Propionigenium and Vibrio species identified as possible component causes of shrimp white feces syndrome (WFS) associated with the microsporidian Enterocytozoon hepatopenaei

White feces syndrome (WFS) in cultivated shrimp is characterized by white shrimp midguts (intestines) and white fecal strings that float as mats on pond surfaces. The etiology of WFS is complex, but one type called EHP-WFS is associated with the microsporidian Enterocytozoon hepatopenaei (EHP). The hepatopancreas (HP), midgut and fecal strings of EHP-WFS shrimp exhibit massive quantities of EHP spores together with mixed, unidentified bacteria. In EHP-WFS ponds, some EHP-infected shrimp show white midguts (WG) and produce white feces while other EHP-infected shrimp in the same pond show grossly normal midguts (NG) and produce no white feces. We hypothesized that comparison of the microbial flora between WG and NG shrimp would reveal probable combinations of microbes significantly associated with EHP-WFS. To test this, we selected a Penaeus vannamei cultivation pond exhibiting severe WFS and used microscopic and microbial profiling analyses to compare WG and NG samples. Histologically, EHP was confirmed in the HP and midgut of both WG and NG shrimp, but EHP burdens were higher and EHP tissue damage was more severe in WG shrimp. Further, intestinal microbiomes in WG shrimp were less diverse and had higher abundance of bacteria from the genera Vibrio and Propionigenium. Propionigenium burden in the HP of WG shrimp (9364 copies/100ng DNA) was significantly higher (P = 1.1 x 10^-5) than in NG shrimp (12 copies/100ng DNA). These findings supported our hypothesis by revealing two candidate bacterial genera that should be tested in combination with EHP as potential component causes of EHP-WFS in P. vannamei.

Research progress on viral accommodation 2009 to 2019

The viral accommodation hypothesis for crustaceans and insects was first proposed in 1998/2001, stimulated by observations that shrimp and insects or insect cell lines can coexist with both DNA or RNA viruses without showing any signs of disease (i.e., they tolerate, single to multiple, persistent infections, sometimes for a lifetime). A review of tests of the hypothesis up to 2007 was previously published in DCI. This was followed by a major revision in 2009 when the elusive memory element required by the hypothesis was proposed to reside in non-retroviral fragments of extant viruses, now called endogenous viral elements (EVE) that are autonomously inserted into the host genome as cDNA copied from viral mRNA. Here, progress in research on viral accommodation in crustaceans and insects over the decade following 2009 is reviewed. It culminates with a discussion of exiting research results from insects in 2019 that prove the existence of specific, adaptive and heritable immunity, at least in mosquitoes. It remains to be determined whether the same mechanisms also govern EVE acquisition and its protective RNA production in shrimp. The wide-ranging consequences of the revealed mechanisms for viral disease control in economic crustaceans and insects is discussed.

The shrimp microsporidian Enterocytozoon hepatopenaei (EHP): Biology, pathology, diagnostics and control

Disease is a major limiting factor in the global production of cultivated shrimp. The microsporidian parasite Enterocytozoon hepatopenaei (EHP) was formally characterized in 2009 as a rare infection of the black tiger shrimp Penaeus monodon. It remained relatively unstudied until mid-2010, after which infection with EHP became increasingly common in the Pacific whiteleg shrimp Penaeus vannamei, by then the most common shrimp species farmed in Asia. EHP infects the hepatopancreas of its host, causing hepatopancreatic microsporidiosis (HPM), a condition that has been associated with slow growth of the host in aquaculture settings. Unlike other infectious disease agents that have caused economic losses in global shrimp aquaculture, EHP has proven more challenging because too little is still known about its environmental reservoirs and modes of transmission during the industrial shrimp production process. This review summarizes our current knowledge of the EHP life cycle and the molecular strategies that it employs as an obligate intracellular parasite. It also provides an analysis of available and new methodologies for diagnosis since most of the current literature on EHP focuses on that topic. We summarize current knowledge of EHP infection and transmission dynamics and currently recommended, practical control measures that are being applied to limit its negative impact on shrimp cultivation. We also point out the major gaps in knowledge that urgently need to be bridged in order to improve control measures.

Shewanella khirikhana sp. nov. - a shrimp pathogen isolated from a cultivation pond exhibiting early mortality syndrome

Early mortality syndrome (EMS) in cultivated shrimp is of complex aetiology. One of the causes is acute hepatopancreatic necrosis disease (AHPND) caused by unique Vibrio isolates that carry two Pirvp toxin genes, but other causes of EMS remain mostly unexplained. Here, we describe the discovery of a Shewanella isolate TH2012T from an EMS/AHPND outbreak pond and demonstrate its virulence for shrimp (the mean lethal concentration of 105 colony-forming units per millilitre by immersion challenge) accompanied by distinctive histopathology, particularly of the ventral nerve cord and lymphoid organ but also including the digestive tract. On the basis of its complete genome sequence, multilocus phylogenetic trees, digital DNA-DNA hybridization analysis and differential phenotypic characteristics, we propose that Shewanella isolate TH2012T represents a novel species, separated sufficiently from the type strains S. litorisediminis and S. amazonensis to justify naming it Shewanella khirikhana sp. nov. Analysis of the TH2012T genome revealed no homologues of the Pirvp toxin genes but revealed a number of other potential virulence factors. It constitutes the first Shewanella isolate reported to be pathogenic to shrimp.

Mendelian inheritance of endogenous viral elements (EVE) of white spot syndrome virus (WSSV) in shrimp

Previous work has shown that non-retroviral endogenous viral elements (EVE) are common in crustaceans, including penaeid shrimp. So far, they have been reported for infectious hypodermal and hematopoietic necrosis virus (IHHNV) and white spot syndrome virus (WSSV). For the latter, it was shown that shrimp sperm were positive for an EVE of WSSV called EVE₃₆₆, suggesting that it was heritable, since shrimp sperm (non-motile) do not contain mitochondria. However, to prove this hypothesis that EVE₃₆₆ was heritable and located in chromosomal DNA, it was necessary to carry out mating tests to show that EVE₃₆₆ could be detected in parental shrimp and distributed in their offspring in a Mendelian fashion. To do this, we analyzed two shrimp crosses using polyacrylamide gels with a multiple-allele, microsatellite marker Pmo11 as a quality control for single allele detection. In both crosses, all of the shrimp (parents and siblings) were positive for 2 Pmo11 alleles as expected. In Cross 1, the female was PCR-positive for EVE₃₆₆ while the male was negative, and in Cross 2, both the female and male were PCR-positive for EVE₃₆₆. Individual analysis of the offspring of Cross 1 revealed a distribution of 1:1 for EVE₃₆₆, indicating that the EVE₃₆₆-positive female parent was heterozygous for EVE₃₆₆. In the second cross, the distribution of EVE₃₆₆ in the offspring was 3:1, indicating that both PCR-positive parents were heterozygous for EVE₃₆₆. These results supported the hypothesis that EVE₃₆₆ was present in shrimp chromosomal DNA and was heritable in a Mendelian fashion. This work provides a model to screen for heritable EVE in shrimp and shows that selection of one parent heterozygous for an EVE and the other negative for it can result in approximately half of the siblings positive and half negative for that EVE as expected. Dividing the siblings of such a cross into an EVE positive group and an EVE negative group followed by challenge with the originating lethal virus should reveal whether or not possession of that specific EVE results in any significant protection against disease caused by the homologous virus.

Extract from the fermented soybean product Natto inhibits Vibrio biofilm formation and reduces shrimp mortality from Vibrio harveyi infection

Many bacteria, including Vibrio pathogens of shrimp, need to colonize and/or form biofilms in hosts or the environment to cause disease. Thus, one possible control strategy for shrimp Vibriosis is biofilm inhibition. With this objective, an extract from the Japanese fermented soybean product, Natto was tested with the luminescent shrimp pathogen Vibrio harveyi (VH) for its ability to inhibit or degrade biofilm and to interfere with cell growth in broth. Natto is a traditional fermentation product of Bacillus subtilis var Natto (BSN1). Using 96 well microtiter plates coated with 0.4% chitosan, we found that biofilm formation by VH was inhibited, while growth in parallel broth cultures was not. When an extract from Natto prepared using BSN1 was mixed with feed for the whiteleg shrimp Penaeus vannamei before immersion challenge with V. harveyi at 10⁶ cfu/ml, survival was significantly higher (p≤0.05) than for control shrimp given feed without these additives. Further work done to test whether d-amino acids were involved in biofilm formation as previously reported for B. subtilis, Staphylococus aureus and Pseudomonas aeruginosa gave negative results. In conclusion, we discovered that Natto extract can inhibit Vibrio biofilm formation and that it or BSN1 alone added to shrimp feed can significantly reduce shrimp mortality in immersion challenges with pathogenic VH. This shows some promise for possible application against Vibriosis in shrimp since Natto is generally regarded as safe (GRAS) for human consumption.

Variable RNA expression from recently acquired, endogenous viral elements (EVE) of white spot syndrome virus (WSSV) in shrimp

The viral accommodation hypothesis proposes that endogenous viral elements (EVE) from both RNA and DNA viruses are being continually integrated into the shrimp genome by natural host processes and that they can result in tolerance to viral infection by fortuitous production of antisense, immunospecific RNA (imRNA). Thus, we hypothesized that previously reported microarray results for the presence of white spot syndrome virus (WSSV) open reading frames (ORFs) formerly called 151, 366 and 427 in a domesticated giant tiger shrimp (Penaeus monodon) breeding stock might have represented expression from EVE, since the stock had shown uninterrupted freedom from white spot disease (WSD) for many generations. To test this hypothesis, 128 specimens from a current stock generation were confirmed for freedom from WSSV infection using two nested PCR detection methods. Subsequent nested-PCR testing revealed 33/128 specimens (26%) positive for at least one of the ORF at very high sequence identity (95-99%) to extant WSSV. Positive results for ORF 366 (now known to be a fragment of the WSSV capsid protein gene) dominated (28/33 = 84.8%), so 9 arbitrarily selected 366-positive specimens were tested by strand-specific, nested RT-PCR using DNase-treated RNA templates. This revealed variable RNA expression in individual shrimp including no RNA transcripts (n = 1), sense transcripts only (n = 1), antisense transcripts only (n = 2) or transcripts of both sense (n = 5). The latter 7 expression products indicated specimens producing putative imRNA. The variable types and numbers of the EVE and the variable RNA expression (including potential imRNA) support predictions of the viral accommodation hypothesis that EVE are randomly produced and expressed. Positive nested PCR test results for EVE of ORF 366 using DNA templates derived from shrimp sperm (germ cells), indicated that they were heritable.

A Nested PCR Assay to Avoid False Positive Detection of the Microsporidian Enterocytozoon hepatopenaei (EHP) in Environmental Samples in Shrimp Farms

Hepatopancreatic microsporidiosis (HPM) caused by Enterocytozoon hepatopenaei (EHP) is an important disease of cultivated shrimp. Heavy infections may lead to retarded growth and unprofitable harvests. Existing PCR detection methods target the EHP small subunit ribosomal RNA (SSU rRNA) gene (SSU-PCR). However, we discovered that they can give false positive test results due to cross reactivity of the SSU-PCR primers with DNA from closely related microsporidia that infect other aquatic organisms. This is problematic for investigating and monitoring EHP infection pathways. To overcome this problem, a sensitive and specific nested PCR method was developed for detection of the spore wall protein (SWP) gene of EHP (SWP-PCR). The new SWP-PCR method did not produce false positive results from closely related microsporidia. The first PCR step of the SWP-PCR method was 100 times (104 plasmid copies per reaction vial) more sensitive than that of the existing SSU-PCR method (106 copies) but sensitivity was equal for both in the nested step (10 copies). Since the hepatopancreas of cultivated shrimp is not currently known to be infected with microsporidia other than EHP, the SSU-PCR methods are still valid for analyzing hepatopancreatic samples despite the lower sensitivity than the SWP-PCR method. However, due to its greater specificity and sensitivity, we recommend that the SWP-PCR method be used to screen for EHP in feces, feed and environmental samples for potential EHP carriers.

Interaction study of a novel Macrobrachium rosenbergii effector caspase with B2 and capsid proteins of M. rosenbergii nodavirus reveals their roles in apoptosis

Apoptosis is an essential immune response to protect invertebrates from virus infected cells. In shrimp, virus infection has been reported to induce apoptosis. Macrobrachium rosenbergii (Mr) was considered to be a disease-resistant host when compared to penaeid shrimps. Caspase-3 was classified as an executioner caspase which played a key role in virus-induced apoptosis. In this study, an effector caspase gene of M. rosenbergii (Mrcasp) was cloned and characterized. The open reading frame (ORF) of Mrcasp was 957 nucleotide encoding 318 amino acid with a deduced molecular mass of 35.87 kDa. RT-PCR analysis showed the presence of Mrcasp in all examined tissues. The phylogenetic tree indicated that Mrcasp was closely related with caspase 3 of shrimp. The functions of the Mrcasp, B2 and capsid proteins of M. rosenbergii nodavirus (MrNV) were assayed in Sf-9 cells. The results showed that Mrcasp induce apoptotic morphology cells; however, capsid protein of MrNV could inhibit apoptotic cells whereas B2 could neither induce nor inhibit apoptotic cells by DAPI staining. The protein interaction between Mrcasp and viral MrNV structure revealed that Mrcasp did not bind to B2 or capsid protein whereas B2 and capsid proteins could bind directly to each other. This study reported a novel sequence of a full-length Mrcasp and its functional studies indicated that Mrcasp could induce apoptotic cells. Our data is the first report demonstrating the direct protein-protein interaction between capsid protein and B2 protein of MrNV.

Characterization and PCR Detection Of Binary, Pir-Like Toxins from Vibrio parahaemolyticus Isolates that Cause Acute Hepatopancreatic Necrosis Disease (AHPND) in Shrimp

Unique isolates of Vibrio parahaemolyticus (VPAHPND) have previously been identified as the causative agent of acute hepatopancreatic necrosis disease (AHPND) in shrimp. AHPND is characterized by massive sloughing of tubule epithelial cells of the hepatopancreas (HP), proposed to be induced by soluble toxins released from VP_AHPND that colonize the shrimp stomach. Since these toxins (produced in broth culture) have been reported to cause AHPND pathology in reverse gavage bioassays with shrimp, we used ammonium sulfate precipitation to prepare protein fractions from broth cultures of VP_AHPND isolates for screening by reverse gavage assays. The dialyzed 60% ammonium sulfate fraction caused high mortality within 24–48 hours post-administration, and histological analysis of the moribund shrimp showed typical massive sloughing of hepatopancreatic tubule epithelial cells characteristic of AHPND. Analysis of the active fraction by SDS-PAGE revealed two major bands at marker levels of approximately 16 kDa (ToxA) and 50 kDa (ToxB). Mass spectrometry analysis followed by MASCOT analysis revealed that both proteins had similarity to hypothetical proteins of V. parahaemolyticus M0605 (contig034 GenBank accession no. JALL01000066.1) and similarity to known binary insecticidal toxins called 'Photorhabdus insect related' proteins A and B (Pir-A and Pir-B), respectively, produced by the symbiotic, nematode bacterium Photorhabdus luminescens. In in vivo tests, it was shown that recombinant ToxA and ToxB were both required in a dose dependent manner to cause AHPND pathology, indicating further similarity to Pir-A and -B. A single-step PCR method was designed for detection of the ToxA gene and was validated using 104 bacterial isolates consisting of 51 VPAHPND isolates, 34 non-AHPND VP isolates and 19 other isolates of bacteria commonly found in shrimp ponds (including other species of Vibrio and Photobacterium). The results showed 100% specificity and sensitivity for detection of VP_AHPND isolates in the test set.

Identification and characterization of a QM protein as a possible peptidoglycan recognition protein (PGRP) from the giant tiger shrimp Penaeus monodon

In an attempt to identify a peptidoglycan recognition protein (PGRP) in Penaeus (Penaeus) monodon, in vitro pull-down binding assays were used between shrimp proteins and purified peptidoglycan (PG). By gel electrophoresis and mass spectrometry followed by Mascot program analysis, proteins from shrimp hemocyte peripheral membrane proteins showed significant homology to records for a QM protein, actin and prophenoloxidase 2 precursor (proPO2), while proteins from cell-free plasma showed significant homology to records for a vitellogenin, a fibrinogen related protein (FREP) and a C-type lectin. Due to time and resource limitations, specific binding to PG was examined only for recombinant PmQM protein and PmLec that were synthesized based on sequences reported in the Genbank database (accession numbers FJ766846 and DQ078266, respectively). An in vitro assay revealed that hemocytes would bind with and encapsulate agarose beads coated with recombinant PmQM (rPmQM) or rPmLec and that melanization followed 2h post-encapsulation. ELISA tests confirmed specific binding of rPmQM protein to PG. This is the first time that PmQM has been reported as a potential PGRP in shrimp or any other crustacean. The two other potential PGRP identified (FREP and the vitellin-like protein present in male P. monodon, unlike other vitellin subunits) should also be expressed heterologously and tested for their ability to activate shrimp hemocytes.

ShrimpGPAT: a gene and protein annotation tool for knowledge sharing and gene discovery in shrimp

BACKGROUND

Although captured and cultivated marine shrimp constitute highly important seafood in terms of both economic value and production quantity, biologists have little knowledge of the shrimp genome and this partly hinders their ability to improve shrimp aquaculture. To help improve this situation, the Shrimp Gene and Protein Annotation Tool (ShrimpGPAT) was conceived as a community-based annotation platform for the acquisition and updating of full-length complementary DNAs (cDNAs), Expressed Sequence Tags (ESTs), transcript contigs and protein sequences of penaeid shrimp and their decapod relatives and for in-silico functional annotation and sequence analysis.

DESCRIPTION

ShrimpGPAT currently holds quality-filtered, molecular sequences of 14 decapod species (~500,000 records for six penaeid shrimp and eight other decapods). The database predominantly comprises transcript sequences derived by both traditional EST Sanger sequencing and more recently by massive-parallel sequencing technologies. The analysis pipeline provides putative functions in terms of sequence homologs, gene ontologies and protein-protein interactions. Data retrieval can be conducted easily either by a keyword text search or by a sequence query via BLAST, and users can save records of interest for later investigation using tools such as multiple sequence alignment and BLAST searches against pre-defined databases. In addition, ShrimpGPAT provides space for community insights by allowing functional annotation with tags and comments on sequences. Community-contributed information will allow for continuous database enrichment, for improvement of functions and for other aspects of sequence analysis.

CONCLUSIONS

ShrimpGPAT is a new, free and easily accessed service for the shrimp research community that provides a comprehensive and up-to-date database of quality-filtered decapod gene and protein sequences together with putative functional prediction and sequence analysis tools. An important feature is its community-based functional annotation capability that allows the research community to contribute knowledge and insights about the properties of molecular sequences for better, shared, functional characterization of shrimp genes. Regularly updated and expanded with data on more decapods, ShrimpGPAT is publicly available at http://shrimpgpat.sc.mahidol.ac.th/.

Two new anti-apoptotic proteins of white spot syndrome virus that bind to an effector caspase (PmCasp) of the giant tiger shrimp Penaeus (Penaeus) monodon

White spot syndrome virus proteins WSSV134 and WSSV322 have been shown to bind with the p20 domain (residues 55-214) of Penaeus monodon caspase (PmCasp) protein through yeast two-hybrid screening. Binding was confirmed for the p20 domain and the full-length caspase by co-immunoprecipitation. WSSV134 is also known as the WSSV structural protein VP36A, but no function or conserved domains have been ascribed to WSSV322. Discovery of the caspase binding activity of these two proteins led to an investigation of their possible anti-apoptotic roles. Full-length PmCasp was confirmed to be an effector caspase by inducing apoptosis in transfected Sf-9 cells as assessed by DAPI staining. Using the same cell model, comparison of cells co-transfected with PmCasp and either WSSV134 or WSSV322 revealed that both of the binding proteins had anti-apoptotic activity. However, using the same Sf-9 protocol with anti-apoptosis protein-1 (AAP-1; also called WSSV449) previously shown to bind and inactivate a different effector caspase from P. monodon (Pm caspase) did not block apoptosis induced by PmCasp. The results revealed diversity in effector caspases and their viral protein inhibitors in P. monodon.

Construction and application of a protein interaction map for white spot syndrome virus (WSSV)

White spot syndrome virus (WSSV) is currently the most serious global threat for cultured shrimp production. Although its large, double-stranded DNA genome has been completely characterized, most putative protein functions remain obscure. To provide more informative knowledge about this virus, a proteomic-scale network of WSSV-WSSV protein interactions was carried out using a comprehensive yeast two-hybrid analysis. An array of yeast transformants containing each WSSV open reading frame fused with GAL4 DNA binding domain and GAL4 activation domain was constructed yielding 187 bait and 182 prey constructs, respectively. On screening of ∼28,000 pairwise combinations, 710 interactions were obtained from 143 baits. An independent coimmunoprecipitation assay (co-IP) was performed to validate the selected protein interaction pairs identified from the yeast two-hybrid approach. The program Cytoscape was employed to create a WSSV protein-protein interaction (PPI) network. The topology of the WSSV PPI network was based on the Barabási-Albert model and consisted of a scale-free network that resembled other established viral protein interaction networks. Using the RNA interference approach, knocking down either of two candidate hub proteins gave shrimp more protection against WSSV than knocking down a nonhub gene. The WSSV protein interaction map established in this study provides novel guidance for further studies on shrimp viral pathogenesis, host-viral protein interaction and potential targets for therapeutic and preventative antiviral strategies in shrimp aquaculture.

Discovery and partial characterization of a non-LTR retrotransposon that may be associated with abdominal segment deformity disease (ASDD) in the whiteleg shrimp Penaeus (Litopenaeus) vannamei

BACKGROUND

Abdominal segment deformity disease (ASDD) of cultivated whiteleg shrimp Penaeus (Litopenaeus) vannamei causes economic loss of approximately 10% in affected specimens because of the unsightliness of distorted abdominal muscles. It is associated with the presence of viral-like particles seen by electron microscopy in the ventral nerve cords of affected shrimp. Thus, shotgun cloning was carried out to seek viral-like sequences in affected shrimp.

RESULTS

A new retrovirus-like element of 5052 bp (named abdominal segment deformity element or ASDE) was compiled by shotgun cloning and 3' and 5' RACE using RNA and DNA extracted from ventral nerve cords of ASDD shrimp. ASDE contained 7 putative open reading frames (ORF). One ORF (called the PENS sub-domain), had a deduced amino acid (aa) sequence homologous to the GIY-YIG endonuclease domain of penelope-like retrotransposons while two others were homologous to the reverse transcriptase (RT) and RNaseH domains of the pol gene of non-long terminal repeat (non-LTR) retrotransposons (called the NLRS sub-domain). No single amplicon of 5 kb containing both these elements was obtained by PCR or RT-PCR from ASDD shrimp. Subsequent analysis indicated that PENS and NLRS were not contiguous and that NLRS was a host genetic element. In situ hybridization using a dioxygenin-labeled NLRS probe revealed that NLRS gave positive reactions in abdominal-ganglion neurons of ASDD shrimp but not normal shrimp. Preliminary analysis indicated that long-term use of female broodstock after eyestalk ablation in the hatchery increased the intensity of RT-PCR amplicons for NLRS and also the prevalence of ASDD in mysis 3 offspring of the broodstock. The deformities persist upon further cultivation until shrimp harvest but do not increase in prevalence and do not affect growth or survival.

CONCLUSIONS

Our results suggested that NLRS is a shrimp genetic element associated with ASDD and that immediate preventative measures could include shorter-term use of broodstock after eyestalk ablation and/or discard of broodstock that give strong RT-PCR reactions for NLRS. In the longer term, it is recommended, if possible, that currently used, domesticated shrimp lines be selected for freedom from NLRS. The molecular tools developed in this work will facilitate the management and further study of ASDD.

The microsporidian Enterocytozoon hepatopenaei is not the cause of white feces syndrome in whiteleg shrimp Penaeus (Litopenaeus) vannamei

Background

The microsporidian Enterocytozoon hepatopenaei was first described from Thailand in 2009 in farmed, indigenous giant tiger shrimp Penaeus (Penaeus) monodon. The natural reservoir for the parasite is still unknown. More recently, a microsporidian closely resembling it in morphology and tissue preference was found in Thai-farmed, exotic, whiteleg shrimp Penaeus (Litopenaeus) vannamei exhibiting white feces syndrome (WFS). Our objective was to compare the newly found pathogen with E. hepatopenaei and to determine its causal relationship with WFS.

Results

Generic primers used to amplify a fragment of the small subunit ribosomal RNA (ssu rRNA) gene for cloning and sequencing revealed that the new parasite from WFS ponds had 99% sequence identity to that of E. hepatopenaei, suggesting it was conspecific. Normal histological analysis using tissue sections stained with hematoxylin and eosin (H&E) revealed that relatively few tubule epithelial cells exhibited spores, suggesting that the infections were light. However, the H&E results were deceptive since nested PCR and in situ hybridization analysis based on the cloned ssu rRNA gene fragment revealed very heavy infections in tubule epithelial cells in the central region of the hepatopancreas in the absence of spores. Despite these results, high prevalence of E. hepatopenaei in shrimp from ponds not exhibiting WFS and a pond that had recovered from WFS indicated no direct causal association between these infections and WFS. This was supported by laboratory oral challenge trials that revealed direct horizontal transmission to uninfected shrimp but no signs of WFS.

Conclusions

The microsporidian newly found in P. vannamei is conspecific with previously described E. hepatopenaei and it is not causally associated with WFS. However, the deceptive severity of infections (much greater than previously reported in P. monodon) would undoubtedly have a negative effect on whiteleg shrimp growth and production efficiency and this could be exacerbated by the possibility of horizontal transmission revealed by laboratory challenge tests. Thus, it is recommended that the PCR and in situ hybridization methods developed herein be used to identify the natural reservoir species so they can be eliminated from the shrimp rearing system.

Novel, anionic, antiviral septapeptides from mosquito cells also protect monkey cells against dengue virus

We have shown previously that ultrafiltrates (5 kDa cutoff) of cell-free medium from mosquito cell cultures persistently infected with DENV serotype 2 (DENV-2) contained a novel antiviral agent (called viprolaxikine) that could protect pre-treated, naïve mosquito cells from DENV infection. Here, we show that viprolaxikine also reduced DENV-2 titers by almost 4 logs (>99.9%) when compared to Vero cells mock-treated with ultrafiltrates from cultures of uninfected mosquito cells. Protease treatment removed the anti-DENV-2 activity. Pre-incubation for 48-h was required to obtain the maximum, dose-dependent protection against DENV-2, indicating that the antiviral activity was based on the interaction between Vero cells and viprolaxikine rather than direct action of viprolaxikine on DENV-2. Activity was highest against DENV-2, but there was also significant activity against the 3 other DENV serotypes. LC-MS-MS analysis revealed that the active viprolaxikine fraction contained anionic, antiviral peptides, each comprised of 7 amino acids (DDHELQD, DETELQD and DEVMLQD or DEVLMQD) and with a common sequence motif of D-D/E-X-X-X-Q-D. These sequences do not occur in the dengue virus genome, suggesting that the peptides are produced by the host insect cells when persistently infected with DENV-2. These peptides represent a new class of anionic, insect-derived, antiviral peptides with activity against a flavivirus in both mammalian and insect cells.

Structure and expression of a shrimp prohormone convertase 2

Although many crustacean neuroendocrine hormones have been reported, the enzymes responsible for post-translational modification of neuroendocrine hormones have rarely been characterized. A prohormone convertase 2 (PC2)-like enzyme has been isolated from the optic lobe of the giant tiger shrimp, Penaeus monodon and referred as PmPC2. The full length cDNA sequence of PmPC2 has been identified and found to resemble evolutionarily conserved PC2 enzymes of vertebrates and invertebrates. PmPC2 was expressed in all larval developmental stages and in neuroendrocrine cells in the adult optic lobe. Its expression was found to be negatively related with shrimp body weight by qPCR (P<0.05). Immunohistochemistry results using an anti-rPmPC2 antibody with adult shrimp revealed high staining intensity in specific neurosecretory cells including the sinus gland, the organ of Hanström (also referred to as the medullar terminalis X-organ) and the organ of Bellonci (also referred to as the sensory or X-organ). By using the yeast two hybrid technique, PmPC2 was found to bind with P. monodon hyperglycemic hormone (Pem-CHH1) that plays an important role in glucose metabolism. Since PmPC2 is a subtilisin-like serine proteinase, it is expected to cleave the synthetic substrate, pyr-RTKR-MCA, but the expressed recombinant catalytic domain of PmPC2 (rPmPC2-cat) showed no enzymatic activity as expected. In vivo injection of dsRNA-PmPC2 resulted in reduced transcripts for both PmPC2 and Pem-CHH1 on day 3 post injection, but there was no accompanying reduction of glucose level in the hemolymph. Taken together, PmPC2 localization, expression and activity suggest that it has a function(s) in the shrimp neuroendrocrine system and that it may not only activate Pem-CHH1 but also affect its expression. However, there is no obvious explanation for the negative correlation between PmPC2 expression level and shrimp body weight.

A novel lectin domain-containing protein (LvCTLD) associated with response of the whiteleg shrimp Penaeus (Litopenaeus) vannamei to yellow head virus (YHV)

When using mRNA from gills of normal whiteleg shrimp Penaeus (Litopenaeus) vannamei as the tester and mRNA from yellow head virus (YHV)-infected shrimp as the driver, subtractive suppression hybridization (SSH) revealed that a novel EST clone of 198 bp with a putative C-type lectin-like domain (CTLD) was downregulated in YHV-infected shrimp. The clone nucleotide sequence had 99% identity with one contig MGID1052359 (1,380 bp) reported in an EST database of P. vannamei, and the presence of this target in normal shrimp was confirmed by RT-PCR using primers designed from the MGID1052359 sequence. Analysis of the primary structure of the deduced amino acid (a.a.) sequence of the contig revealed a short portion (40 a.a. residues) at its N-terminus with high similarity to a low density lipoprotein receptor (LDLR) class A domain and another 152 a.a. residues at its C-terminus with high similarity to a C-type lectin domain. Thus, the clone was named LvCTLD and three recombinant proteins (LvCTLD, the LDLR domain and the CTLD domain) were synthesized in a bacterial system based on its sequence. An in vitro encapsulation assay revealed that Sepharose 4B beads coated with rLvCTLD were encapsulated by shrimp hemocytes and that melanization followed by 24 h post-encapsulation. The encapsulation activity of rLvCTLD was inhibited by 100 mM galactose, but not mannose or EDTA. In vivo injection of rLvCTLD or rLvCTLD plus YHV resulted in a significant elevation of PO activity in the hemolymph of the challenged shrimp when compared to shrimp injected with buffer, suggesting that rLvCTLD could activate the proPO system. An ELISA test revealed that rLvCTLD could bind to YHV particles in the presence of shrimp hemolymph. Phylogenetic analysis suggested that the LvCTLD sequence was more closely related to an antiviral gene found in Penaeus monodon (PmAV) than to other reported shrimp lectins. Taken together, we conclude that a novel shrimp LvCTLD is a host recognition molecule involved in the shrimp defense mechanism against YHV via recruitment of hemocytes, probably at the site of viral infection, and via activation of the proPO system.

Chitosan and its quaternized derivative as effective long dsRNA carriers targeting shrimp virus in Spodoptera frugiperda 9 cells

RNA interference (RNAi) is a promising strategy to combat shrimp viral pathogens at lab-scale experiments. Development of effective orally delivered agents for double-stranded (ds)RNA is necessary for RNAi application at farm level. Since continuous shrimp cell lines have not been established, we are developing a dsRNA-delivery system in Spodoptera frugiperda (Sf9) cells for studying in vitro RNAi-mediated gene silencing of shrimp virus. Sf9 cells challenged with yellow head virus (YHV) were used for validating nanoparticles as effective dsRNA carriers. Inexpensive and biodegradable polymers, chitosan and its quarternized derivative (QCH4), were formulated with long dsRNA (>100 bp) targeting YHV. Their morphology and physicochemical properties were examined. When treated with chitosan- and QCH4-dsRNA complexes, at least 50% reduction in YHV infection in Sf9 cells relative to the untreated control was evident at 24h post infection with low cytoxicity. Inhibitory effects of chitosan- and QCH4-dsRNA complexes were comparable to that of dsRNA formulated with Cellfectin(®), a commercial lipid-based transfection reagent. The natural and quaternized chitosan prepared in this study can be used for shrimp virus-specific dsRNA delivery in insect cultures, and have potential for future development of dsRNA carriers in shrimp feed.

Disease will limit future food supply from the global crustacean fishery and aquaculture sectors

Seafood is a highly traded food commodity. Farmed and captured crustaceans contribute a significant proportion with annual production exceeding 10 M metric tonnes with first sale value of $40bn. The sector is dominated by farmed tropical marine shrimp, the fastest growing sector of the global aquaculture industry. It is significant in supporting rural livelihoods and alleviating poverty in producing nations within Asia and Latin America while forming an increasing contribution to aquatic food supply in more developed countries. Nations with marine borders often also support important marine fisheries for crustaceans that are regionally traded as live animals and commodity products. A general separation of net producing and net consuming nations for crustacean seafood has created a truly globalised food industry. Projections for increasing global demand for seafood in the face of level or declining fisheries requires continued expansion and intensification of aquaculture while ensuring best utilisation of captured stocks. Furthermore, continued pressure from consuming nations to ensure safe products for human consumption are being augmented by additional legislative requirements for animals (and their products) to be of low disease status. As a consequence, increasing emphasis is being placed on enforcement of regulations and better governance of the sector; currently this is a challenge in light of a fragmented industry and less stringent regulations associated with animal disease within producer nations. Current estimates predict that up to 40% of tropical shrimp production (>$3bn) is lost annually, mainly due to viral pathogens for which standard preventative measures (e.g. such as vaccination) are not feasible. In light of this problem, new approaches are urgently required to enhance yield by improving broodstock and larval sourcing, promoting best management practices by farmer outreach and supporting cutting-edge research that aims to harness the natural abilities of invertebrates to mitigate assault from pathogens (e.g. the use of RNA interference therapeutics). In terms of fisheries losses associated with disease, key issues are centred on mortality and quality degradation in the post-capture phase, largely due to poor grading and handling by fishers and the industry chain. Occurrence of disease in wild crustaceans is also widely reported, with some indications that climatic changes may be increasing susceptibility to important pathogens (e.g. the parasite Hematodinium). However, despite improvements in field and laboratory diagnostics, defining population-level effects of disease in these fisheries remains elusive. Coordination of disease specialists with fisheries scientists will be required to understand current and future impacts of existing and emergent diseases on wild stocks. Overall, the increasing demand for crustacean seafood in light of these issues signals a clear warning for the future sustainability of this global industry. The linking together of global experts in the culture, capture and trading of crustaceans with pathologists, epidemiologists, ecologists, therapeutics specialists and policy makers in the field of food security will allow these issues to be better identified and addressed.

Historic emergence, impact and current status of shrimp pathogens in Asia

It is estimated that approximately 60% of disease losses in shrimp aquaculture have been caused by viral pathogens and 20% by bacterial pathogens. By comparison, losses to fungi and parasites have been relatively small. For bacterial pathogens, Vibrio species are the most important while for viral pathogens importance has changed since 2003 when domesticated and genetically selected stocks of the American whiteleg shrimp Penaeus (Litopenaeus) vannamei (Boone 1931) replaced the formerly dominant giant tiger or black tiger shrimp Penaeus (Penaeus) monodon (Fabricius 1798) as the dominant cultivated species. For both species, white spot syndrome virus (WSSV) and yellow head virus (YHV) are the most lethal. Next most important for P. vannamei is infectious myonecrosis virus (IMNV), originally reported from Brazil, but since 2006 from Indonesia where it was probably introduced by careless importation of shrimp aquaculture stocks. So far, IMNV has not been reported from other countries in Asia. Former impacts of Taura syndrome virus (TSV) and infectious hypodermal and hematopoietic necrosis virus (IHHNV) on this species have dramatically declined due to the introduction of tolerant stocks and to implementation of good biosecurity practices. Another problem recently reported for P. vannamei in Asia is abdominal segment deformity disease (ASDD), possibly caused by a previously unknown retrovirus-like agent. Next most important after WSSV and YHV for P. monodon is monodon slow growth syndrome (MSGS) for which component causes appear to be Laem Singh virus (LSNV) and a cryptic integrase containing element (ICE). Hepatopancreatic parvovirus (HPV) and monodon baculovirus (MBV) may be problematic when captured P. monodon are used to produce larvae, but only in the absence of proper preventative measures. Since 2009 increasing losses with P. vannamei in China, Vietnam and now Thailand are associated with acute hepatopancreatic necrosis syndrome (AHPNS) of presently unknown cause. Despite these problems, total production of cultivated penaeid shrimp from Asia will probably continue to rise as transient disease problems are solved and use of post larvae originating from domesticated SPF shrimp stocks in more biosecure settings expands.

Co-interactive DNA-binding between a novel, immunophilin-like shrimp protein and VP15 nucleocapsid protein of white spot syndrome virus

White spot syndrome virus (WSSV) is one of the most serious pathogens of penaeid shrimp. Although its genome has been completely characterized, the functions of most of its putative proteins are not yet known. It has been suggested that the major nucleocapsid protein VP15 is involved in packaging of the WSSV genome during virion formation. However, little is known in its relationship with shrimp host cells. Using the yeast two-hybrid approach to screen a shrimp lymphoid organ (LO) cDNA library for proteins that might interact with VP15, a protein named PmFKBP46 was identified. It had high sequence similarity to a 46 kDa-immunophilin called FKBP46 from the lepidopteran Spodoptera frugiperda (the fall armyworm). The full length PmFKBP46 consisted of a 1,257-nucleotide open reading frame with a deduced amino acid sequence of 418 residues containing a putative FKBP-PPIase domain in the C-terminal region. Results from a GST pull-down assay and histological co-localization revealed that VP15 physically interacted with PmFKBP46 and that both proteins shared the same subcellular location in the nucleus. An electrophoretic mobility shift assay indicated that PmFKBP46 possessed DNA-binding activity and functionally co-interacted with VP15 in DNA binding. The overall results suggested that host PmFKBP46 might be involved in genome packaging by viral VP15 during virion assembly.

False rumours of disease outbreaks caused by infectious myonecrosis virus (IMNV) in the whiteleg shrimp in Asia

Background

Infectious myonecrosis virus (IMNV) disease outbreaks in cultivated whiteleg shrimp Penaeus (Litopenaeus) vannamei are characterized by gross signs of whitened abdominal muscles and by slow mortality reaching up to 70%. In 2006 the first disease outbreaks caused by IMNV in Asia occurred in Indonesia. Since then rumours have periodically circulated about IMNV disease outbreaks in other Asian countries. Our findings indicate that these are false rumours.

Findings

Our continual testing by nested RT-PCR of shrimp samples suspected of IMNV infection from various Asian countries since 2006 has yielded negative results, except for samples from Indonesia. Our results are supported by the lack of official reports of IMNV outbreaks since January 2007 in the Quarterly Report on Aquatic Animal Diseases (QAAD) from the Network of Aquaculture Centers in Asia Pacific (NACA). In most cases, our shrimp samples for which tissue sections were possible showed signs of muscle cramp syndrome that also commonly causes muscle whitening in stressed whiteleg shrimp. Thus, we suspect that most of the false rumours in Asia about IMNV outside of Indonesia have resulted because of muscle cramp syndrome.

Conclusions

Results from continual testing of suspected IMNV outbreaks in Asian countries other than Indonesia since 2006 and the lack of official country reports of IMNV outbreaks since January 2007, indicate that rumours of IMNV outbreaks in Asian countries outside of Indonesia are false. We suspect that confusion has arisen because muscle cramp syndrome causes similar signs of whitened tail muscles in whiteleg shrimp.

Shrimp molecular responses to viral pathogens

From almost negligible amounts in 1970, the quantity of cultivated shrimp (~3 million metric tons in 2007) has risen to approach that of the capture fishery and it constitutes a vital source of export income for many countries. Despite this success, viral diseases along the way have caused billions of dollars of losses for shrimp farmers. Desire to reduce the losses to white spot syndrome virus in particular, has stimulated much research since 2000 on the shrimp response to viral pathogens at the molecular level. The objective of the work is to develop novel, practical methods for improved disease control. This review covers the background and limitations of the current work, baseline studies and studies on humoral responses, on binding between shrimp and viral structural proteins and on intracellular responses. It also includes discussion of several important phenomena (i.e., the quasi immune response, viral co-infections, viral sequences in the shrimp genome and persistent viral infections) for which little or no molecular information is currently available, but is much needed.

A novel integrase-containing element may interact with Laem-Singh virus (LSNV) to cause slow growth in giant tiger shrimp

Background

From 2001-2003 monodon slow growth syndrome (MSGS) caused severe economic losses for Thai shrimp farmers who cultivated the native, giant tiger shrimp, and this led them to adopt exotic stocks of the domesticated whiteleg shrimp as the species of cultivation choice, despite the higher value of giant tiger shrimp. In 2008, newly discovered Laem-Singh virus (LSNV) was proposed as a necessary but insufficient cause of MSGS, and this stimulated the search for the additional component cause(s) of MSGS in the hope that discovery would lead to preventative measures that could revive cultivation of the higher value native shrimp species.

Results

Using a universal shotgun cloning protocol, a novel RNA, integrase-containing element (ICE) was found in giant tiger shrimp from MSGS ponds (GenBank accession number FJ498866). In situ hybridization probes and RT-PCR tests revealed that ICE and Laem-Singh virus (LSNV) occurred together in lymphoid organs (LO) of shrimp from MSGS ponds but not in shrimp from normal ponds. Tissue homogenates of shrimp from MSGS ponds yielded a fraction that gave positive RT-PCR reactions for both ICE and LSNV and showed viral-like particles by transmission electron microscopy (TEM). Bioassays of this fraction with juvenile giant tiger shrimp resulted in retarded growth with gross signs of MSGS, and in situ hybridization assays revealed ICE and LSNV together in LO, eyes and gills. Viral-like particles similar to those seen in tissue extracts from natural infections were also seen by TEM.

Conclusions

ICE and LSNV were found together only in shrimp from MSGS ponds and only in shrimp showing gross signs of MSGS after injection with a preparation containing ICE and LSNV. ICE was never found in the absence of LSNV although LSNV was sometimes found in normal shrimp in the absence of ICE. The results suggest that ICE and LSNV may act together as component causes of MSGS, but this cannot be proven conclusively without single and combined bioassays using purified preparations of both ICE and LSNV. Despite this ambiguity, it is recommended in the interim that ICE be added to the agents such as LSNV already listed for exclusion from domesticated stocks of the black tiger shrimp.

Response to Dengue virus infections altered by cytokine-like substances from mosquito cell cultures

Background

With both shrimp and commercial insects such as honey bees, it is known that stable, persistent viral infections characterized by absence of disease can sometimes shift to overt disease states as a result of various stress triggers and that this can result in serious economic losses. The main research interest of our group is to understand the dynamics of stable viral infections in shrimp and how they can be destabilized by stress. Since there are no continuous cell lines for crustaceans, we have used a C6/36 mosquito cell line infected with Dengue virus to test hypotheses regarding these interactions. As a result, we accidentally discovered two new cytokine-like substances in 5 kDa extracts from supernatant solutions of acutely and persistently infected mosquito cells.

Results

Naïve C6/36 cells were exposed for 48 h to 5 kDa membrane filtrates prepared from the supernatant medium of stable C6/36 mosquito cell cultures persistently-infected with Dengue virus. Subsequent challenge of naïve cells with a virulent stock of Dengue virus 2 (DEN-2) and analysis by confocal immunofluorescence microscopy using anti-DEN-2 antibody revealed a dramatic reduction in the percentage of DEN-2 infected cells when compared to control cells. Similar filtrates prepared from C6/36 cells with acute DEN-2 infections were used to treat stable C6/36 mosquito cell cultures persistently-infected with Dengue virus. Confocal immunofluorescence microscopy revealed destabilization in the form of an apoptosis-like response. Proteinase K treatment removed the cell-altering activities indicating that they were caused by small polypeptides similar to those previously reported from insects.

Conclusions

This is the first report of cytokine-like substances that can alter the responses of mosquito cells to Dengue virus. This simple model system allows detailed molecular studies on insect cytokine production and on cytokine activity in a standard insect cell line.

Knocking down a Taura syndrome virus (TSV) binding protein Lamr is lethal for the whiteleg shrimp Penaeus vannamei

A cDNA encoding a laminin receptor protein (Lamr) has been isolated from hemocytes of the Pacific white shrimp Penaeus (Litopenaeus) vannamei (Pv), based on primers designed from a previously published Lamr sequence of a Taura syndrome virus (TSV) binding protein of the black tiger shrimp Penaeus monodon (Pm). The deduced amino acid sequence of PvLamr shares 97% identity with PmLamr and has significant homology to laminin receptors and ribosomal protein p40 from various organisms. Tissue distribution analysis by RT-PCR revealed that Lamr transcripts were widely expressed in all tested tissues of P. monodon and Penaeus vannamei. PmLamr was constructed and expressed in Escherichia coli, and the recombinant protein was purified and used to raise a polyclonal antibody. The antiserum reacted with purified recombinant PmLamr and crude muscle tissue proteins from both P. monodon and P. vannamei, but not with hemocyte-free shrimp hemolymph. Examination of protein localization by immunohistochemical analysis revealed the presence of Lamr positive cytoplasm in subcuticular epithelial cells, hematopoietic tissues, epithelial cells of the stomach, epithelial cells of the anterior midgut cecum, antennal gland epithelial cells, F cells of the hepatopancreas, cells in the ovarian zone of proliferation and spheroid cells in the lymphoid organ. RNA interference-mediated silencing of the messenger from Lamr in P. vannamei led to shrimp mortality and indicated an essential function of Lamr for shrimp viability. A negative consequence was that the effect of Lamr knockdown on shrimp infection by Taura syndrome virus could not be assessed.

Rapid and sensitive detection of Macrobrachium rosenbergii nodavirus in giant freshwater prawns by reverse transcription loop-mediated isothermal amplification combined with a lateral flow dipstick

Loop-mediated isothermal amplification (LAMP) allows rapid amplification of nucleic acids under isothermal conditions. It can be combined with a chromatographic lateral flow dipstick (LFD) for much more efficient, field-friendly detection of MrNV. In this work, RT-LAMP was performed at 65 degrees C for 40 min, followed by 5 min for hybridization with an FITC-labeled DNA probe and 5 min for LFD resulted in visualization of DNA amplicons trapped at the LFD test line. Thus, total assay time, including 10 min for rapid RNA extraction was approximately 60 min. In addition to advantages of short assay time, confirmation of amplicon identity by hybridization and elimination of electrophoresis with carcinogenic ethidium bromide, the RT-LAMP-LFD was more sensitive than an existing RT-PCR method for detection of MrNV. The RT-LAMP-LFD method gave negative test results with nucleic acid extracts from normal shrimp and from shrimp infected with other viruses including DNA viruses [PstDNV (IHHNV), PemoNPV (MBV), PmDNV (HPV), WSSV] and RNA viruses (TSV, IMNV, YHV/GAV).

Identification and characterization of Alix/AIP1 interacting proteins from the black tiger shrimp, Penaeus monodon

Apoptosis is proposed to be a major cause of death in shrimp viral infections. From our previous study, an apoptosis-related gene, Pm-Alix, was identified from the black tiger shrimp. Its expression was high in defence-related tissues including haemocytes and the lymphoid organ. To clarify its possible role in shrimp, we used Pm-Alix as bait in a yeast two-hybrid analysis to search for Alix interacting proteins in shrimp. Two cDNA sequences discovered had homology to a predicted ubiquitin C of the purple sea urchin, Strongylocentrotus purpuratus, and to a guanylyl cyclase of the red swamp crayfish, Procambarus clarkii. In vitro pull-down assays confirmed positive interaction between Pm-Alix and both proteins. Tissue distribution analysis revealed that Pm-Alix and the two binding partners were widely expressed in various tissues but more highly expressed in haemocytes. However, no significant positive or negative correlation was found in the expression of these genes as shrimp approached morbidity and death after challenge with white spot syndrome virus. Thus, the results suggested that Alix and its interacting partners did not play a direct role related to shrimp death.

Successful propagation of shrimp yellow head virus in immortal mosquito cells

Research on crustacean viruses is hampered by the lack of continuous cell lines susceptible to them. To overcome this problem, we previously challenged immortal mosquito and lepidopteran cell lines with shrimp yellow head virus (YHV), followed by serial, split-passage of whole cells, and showed that this produced cells that persistently expressed YHV antigens. To determine whether such insect cultures positive for YHV antigens could be used to infect shrimp Penaeus monodon with YHV, culture supernatants and whole-cell homogenates were used to challenge shrimp by injection. Shrimp injected with culture supernatants could not be infected. However, shrimp injection-challenged with whole-cell homogenates from Passage 5 (early-passage) of such cultures died with histological and clinical signs typical for yellow head disease (YHD), while homogenates of mock-passaged, YHV-challenged cells did not. By contrast, shrimp challenged with cell homogenates of late-passage cultures became infected with YHV, but survived, suggesting that YHV attenuation had occurred during its long-term serial passage in insect cells. Thus, YHV could be propagated successfully in C6/36 mosquito cells and used at low passage numbers as a source of inoculum to initiate lethal infections in shrimp. This partially solves the problem of lack of continuous shrimp cell lines for cultivation of YHV.

Impact of yellow head virus outbreaks in the whiteleg shrimp, Penaeus vannamei (Boone), in Thailand

Yellow head virus (YHV) is known as a major pathogen in the black tiger shrimp, Penaeus (Penaeus) monodon. It can also cause serious mortality in farmed whiteleg shrimp, Penaeus (Litopenaeus) vannamei. However, there is no published information on the economic and/or production impact of the disease in P. vannamei. Shrimp with gross signs of YHV disease (faded body colour and 60-70% mortality) were observed in 20 study farms rearing P. vannamei in the central part of Thailand from the end of 2007 through early 2008. The estimated economic loss for these farms according to the Thai Animal Aquaculture Association was approximately US$3 million. Detailed sequence analysis of RT-PCR amplicons from shrimp in all the study ponds revealed the presence of YHV Type 1b (YHV-1b) alone (characterized by a 162-bp deletion in the ORF3 region encoding the structural gene for gp116) and the absence of YHV Type 1a (YHV-1a), the original YHV type reported from Thailand. Despite the large 162-bp deletion (= 54 deduced amino acids) in the gp116 structural gene, histopathology of YHV-1b infections was identical to that of YHV-1a infections, and electron microscopy revealed that YHV-1b virions were morphologically indistinguishable from those previously reported for YHV-1a. In addition, an existing commercial RT-PCR detection kit and an immunochromatographic test strip for the detection of YHV were proven to have been valid tests for both YHV-1b and YHV-1a. The source of the virus for these outbreaks was unlikely to have been the post-larvae used to stock the ponds, as they were derived from domesticated specific pathogen-free stocks free of YHV. Thus, it is possible that they originated from an unknown, natural reservoir.

Persistent, triple-virus co-infections in mosquito cells

Background

It is known that insects and crustaceans can carry simultaneous, active infections of two or more viruses without showing signs of disease, but it was not clear whether co-infecting viruses occupied the same cells or different cells in common target tissues. Our previous work showed that successive challenge of mosquito cell cultures followed by serial, split-passage resulted in stabilized cultures with 100% of the cells co-infected with Dengue virus (DEN) and an insect parvovirus (densovirus) (DNV). By addition of Japanese encephalitis virus (JE), we tested our hypothesis that stable, persistent, triple-virus co-infections could be obtained by the same process.

Results

Using immunocytochemistry by confocal microscopy, we found that JE super-challenge of cells dually infected with DEN and DNV resulted in stable cultures without signs of cytopathology, and with 99% of the cells producing antigens of the 3 viruses. Location of antigens for all 3 viruses in the triple co-infections was dominant in the cell nuclei. Except for DNV, this differed from the distribution in cells persistently infected with the individual viruses or co-infected with DNV and DEN. The dependence of viral antigen distribution on single infection or co-infection status suggested that host cells underwent an adaptive process to accommodate 2 or more viruses.

Conclusions

Individual mosquito cells can accommodate at least 3 viruses simultaneously in an adaptive manner. The phenomenon provides an opportunity for genetic exchange between diverse viruses and it may have important medical and veterinary implications for arboviruses.

Hypothesis for heritable, anti-viral immunity in crustaceans and insects

Background

It is known that crustaceans and insects can persistently carry one or more viral pathogens at low levels, without signs of disease. They may transmit them to their offspring or to naïve individuals, often with lethal consequences. The underlying molecular mechanisms have not been elucidated, but the process has been called viral accommodation. Since tolerance to one virus does not confer tolerance to another, tolerance is pathogen-specific, so the requirement for a specific pathogen response mechanism (memory) was included in the original viral accommodation concept. Later, it was hypothesized that specific responses were based on the presence of viruses in persistent infections. However, recent developments suggest that specific responses may be based on viral sequences inserted into the host genome.

Presentation of the hypothesis

Non-retroviral fragments of both RNA and DNA viruses have been found in insect and crustacean genomes. In addition, reverse-transcriptase (RT) and integrase (IN) sequences are also common in their genomes. It is hypothesized that shrimp and other arthropods use these RT to recognize "foreign" mRNA of both RNA and DNA viruses and use the integrases (IN) to randomly insert short cDNA sequences into their genomes. By chance, some of these sequences result in production of immunospecific RNA (imRNA) capable of stimulating RNAi that suppresses viral propagation. Individuals with protective inserts would pass these on to the next generation, together with similar protective inserts for other viruses that could be amalgamated rapidly in individual offspring by random assortment of chromosomes. The most successful individuals would be environmentally selected from billions of offspring.

Conclusion

This hypothesis for immunity based on an imRNA generation mechanism fits with the general principle of invertebrate immunity based on a non-host, "pattern recognition" process. If proven correct, understanding the process would allow directed preparation of vaccines for selection of crustacean and insect lines applicable in commercial production species (e.g., shrimp and bees) or in control of insect-borne diseases. Arising from a natural host mechanism, the resulting animals would not be artificially, genetically modified (GMO).

Reviewers

This article was reviewed by Akria Shibuya, Eugene V. Koonin and L. Aravind.

Parvo-like virus in the hepatopancreas of freshwater prawns Macrobrachium rosenbergii cultivated in Thailand

A survey of cultivated giant freshwater prawns Macrobrachium rosenbergii from Thailand revealed the presence of unusual spherical to ovoid inclusions in nuclei of hepatopancreas tubule epithelial cells. These began as small eosinophilic inclusions that became more basophilic as they increased in size. They were present in both R-cells and E-cells but were largest and deeply basophilic only in the E-cells. Confocal laser microscopy revealed that stained nucleic acid fluorescence from the inclusions was lost by treatment with DNase I specific for double- and single-stranded DNA and also lost or reduced by treatment with mungbean nuclease specific for single-stranded nucleic acids. Transmission electron microscopy (TEM) revealed that the inclusions contained tightly packed, unenveloped, viral-like particles of approximately 25 to 30 nm diameter, resembling those produced by shrimp parvoviruses. However, PCR, in situ hybridization and immunohistochemical tests for shrimp parvoviruses previously reported from Thailand were all negative. These results suggested that the inclusions contained a parvo-like virus, not previously reported from M. rosenbergii in Thailand.

Hypothesis for heritable, anti-viral immunity in crustaceans and insects

Correction to Flegel, TW: Hypothesis for heritable, anti-viral immunity in crustaceans and insects. Biology Direct 2009, 4:32.

Rapid and sensitive detection of white spot syndrome virus by loop-mediated isothermal amplification combined with a lateral flow dipstick

Loop-mediated isothermal amplification (LAMP) allows rapid amplification of nucleic acids under isothermal conditions using a set of four specifically designed primers that recognize six distinct target sequences. It can be combined with a chromatographic lateral flow dipstick (LFD) for highly specific, rapid and simple visual detection of WSSV-specific amplicons. Using this protocol, a 30-min amplification followed by 5 min hybridization with an FITC-labeled DNA probe and 5 min LFD resulted in visualization of DNA amplicons trapped at the LFD test line. Thus, 10 min for rapid DNA extraction followed by LAMP combined with LFD detection resulted in a total assay time of approximately 50 min. Detection sensitivity was comparable to other commonly-used methods for nested PCR detection of WSSV but had the additional advantages of reduced assay time, confirmation of amplicon identity by hybridization and elimination of electrophoresis with carcinogenic ethidium bromide.

Detection of shrimp infectious myonecrosis virus by reverse transcription loop-mediated isothermal amplification combined with a lateral flow dipstick

Infectious myonecrosis virus (IMNV) has caused a slowly progressive disease with cumulative mortalities of up to 70% or more in cultured Penaeus (Litopenaeus) vannamei in Northeast Brazil and Indonesia. Rapid detection of viruses by loop-mediated isothermal amplification (LAMP) of genomic material with high specificity and sensitivity can be applied for diagnosis, monitoring and control of diseases in shrimp aquaculture. Using an IMNV template, successful detection was achieved after a 60-min RT-LAMP reaction using biotin-labeled primers followed by 5min hybridization with an FITC-labeled DNA probe and 5min assay using a chromatographic lateral flow dipstick (LFD). Thus, the combined system of RT-LAMP and LFD required a total assay interval of less than 75min, excluding the RNA extraction time. The sensitivity of detection was comparable to that of other commonly used methods for nested RT-PCR detection of IMNV. In addition to reducing amplicon detection time when compared to electrophoresis, LFD confirmed amplicon identity by hybridization and eliminated the need to handle carcinogenic ethidium bromide. The RT-LAMP-LFD method gave negative test results with nucleic acid extracts from normal shrimp and from shrimp infected with other viruses including infectious hypodermal hematopoietic necrosis virus (IHHNV), monodon baculovirus (MBV), a hepatopancreatic parvovirus from P. monodon (PmDNV), white spot syndrome virus (WSSV), yellow head virus (YHV), Taura syndrome virus (TSV), Macrobrachium rosenbergii nodavirus (MrNV) and gill associated virus (GAV).

Up-regulation of ribophorin I after yellow head virus (YHV) challenge in black tiger shrimp Penaeus monodon

This work constitutes the second report from a continuing investigation of shrimp genes that may be involved in apoptosis associated death resulting from yellow head virus (YHV) infection. Here, we describe from the black tiger shrimp Penaeus monodon, a ribophorin I-like gene that is probably a subunit of the oligosaccharyltransferase complex (OST), a key enzyme in N-linked glycosylation that occurs in the endoplasmic reticulum. The OST complex also contains DAD1 (defender against apoptotic death 1) that has been reported to control apoptosis and that we have previously reported from P. monodon. The full length ribophorin I of P. monodon comprised 2157 bp with the ORF of 1806 bp corresponding to 601 deduced amino acids and three putative N-linked glycosylation sites. Analysis revealed hydrophobic properties implying that it could be a membrane protein. Tissue distribution analysis using real-time RT-PCR with SYBR Green revealed that ribophorin I was endogenously expressed in all examined tissues of normal shrimp. However, unlike DAD1 that was down-regulated after YHV challenge, ribophorin I expression was up-regulated and remained high until the moribund stage.

Mosquito cells accommodate balanced, persistent co-infections with a densovirus and Dengue virus

To study persistent viral co-infections in arthropods, we first produced stable, persistently infected C6/36 mosquito cell cultures by serial passage of exponentially growing whole cells infected with either a densovirus (AalDNV) or Dengue virus (DEN-2). We then obtained stable, persistent co-infections by reciprocal super-challenge and similar passaging. Persistently infected cultures did not differ from naïve-cell cultures in growth rate and cell morphology. Nor did they differ in high production of both viruses with high infection rates for naïve C6/36 cells. Immunocytochemistry revealed that 99-100% of the cells were coinfected but that super-infection order had some effect on antigen distribution for the two viruses. Our results combined with existing field information and previously published experimental work suggest that the capacity to support stable, viral co-infections may be a general phenomenon for arthropod cells, and that they may be achieved easily and rapidly by serial passaging of whole cultured cells. Such persistent infections would facilitate studies on interactions between co-infecting viruses.