55% trimers. A total of 13172 and 5087 differentially expressed transcripts were found in the V. alginolyticus-infected group and WSSV-infected group, respectively. Of these, 5920 transcripts were up-regulated and 7252 were down-regulated in the V. alginolyticus-infected crabs and 2302 transcripts were up-regulated and 2785 were down-regulated in the WSSV-infected crabs. Additionally, 3096 transcripts were differentially expressed simultaneously in the V. alginolyticus-infected crabs and the WSSV-infected crabs. Several known immune-related genes such as heat shock protein, Janus kinase, STAT, relish, caspase, Ca²⁺-transporting ATPase and lysosomal alpha-mannosidase were found among the differentially expressed transcripts. Transcription and its regulation were significant biological processes, and ATP binding and zinc ion binding were significant molecular functions. This is the first report of comparative transcriptomic analysis of crab hemocytes in response to WSSV or V. alginolyticus infection. These findings will contribute to our understanding of the immune response to WSSV and V. alginolyticus infection in crustaceans.

Experimental inoculation of oriental river prawn Macrobrachium nipponense with white spot syndrome virus (WSSV)

The oriental river prawn Macrobrachium nipponense is an economically important species that is widely farmed in China. White spot syndrome virus (WSSV) is one of the most devastating pathogens of the cultured shrimp Litopenaeus vannamei, responsible for massive loss of its commercial products worldwide. We investigated the infectivity and pathogenicity of WSSV in adult M. nipponense using standardized conditions for L. vannamei. The median lethal dose of WSSV in adult M. nipponense was 103.84±0.06 copies g-1, which was about 1000-fold higher than in L. vannamei (100.59±0.22 copies g-1). WSSV was detected by 2-step PCR in the gills, hepatopancreas, muscle, stomach, heart, gut, nerve, integument, pereopod, eyestalk, testis, and ovary of experimentally infected dead M. nipponense. Lesions were observed histologically following WSSV injection, showing basophilic intranuclear inclusion bodies in the hepatopancreas and subsequently in the gills. The clearance of WSSV was observed in hepatopancreas and gills at 48 and 96 h post-inoculation, respectively. No histological lesions were detected in muscle from 0-96 h post-injection. The results show that the oriental river prawn M. nipponense can be infected by WSSV and the infections are self limiting over time; therefore, M. nipponense may serve as a useful model for studying resistance to WSSV.

A Proteomic Study of Hemocyte Proteins from Mud Crab (Scylla paramamosain) Infected with White Spot Syndrome Virus or Vibrio alginolyticus

In this study, we investigated the hemocytes' immune response to white spot syndrome virus (WSSV) or Vibrio alginolyticus infection at the protein level. The differential proteomes from crab hemocytes infected with WSSV or V. alginolyticus were analyzed using the isobaric tags for relative and absolute quantitation approach immediately after infection. Using this approach, we identified 1,799 proteins by their by LC-MS/MS spectra and sequencing data. These included 157 upregulated proteins and 164 downregulated proteins after WSSV infection. Similarly, 243 proteins were determined to be differentially expressed during V. alginolyticus infection, of these, 121 were upregulated and 122 were downregulated after infection. Interestingly, among these differentially expressed proteins, 106 were up- or downregulated significantly in both WSSV and V. alginolyticus infection. Six genes, β-actin, myosin-9, anti-lipopolysaccharide factor isoform 4, anti-lipopolysaccharide factor 4, transketolase-like protein 2-like isoform 1, and sarcoplasmic calcium-binding protein 1 were chosen for further study. The expression of these genes all showed a trend of upregulation at 24 h post-WSSV or V. alginolyticus infection except for myosin-9 in response to WSSV. To confirm the protective effects of the six genes, crabs were injected with specific dsRNAs before WSSV or V. alginolyticus challenge. The results showed that the knockdown of these genes led to an increase in the morbidity and mortality (P < 0.01) rate, and a decrease in infection time in WSSV-infected crabs. During the first 84 h, knockdown of these genes also led to an increase in the morbidity rates in V. alginolyticus -infected crabs, and results of four genes showed a higher mortality rate than that of the control after they were knocked down. This is the first report of the proteome response in crab hemocytes during WSSV or V. alginolyticus infection. These findings will contribute to our understanding of the immune response to WSSV and V. alginolyticus infection in crabs.

Experimental inoculation of Louisiana red swamp crayfish Procambarus clarkii with white spot syndrome virus (WSSV)

The red swamp crayfish Procambarus clarkii represents an important aquaculture species responsible for over half of all commercial aquaculture profits in Louisiana, USA. White spot syndrome virus (WSSV) is highly pathogenic in crustacean species and induces mass mortality in aquaculture operations worldwide. Natural outbreaks of WSSV occur yearly in cultured populations of crayfish in Louisiana. The goal of this study was to better understand the infectivity of WSSV in P. clarkii, by determining the minimum lethal dose necessary to initiate infection and to measure the resulting cumulative mortality following infection with different doses. A real time quantitative PCR (qPCR) method was used to detect WSSV in DNA extracted from gill tissue to ensure P. clarkii study populations were WSSV-free before the start of trials. Viable viral particles were isolated from naturally infected P. clarkii gill tissue and quantified using a novel digital PCR approach. Three infectivity trials were performed, and WSSV inocula were created by serial dilution, generating 5 treatments per trial. Five crayfish (weighing ~25 g) per dilution per trial received viral inoculations. Mortality was monitored daily for the duration of the trial in order to construct a median lethal dose (LD50) curve, and probit regression analysis was used to determine LD50 concentrations of viral particles. Knowledge of the infectivity of WSSV in native crayfish populations is of critical importance to the management of the commercial crayfish aquaculture industry in Louisiana. This is the first study to investigate the infectivity and to determine the LD50 of the Louisiana strain of WSSV in native crayfish.

Molecular Mechanisms of White Spot Syndrome Virus Infection and Perspectives on Treatments

Since its emergence in the 1990s, White Spot Disease (WSD) has had major economic and societal impact in the crustacean aquaculture sector. Over the years shrimp farming alone has experienced billion dollar losses through WSD. The disease is caused by the White Spot Syndrome Virus (WSSV), a large dsDNA virus and the only member of the Nimaviridae family. Susceptibility to WSSV in a wide range of crustacean hosts makes it a major risk factor in the translocation of live animals and in commodity products. Currently there are no effective treatments for this disease. Understanding the molecular basis of disease processes has contributed significantly to the treatment of many human and animal pathogens, and with a similar aim considerable efforts have been directed towards understanding host-pathogen molecular interactions for WSD. Work on the molecular mechanisms of pathogenesis in aquatic crustaceans has been restricted by a lack of sequenced and annotated genomes for host species. Nevertheless, some of the key host-pathogen interactions have been established: between viral envelope proteins and host cell receptors at initiation of infection, involvement of various immune system pathways in response to WSSV, and the roles of various host and virus miRNAs in mitigation or progression of disease. Despite these advances, many fundamental knowledge gaps remain; for example, the roles of the majority of WSSV proteins are still unknown. In this review we assess current knowledge of how WSSV infects and replicates in its host, and critique strategies for WSD treatment.

Characterization of fossilized relatives of the White Spot Syndrome Virus in genomes of decapod crustaceans

Background

The White Spot Syndrome Virus (WSSV) is an important pathogen that infects a variety of decapod species and causes a highly contagious disease in penaeid shrimps. Mass mortalities caused by WSSV have pronounced commercial impact on shrimp aquaculture. Until now WSSV is the only known member of the virus family Nimaviridae, a group with obscure phylogenetic affinities. Its isolated position makes WSSV studies challenging due to large number of genes without homology in other viruses or cellular organisms.

Results

Here we report the discovery of an unusually large amount of sequences with high similarity to WSSV in a genomic library from the Jamaican bromeliad crab Metopaulias depressus. De novo assembly of these sequences allowed for the partial reconstruction of the genome of this endogenized virus with total length of 200 kbp encompassed in three scaffolds. The genome includes at least 68 putative open reading frames with homology in WSSV, most of which are intact. Among these, twelve orthologs of WSSV genes coding for non-structural proteins and nine genes known to code for the major components of the WSSV virion were discovered. Together with reanalysis of two similar cases of WSSV-like sequences in penaeid shrimp genomic libraries, our data allowed comparison of gene composition and gene order between different lineages related to WSSV. Furthermore, screening of published sequence databases revealed sequences with highest similarity to WSSV and the newly described virus in genomic libraries of at least three further decapod species. Analysis of the viral sequences detected in decapods suggests that they are less a result of contemporary WSSV infection, but rather originate from ancestral infection events. Phylogenetic analyses suggest that genes were acquired repeatedly by divergent viruses or viral strains of the Nimaviridae.

Conclusions

Our results shed new light on the evolution of the Nimaviridae and point to a long association of this viral group with decapod crustaceans.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0380-7) contains supplementary material, which is available to authorized users.

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.

Rapid detection of shrimp white spot syndrome virus by real time, isothermal recombinase polymerase amplification assay

White spot syndrome virus (WSSV) causes large economic losses to the shrimp aquaculture industry, and thus far there are no efficient therapeutic treatments available against this lethal virus. In this study, we present the development of a novel real time isothermal recombinase polymerase amplification (RPA) assay for WSSV detection on a small ESEQuant Tube Scanner device. The RPA sensitivity, specificity and rapidity were evaluated by using a plasmid standard as well as viral and shrimp genomic DNAs. Compared with qPCR, the RPA assay revealed more satisfactory performance. It reached a detection limit up to 10 molecules in 95% of cases as determined by probit analysis of 8 independent experiments within 6.41 ± 0.17 min at 39 °C. Consequently, this rapid RPA method has great application potential for field use or point of care diagnostics.

Production of recombinant capsid protein of Macrobrachium rosenbergii nodavirus (r-MCP43) of giant freshwater prawn, M. rosenbergii (de Man) for immunological diagnostic methods

White tail disease (WTD) caused by Macrobrachium rosenbergii nodavirus (MrNV) and extra small virus (XSV) is a serious problem in prawn hatcheries. The gene for capsid protein of MrNV (MCP43) was cloned into pRSET B expression vector. The MCP43 protein was expressed as a protein with a 6-histidine tag in Escherichia coli GJ1158 with NaCl induction. This recombinant protein, which was used to raise the antiserum in rabbits, recognized capsid protein in different WTD-infected post-larvae and adult prawn. Various immunological methods such as Western blot, dot blot and ELISA techniques were employed to detect MrNV in infected samples using the antiserum raised against recombinant MCP43 of MrNV. The dot blot assay using anti-rMCP43 was found to be capable of detecting MrNV in WTD-infected post-larvae as early as at 24 h post-infection. The antiserum raised against r-MCP43 could detect the MrNV in the infected samples at the level of 100 pg of total protein. The capsid protein of MrNV estimated by ELISA using anti-rMCP43 and pure r-MCP43 as a standard was found to increase gradually during the course of infection from 24 h p.i. to moribund stage. The results of immunological diagnostic methods employed in this study were compared with that of RT-PCR to test the efficiency of antiserum raised against r-MCP43 for the detection of MrNV. The Western blot, dot blot and ELISA detected all MrNV-positive coded samples as detected by RT-PCR.

Variations in biochemical and histological characteristics of WSSV infected green tiger shrimp Penaeus semisulcatus

White Spot Syndrome Virus causes viral disease in crustaceans and generates a significant burden in the developing nations. Biochemical and immunological assays were performed in WSSV infected Penaeus semisulcatus which were monitored in different salinity conditions. Continuous exposure of shrimps to WSSV showed a reduced life span, indicating the pathogenicity in Penaeidae species. Hence, this study is intended to investigate the protective antioxidant potential of the innate immune system consisting biochemical and morphological alterations. Penaeus semisulcatus challenged with white spot syndrome virus (5.5 × 10(4) copy number; WSSV) reared at different salinity 5, 15, 25 (control) and 35 g/L were examined after 0-120 h for immunological parameters such as total hemocyte count (THC), phenoloxidase (PO) and respiratory burst (RB) and alkaline and acid phosphatase activities. After 72 h, the WSSV injected P. semisulcatus tissues were histopathologically sectioned and stained. This study would be helpful to understand host-pathogen interaction and envisages the improvement of better management practices in shrimp aquaculture system.

White spot syndrome virus epizootic in cultured Pacific white shrimp Litopenaeus vannamei (Boone) in Taiwan

White spot syndrome virus (WSSV) has caused significant losses in shrimp farms worldwide. Between 2004 and 2006, Pacific white shrimp Litopenaeus vannamei (Boone) were collected from 220 farms in Taiwan to determine the prevalence and impact of WSSV infection on the shrimp farm industry. Polymerase chain reaction (PCR) analysis detected WSSV in shrimp from 26% of farms. Juvenile shrimp farms had the highest infection levels (38%; 19/50 farms) and brooder shrimp farms had the lowest (5%; one of 20 farms). The average extent of infection at each farm was as follows for WSSV-positive farms: post-larvae farms, 71%; juvenile farms, 61%; subadult farms, 62%; adult farms, 49%; and brooder farms, 40%. Characteristic white spots, hypertrophied nuclei and basophilic viral inclusion bodies were found in the epithelia of gills and tail fans, appendages, cephalothorax and hepatopancreas, and virions of WSSV were observed. Of shrimp that had WSSV lesions, 100% had lesions on the cephalothorax, 96% in gills and tail fans, 91% on appendages and 17% in the hepatopancreas. WSSV was also detected in copepoda and crustaceans from the shrimp farms. Sequence comparison using the pms146 gene fragment of WSSV showed that isolates from the farms had 99.7-100% nucleotide sequence identity with four strains in the GenBank database--China (AF332093), Taiwan (AF440570 and U50923) and Thailand (AF369029). This is the first broad study of WSSV infection in L. vannamei in Taiwan.

Protection of Macrobrachium rosenbergii against white tail disease by oral administration of bacterial expressed and encapsulated double-stranded RNA

White tail disease (WTD) of cultured Macrobrachium rosenbergii is caused by M. rosenbergii nodavirus (MrNV) and an extra small virus (XSV), both present together, and the mortality rate can be as high as 100% within 2 or 3 days of infection. Possible protection of M. rosenbergii against WTD by oral administration of bacterial expressed and encapsulated double-stranded RNA (dsRNA) was studied. Juvenile M. rosenbergii were fed with the feed coated with inactivated bacteria encapsulated dsRNA of MrNV and XSV genes individually and in combination for 7 days followed by challenge with WTD causing agents at 24 h and 72 h post-feeding. Test animals fed with a combination of dsRNA of MrNV and XSV capsid genes showed the highest relative percent survival (RPS) when compared to other treatments with RPS of 80% and 75% at 24 and 72 h respectively. One hundred percent mortality was observed in test animals fed with control dsRNA coated feed. Although in the literature, injection is the most common method used to deliver dsRNA, this study shows that oral administration is effective, feasible and economical.

Transcriptome analysis on Chinese shrimp Fenneropenaeus chinensis during WSSV acute infection

Previous studies have discovered a lot of immune-related genes responding to white spot syndrome virus (WSSV) infection in crustacean. However, little information is available in relation to underlying mechanisms of host responses during the WSSV acute infection stage in naturally infected shrimp. In this study, we employed next-generation sequencing and bioinformatic techniques to observe the transcriptome differences of the shrimp between latent infection stage and acute infection stage. A total of 64,188,426 Illumina reads, including 31,685,758 reads from the latent infection group and 32,502,668 reads from the acute infection group, were generated and assembled into 46,676 unigenes (mean length: 676 bp; range: 200-15,094 bp). Approximately 24,000 peptides were predicted and classified based on homology searches, gene ontology, clusters of orthologous groups of proteins, and biological pathway mapping. Among which, 805 differentially expressed genes were identified and categorized into 11 groups based on their possible function. Genes in the Toll and IMD pathways, the Ras-activated endocytosis process, the RNA interference pathway, anti-lipopolysaccharide factors and many other genes, were found to be activated in shrimp from latent infection stage to acute infection stage. The anti-bacterially proPO-activating cascade was firstly uncovered to be probably participated in antiviral process. These genes contain not only members playing function in host defense against WSSV, but also genes utilized by WSSV for its rapid proliferation. In addition, the transcriptome data provides detail information for identifying novel genes in absence of the genome database of shrimp.

In vitro propagation of hepatopancreatic parvo-like virus (HPV) of shrimp in C6/36 (Aedes albopictus) cell line

Hepatopancreatic parvovirus (HPV) which causes infection in many species of penaeid shrimp is a serious viral pathogen in the young life stages of shrimp. An attempt was made to develop an in vitro system using C6/36 subclone of Aedes albopictus cell line for propagation of HPV. The results revealed that C6/36 cells were susceptible to this virus and the infected cells showed CPE in the form of vacuole formation. The results of PCR, immunocytochemistry and Western blot revealed the HPV-infection in C6/36 cell line. The RT-PCR analysis confirmed the replication of HPV in C6/36 cell line. The HPV load was quantified at different time intervals by ELISA and real time PCR, and the results showed the increase of viral load in C6/36 cell line in time course of infection. HPV propagated in C6/36 cell line was used to infect post-larvae of shrimp and the results showed that the twentieth passage of HPV propagated in C6/36 cell line caused 100% mortality in post-larvae after 6 weeks post infection (d.p.i.). The infected post-larvae showed clinical signs of reduced growth, reduced preening, muscle opacity and atrophy of hepatopancreas. The HPV-infection was confirmed by PCR. The results of the present study showed that C6/36 cell line can be used as an in vitro model for HPV replication instead of whole animal.

Exploring Pandora's box: potential and pitfalls of low coverage genome surveys for evolutionary biology

High throughput sequencing technologies are revolutionizing genetic research. With this “rise of the machines”, genomic sequences can be obtained even for unknown genomes within a short time and for reasonable costs. This has enabled evolutionary biologists studying genetically unexplored species to identify molecular markers or genomic regions of interest (e.g. micro- and minisatellites, mitochondrial and nuclear genes) by sequencing only a fraction of the genome. However, when using such datasets from non-model species, it is possible that DNA from non-target contaminant species such as bacteria, viruses, fungi, or other eukaryotic organisms may complicate the interpretation of the results. In this study we analysed 14 genomic pyrosequencing libraries of aquatic non-model taxa from four major evolutionary lineages. We quantified the amount of suitable micro- and minisatellites, mitochondrial genomes, known nuclear genes and transposable elements and searched for contamination from various sources using bioinformatic approaches. Our results show that in all sequence libraries with estimated coverage of about 0.02–25%, many appropriate micro- and minisatellites, mitochondrial gene sequences and nuclear genes from different KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways could be identified and characterized. These can serve as markers for phylogenetic and population genetic analyses. A central finding of our study is that several genomic libraries suffered from different biases owing to non-target DNA or mobile elements. In particular, viruses, bacteria or eukaryote endosymbionts contributed significantly (up to 10%) to some of the libraries analysed. If not identified as such, genetic markers developed from high-throughput sequencing data for non-model organisms may bias evolutionary studies or fail completely in experimental tests. In conclusion, our study demonstrates the enormous potential of low-coverage genome survey sequences and suggests bioinformatic analysis workflows. The results also advise a more sophisticated filtering for problematic sequences and non-target genome sequences prior to developing markers.

Biology, Host Range, Pathogenesis and Diagnosis of White spot syndrome virus

White spot syndrome virus (WSSV) is the most serious viral pathogen of cultured shrimp. It is a highly virulent virus that can spread quickly and can cause up to 100 % mortality in 3-10 days. WSSV is a large enveloped double stranded DNA virus belonging to genus Whispovirus of the virus family Nimaviridae. It has a wide host range among crustaceans and mainly affects commercially cultivated marine shrimp species. The virus infects all age groups causing large scale mortalities and the foci of infection are tissues of ectodermal and mesodermal origin, such as gills, lymphoid organ and cuticular epithelium. The whole genome sequencing of WSSV from China, Thailand and Taiwan have revealed minor genetic differences among different strains. There are varying reports regarding the factors responsible for WSSV virulence which include the differences in variable number of tandem repeats, the genome size and presence or absence of different proteins. Aim of this review is to give current information on the status, host range, pathogenesis and diagnosis of WSSV infection.

White spot syndrome virus: Genotypes, Epidemiology and Evolutionary Studies

White spot syndrome virus (WSSV) is a pathogen that has emerged globally affecting shrimp populations. Comparison of WSSV genome have shown the virus to share a high genetic similarity except for a few variable genomic loci that has been employed as markers in molecular epidemiology studies for determining the origin, evolution and spread in different geographical regions. Molecular genotyping of WSSV are based on genomic deletions associated with ORF23/24 and ORF14/15 variable regions and the three variable number of tandem repeat regions, ORF75, ORF94 and ORF125. Studies show the prevalence of several genotypes for WSSV with particular genotypes being more prevalent than others in a given geographical area. Deletions associated with ORF23/24 and ORF14/15 variable regions have proven to be of evolutionary significance. Fitness and virulence studies on different genotypes of WSSV suggest that all the strains of WSSV are equally virulent, but the one with smaller genomic size is the fittest. Studies also have shown that mixed genotype infection of WSSV correlates with lower disease outbreaks. This review focuses on the genotyping studies that were undertaken in elucidating WSSV evolution and epidemiology.

Sodium alginate from Sargassum wightii retards mortalities in Penaeus monodon postlarvae challenged with white spot syndrome virus

Sodium alginate extracted from brown seaweed Sargassum wightii (16.35 ± 1.42%, mean [±SD] yield from 5 extractions) was prepared as a powder or beads and used to enrich Artemia nauplii at concentrations of 100, 200, 300 and 400 mg l-1. The alginate-enriched nauplii were fed to Penaeus monodon shrimp postlarvae (PL) stage 15 (PL15, i.e. 15 d old) for 20 d. Mean weight gain and specific growth rate over this period were 0.24 g and 15.8%, respectively, in PL groups not fed alginate, and 0.20-0.28 g and 14.7-16.5%, respectively, in PL groups fed alginate. Amongst PL35 then challenged with white spot syndrome virus (WSSV) by immersion, all PL not fed alginate died within 9 d. However, amongst PL fed the 4 concentrations of alginate powder or beads, mortality rates reduced with increasing alginate concentration, and between 25 and 32% PL remained alive when the bioassay was terminated on Day 21. Amongst alginate-fed PL groups compared with the control group, mortality was reduced by 26.5 to 58.4%. Nested PCR detection of WSSV revealed sodium alginate concentration-dependent reductions in infection loads. The data indicate that sodium alginate extracted from brown seaweed and fed to P. monodon can retard progression of WSSV disease.

Susceptibility of testicular cell cultures of crab, Scylla serrata (Forskal) to white spot syndrome virus

Testicular cell culture of crab, Scylla serrata (Forskal) was used to study the effects of White spot syndrome virus (WSSV). We are showing the susceptibility of cell culture of crabs to WSSV. The proliferating cell culture of testes were maintained for more than 4 months in a medium prepared from L15 and crab saline supplemented with epidermal growth factor. The cell cultures inoculated with different concentrations of virus showed distinct cytopathic effects such as change in cell appearance, shrinkage and cell lysis. WSSV infection of cultured cells was confirmed by Nested PCR technique. The incorporation of viral DNA in cultured cells was shown by RAPD profile generated using 10-mer primers. The controls that were not exposed to WSSV did not show cytopathic effects. This work shows the usefulness of proliferating testicular cell culture for studying WSSV infection using molecular tools. Thus, this report gains significance as it opens new vistas for diagnostics and drugs for WSSV.

Effects of white spot syndrome virus infection on immuno-enzyme activities and ultrastructure in gills of Cherax quadricarinatus

In this study, we explored the pathogenic mechanism of white spot syndrome virus (WSSV) in crayfish, Cherax quadricarinatus, by investigating activities of enzymes related to innate immune function during infection. After 6-12 h of exposure to WSSV, the activities of four enzymes, phenoloxidase (PO), peroxidase (POD), superoxide dismutase (SOD) and lysozyme (LSZ), increased in the gills of C. quadricarinatus but then sharply decreased during longer infection times. Except for PO, the activities of other enzymes in the WSSV-infected crayfish (Group II) were significantly lower than those of the controls at 72 h post-exposure (P < 0.01). Interestingly, the enzyme activities in the group treated with polysaccharides before challenge with WSSV (Group III) were higher than those in Group II. This phenomenon demonstrated that the polysaccharides could improve the immuno-enzyme activities and enhance the organism's antiviral defenses. Morphological examination by transmission electron microscopy revealed abundant WSSV particles and significant damage in the gills of infected crayfish. WSSV infection caused parts of the gill epithelium and microvilli to be reduced in number and size or damaged; meanwhile, the mitochondria morphology changed, with parts of the cristae diminished leaving large vacuoles. Moreover, electron dense deposits appeared and heterochromatinized nuclei could be seen in blood cells with ruptured nuclear membranes and outflow of nucleoplasm. The findings of this study furthers our understanding of the biochemical alterations induced by viral infections, including changes in the antioxidant status, oxidative stress and lysozyme activity, which could help to advance strategies for control of WSSV in crayfish.

Crassostrea gigas oysters as a shrimp farm bioindicator of white spot syndrome virus

This study explored whether Crassostrea gigas oysters can be used as a bioindicator of white spot syndrome virus (WSSV) in shrimp farm water canals. Bioassays showed that C. gigas can accumulate WSSV in their gills and digestive glands but do not become infected, either by exposure to seawater containing WSSV or by cohabitation with infected shrimp. The use of a WSSV nested PCR to screen oysters placed in water canals at the entry of a shrimp farm allowed WSSV to be detected 16 d prior to the disease occurring. The finding that C. gigas can concentrate small amounts of WSSV present in seawater without being harmed makes it an ideal sentinel species at shrimp farms.

A vector that expresses VP28 of WSSV can protect red swamp crayfish from white spot disease

White spot disease caused by white spot syndrome virus (WSSV) leads to devastating losses in shrimp farming. The WSSV envelope protein VP28, can be used as subunit vaccines that can efficiently protect shrimp against WSSV disease. However, the function of the envelope protein VP19 was not confirmed, some researches found that VP19 could protect shrimp against WSSV, and other reports found it no any protection. To detect the functions of VP28 and VP19 and find a method to prevent this disease in red swamp crayfish Procambarus clarkii, we constructed the plasmid vectors pIevp28 and pIevp19, which contains the ie1 promoter and coding region of vp28 or vp19 of WSSV, respectively. The results of quantitative real-time PCR and western blot showed that the injected vectors could transcribe corresponding mRNAs and translate to the protein VP28 or VP19 in the crayfish. The vp28 or vp19 signal was detected on the third day post injection, and maintained its expression for 30days. The mortality of the crayfish with pIevp28 showed obvious decline compared with the controls (pIe and PBS injection). However, pIevp19 seems did not affect the mortality of the crayfish compared with the controls. Furthermore, only VP28 was found tightly bound to the host haemocytes under immunocytochemistry. The results suggest that the VP28 protein might protect shrimp from the virus through competitive inhibition. We also found that oral administration of Escherichia coli with pIevp28 could protect crayfish from white spot disease, but the E. coli with pIevp19 was not. Therefore, we think that oral administration of bacteria with pIevp28 is a potentially easy therapeutic way against white spot disease in aquaculture.

White spot syndrome virus (WSSV) concentrations in crustacean tissues: a review of data relevant to assess the risk associated with commodity trade

We have reviewed the available peer reviewed literature on pathogen load for white spot syndrome virus (WSSV) in species susceptible to infection. Data on pathogen load in traded commodities are relevant for undertaking import risk assessments for a specific pathogen. Data were available for several of the major penaeid shrimp species farmed for aquaculture and for one crab and crayfish species. Most data are based on experimental infection, but some data were available for farmed or wild shrimp. Owing to the unavailability of immortal cell lines to determine viral load of viable virus, quantitative PCR was the main method used for quantification. The viral loads measured in shrimp at the onset of mortality events were extremely high (in the order of 10(9) -10(10) copy numbers gram(-1) of tissue). In a farm setting, the onset of increased mortalities will often trigger emergency harvests. Therefore, shrimp obtained from emergency harvests are likely to carry substantial concentrations of viral particles. Viral load did not vary greatly with tissue type. The WSSV load in wild crustaceans, farmed crustaceans not undergoing a mortality event or survivors of a mortality event was significantly lower (usually by multiple logs). Studies have also been undertaken in 'vaccinated' shrimp. One of the 'vaccines' led to a significant reduction of viral load in WSSV-exposed animals. The data obtained from the literature review are put into context with published information on minimal infectious dose and WSSV survival in frozen commodity shrimp.

Reservoirs and alternate hosts for pathogens of commercially important crustaceans: a review

There is a considerable body of literature describing the causative agents of many diseases of crustaceans. Given that many of these crustaceans support commercially important fisheries, it is somewhat surprising that comparatively little information is available regarding the natural transmission pathways and reservoirs of many of the disease-causing agents. In this paper we review what is known about reservoirs and alternate hosts for several important diseases of commercially important crustaceans and provide recommendations on future areas of research.

Class B scavenger receptor, Croquemort from kuruma shrimp Marsupenaeus japonicus: Molecular cloning and characterization

The scavenger receptor, Croquemort is a member of the CD36 superfamily comprising transmembrane proteins involved in the recognition of polyanionic ligands. Various researchers have proved that members of the CD36 superfamily are involved in immunity and developmental processes. In the present study, we report a cDNA encoding the kuruma shrimp, Marsupenaeus japonicus Croquemort scavenger receptor (MjSCRBQ) obtained from a cDNA library of lymphoid organ by RACE amplification. The full-length cDNA of 2098 bp consists an open reading frame of 1596 nucleotides that translates into a 532-amino acid putative protein, with a 5' untranslated region of 323 bp and 3' UTR of 153 bp. The MjSCRBQ is constitutively expressed in gills, heart, hemolymph, hepatopancreas, intestine, lymphoid organ, muscle, nerve, and stomach and at high levels in the brain. Expression analysis in lymphoid organs of shrimp infected with white spot syndrome virus (WSSV) revealed high levels of MjSCRBQ 72 and 120 h post-infection. The MjSCRBQ contains putative functional domains including transmembrane domains and a CD36 domain. Multiple alignments of MjSCRBQ amino acid sequences showed significant identity with Drosophila melanogaster SCRBQ (31%), Salmo salar SCRBQ (29%), Homo sapiens SCRBQ (28%) and Rattus norvegicus SCRBQ (30%). In a phylogenetic analysis, MjSCRBQ was identified in the invertebrate scavenger receptor cluster. This is the first report in crustaceans of the identification and characterization of a Croquemort scavenging receptor.

Two novel antimicrobial peptides, arasin-likeSp and GRPSp, from the mud crab Scylla paramamosain, exhibit the activity against some crustacean pathogenic bacteria

Antimicrobial peptides (AMPs) are some of the important host molecules required to resist pathogen infection. Two novel AMPs (arasin-likeSp and GRPSp) were identified from the hemocytes of the mud crab, Scylla paramamosain, by analysis of a hemocyte expressed sequence tag library. The deduced open reading frames of the arasin-likeSp and GRPSp cDNAs are 198 and 168 bp, and encode for predicted peptides of 65 and 55 amino acid residues, respectively. The calculated molecular mass of the mature peptides was 4373 and 2995 Da with an estimated isoelectric point (pI) of 11.03 and 9.66, respectively. The mature peptide of arasin-likeSp is predicted to contain an N-terminal region rich in glycine and arginine and a C-terminal region containing four cysteine residues. Its amino acid sequence has an overall sequence identity of 53% to arasin-2 from the spider crab, Hyas araneus. The mature protein of GRPSp contains two cysteine residues at the C-terminus and two glycine-rich repeats (GGYG and GYGG). In healthy crabs, both arasin-likeSp and GRPSp transcript levels were found to be high in the hemocytes and were further increased at 3 h after challenge with the bacterium, Aerococcus viridans. A synthetic arasin-likeSp peptide revealed the antimicrobial activity against both Gram-positive and Gram-negative bacteria including some crustacean pathogens (A. viridans, Vibrio harveyi and V. anguillarum), whilst the synthetic GRPSp peptide exhibited antibacterial activity against some Gram-positive (A. viridans and Micrococcus luteus), but not Gram-negative, bacteria. These results indicate that arasin-likeSp and GRPSp are potentially novel AMPs involved in the immune responses of mud crab, S. paramamosain.

Studies on pathogenicity and prevalence of white spot syndrome virus in mud crab, Scylla serrata (Forskal), in Zhejiang Province, China

Mud crab, Scylla serrata (Forskal), is the most commercially important marine crab species in China. In recent years, serious diseases have occurred in major mud crab culture regions in SE China. PCR detection of white spot syndrome virus (WSSV) in diseased mud crabs collected from Zhejiang Province during 2006-2008 showed a prevalence of 34.82%. To study the pathogenicity of WSSV to mud crab, healthy mud crabs were injected intramuscularly with serial 10-fold dilutions of a WSSV inoculum. The cumulative mortalities in groups challenged with 10⁻¹, 10⁻², 10⁻³ and 10⁻⁴ dilutions were 100%, 100%, 66.7% and 38.9% at 10 days post-injection, respectively. All moribund and dead mud crabs except the control group were positive for WSSV by PCR. Based on the viral load of the WSSV inoculum by quantitative real-time PCR, the median lethal dose (LD50) of WSSV in S. serrata was calculated as 1.10 × 10⁶ virus copies/crab, or 7.34 × 10³ virus copies g⁻¹ crab weight. The phenoloxidase, peroxidase and superoxide dismutase activities in haemolymph of WSSV-infected moribund crabs, were significantly lower than the control group, whereas alkaline phosphatase, glutamate-pyruvate transaminase and glutamic-oxaloacetic transaminase were higher than in the control group. WSSV was mainly distributed in gills, subcuticular epithelia, heart, intestine and stomach as shown by immunohistochemical analysis with Mabs against WSSV. The epithelial cells of infected gill showed hypertrophied nuclei with basophilic inclusions. Numerous bacilliform virus particles were observed in nuclei of infected gill cells by transmission electron microscopy. It is concluded that WSSV is a major pathogen of mud crab with high pathogenicity.

Molecular characterization and expression analysis of extracellular copper-zinc superoxide dismutase gene from swimming crab Portunus trituberculatus

An extracellular CuZnSOD cDNA was cloned from the haemocytes of swimming crab Portunus trituberculatus by a reverse-transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA end (RACE) method. Analysis of the nucleotide sequence revealed that the ecCuZnSOD full-length cDNA consisted of 965 bp with an open reading frame of 579 bp. It encoded a polypeptide of 192 amino acids which had a predicted molecular weight of 20.0 kDa and with an estimated pI of 6.23. The deduced amino acid sequence contained a putative signal peptide of 31 amino acids. It is predicted to possess all the expected features of CuZnSOD members, including amino acids responsible for binding Cu and Zn, two putative CuZnSOD signatures, two cysteines and one N-linked glycosylation site. Sequence comparison showed that the CuZnSOD deduced amino acid sequence of P. trituberculatus has similarity of 80%, 76%, 55% and 50% to that of blue crab Callinectes sapidus, mud crab Scylla serrata, crayfish Pacifastacus leniusculus and freshwater prawn Macrobrachium rosenbergii, respectively. The ecCuZnSOD transcripts expressed in all examined tissues, including haemocytes, hepatopancreas, heart, stomach, intestine, gill, ovary and muscle. RT-PCR analysis indicated that ecCuZnSOD transcripts both in haemocytes and hepatopancreas increased in the first 3 h after Vibrio alginolyticus challenging, as the bacterial infection progressed, the challenged crabs decreased to levels significantly lower than control by 96 h post-infection. These facts indicated that ecCuZnSOD was potentially involved in the acute response against invading bacteria in P. trituberculatus.

Detection of major capsid protein of infectious myonecrosis virus in shrimps using monoclonal antibodies

Infectious myonecrosis virus (IMNV) has been causing a progressive disease in farm-reared shrimps in Brazil and Indonesia. Immunodiagnostic methods for IMNV detection, although reliable, are not employed currently because monoclonal antibodies (MAbs) against this virus are not available. In this study, a fragment of the IMNV major capsid protein gene, comprising amino acids 300-527 (IMNV(300-527)), was cloned and expressed in Escherichia coli. The nucleotide sequence of the recombinant IMNV(300-527) fragment displayed a high degree of identity to the major capsid protein of IMNV isolates from Brazil (99%) and Indonesia (98%). Ten MAbs were generated against the expressed fragment, and eight of these, mostly IgG(2a) or IgG(2b), were able to bind to IMNV in tissue extracts from shrimps infected naturally in immunodot-blot assays. Six of these MAbs recognized a approximately 100 kDa protein in a Western-blot, which is the predicted mass of IMNV major capsid protein, and also bound to viral inclusions present in muscle fibroses and in coagulative myonecrosis, as demonstrated by immunohistochemistry. Among all those MAbs created, four did not cross-react with non-infected shrimp tissues; this observation supports their applicability as a sensitive and specific immunodiagnosis of IMNV infection in shrimps.