Multiplex RT-nested PCR differentiation of gill-associated virus (Australia) from yellow head virus (Thailand) of Penaeus monodon

Effect of supersaturated dissolved oxygen on growth-, survival-, and immune-related gene expression of Pacific white shrimp (Litopenaeus vannamei)

Background and Aim

Oxygen concentration is an essential water quality parameter for aquaculture systems. Recently, supersaturated dissolved oxygen (DO) has been widely used in aquaculture systems to prevent oxygen depletion; however, the long-term effects of supersaturated DO exposure on aquatic animals have not been studied. In this study, we examined the effects of supersaturated DO on the growth, survival, and gene expression of Pacific white shrimp (Litopenaeus vannamei).

Materials and Methods

Specific pathogen-free shrimp with a body weight of 8.22 ± 0.03 g were randomly assigned to two groups with four replicates at a density of 15 shrimps per tank. Shrimp were cultivated in recirculating tanks containing 50 L of 15 ppt seawater in each replicate. Oxygen was supplied at 5 mg/L to the control tanks using an air microbubble generator and at 15 mg/L to the treatment tanks using a pure oxygen microbubble generator. Shrimp were fed commercial feed pellets containing 39% protein at 4% of their body weight per day for 30 days. Average daily growth (ADG) and feed conversion ratio (FCR) were determined on days 15 and 30. Shrimp molting was measured every day. Individual hemolymph samples were obtained and analyzed for total hemocyte count, differential hemocyte count, and expression of growth- and immune-related genes at the end of the experiment.

Results

Long-term exposure to supersaturated DO significantly affected shrimp growth. After 30 days of supersaturated DO treatment, the final weight and ADG were 14.73 ± 0.16 g and 0.22 ± 0.04, respectively. Shrimp treated with normal aeration showed significantly lower weight (12.13 ± 0.13 g) and ADG (0.13 ± 0.00) compared with the control group. FCR was 1.55 ± 0.04 in the treatment group and 2.51 ± 0.09 in the control group. Notably, the shrimp molting count was 1.55-fold higher in the supersaturated DO treatment than in the supersaturated DO treatment. The expression of growth-related genes, such as alpha-amylase, cathepsin L, and chitotriosidase, was 1.40-, 1.48-, and 1.35-fold higher, respectively, after supersaturated DO treatment. Moreover, the treatment increased the expression of anti-lipopolysaccharide factor, crustin, penaeidin3, and heat shock protein 70 genes by 1.23-, 2.07-, 4.20-, and 679.04-fold, respectively, compared to the controls.

Conclusion

Supersaturated DO increased growth and ADG production and decreased FCR. Furthermore, enhanced immune-related gene expression by supersaturated DO may improve shrimp health and reduce disease risk during cultivation.

Coinfection with Yellow Head Virus Genotype 8 (YHV-8) and Oriental Wenrivirus 1 (OWV1) in Wild Penaeus chinensis from the Yellow Sea

At present, there are few studies on the epidemiology of diseases in wild Chinese white shrimp Penaeus chinensis. In order to enrich the epidemiological information of the World Organisation for Animal Health (WOAH)-listed and emerging diseases in wild P. chinensis, we collected a total of 37 wild P. chinensis from the Yellow Sea in the past three years and carried out molecular detection tests for eleven shrimp pathogens. The results showed that infectious hypodermal and hematopoietic necrosis virus (IHHNV), Decapod iridescent virus 1 (DIV1), yellow head virus genotype 8 (YHV-8), and oriental wenrivirus 1 (OWV1) could be detected in collected wild P. chinensis. Among them, the coexistence of IHHNV and DIV1 was confirmed using qPCR, PCR, and sequence analysis with pooled samples. The infection with YHV-8 and OWV1 in shrimp was studied using molecular diagnosis, phylogenetic analysis, and transmission electron microscopy. It is worth highlighting that this study revealed the high prevalence of coinfection with YHV-8 and OWV1 in wild P. chinensis populations and the transmission risk of these viruses between the wild and farmed P. chinensis populations. This study enriches the epidemiological information of WOAH-listed and emerging diseases in wild P. chinensis in the Yellow Sea and raises concerns about biosecurity issues related to wild shrimp resources.

The taxonomy, host range and pathogenicity of coronaviruses and other viruses in the Nidovirales order

The frequent emergence of coronavirus (CoV) epidemics has seriously threatened public health and stock farming. The major hosts for CoVs are birds and mammals. Although most CoVs inhabit their specific natural hosts, some may occasionally cross the host barrier to infect livestock and even people, causing a variety of diseases. Since the beginning of the new century, increasing attention has been given to research on CoVs due to the emergence of highly pathogenic and genetically diverse CoVs that have caused several epidemics, including the recent COVID-19 pandemic. CoVs belong to the Coronaviridae family of the Nidovirales order. Recently, advanced techniques for viral detection and viral genome analyses have enabled characterization of many new nidoviruses than ever and have greatly expanded the Nidovirales order with new classification and nomenclature. Here, we first provide an overview of the latest research progress in the classification of the Nidovirales order and then introduce the host range, genetic variation, genomic pattern and pathogenic features of epidemic CoVs and other epidemic viruses. This information will promote understanding of the phylogenetic relationship and infectious transmission of various pathogenic nidoviruses, including epidemic CoVs, which will benefit virological research and viral disease control.

Effective suppression of yellow head virus replication in Penaeus monodon hemocytes using constitutive expression vector for long-hairpin RNA (lhRNA)

Double-stranded RNA (dsRNA) is employed to down-regulate the expression of specific genes of shrimp viral pathogens through the RNA interference (RNAi) pathway. The administration of dsRNA into shrimp has been shown to be an effective strategy to block yellow head virus (YHV) progression. In this study, a vector (pLVX-AcGFP1-N1) was developed to introduce a long-hairpin RNA (lhRNA) silencing cassette under a CMV promoter, so-called "pLVX-lhRdRp", against the RNA-dependent RNA polymerase (RdRp) gene of YHV. A primary culture of hemocytes isolated from Penaeus monodon was transfected with the pLVX-lhRdRp vector, generating transcripts of lhRNAs as early as 12 h post transfection. Twelve hours prior to YHV challenge, the primary hemocyte cell culture was transfected with pLVX-lhRdRp, whereas control groups were transfected with pLVX-AcGFP1-N1 or no transfection. The group treated with pLVX-lhRdRp significantly suppressed YHV replication at 24-72 h after YHV challenge. The results from RT-PCR and immunohistochemistry confirmed that both mRNA and protein expression of YHV were effectively inhibited by the pLVX-lhRdRp vector. Thus, our hemocyte culture and dsRNA expression plasmid with constitutive promoter have potential as a platform to test DNA constructs expressing long-hairpin RNA against pathogenic viral infection and as a RNAi-based DNA vaccine in shrimp.

TaqMan real-time and conventional nested PCR tests specific to yellow head virus genotype 7 (YHV7) identified in giant tiger shrimp in Australia

In 2013, a unique seventh yellow head virus genotype (YHV7) was detected in Black Tiger shrimp (Penaeus monodon) broodstock that suffered high mortality following their capture from Joseph Bonaparte Gulf (JBG) in northern Australia. To assist with its diagnosis and assessment of its distribution, prevalence and pathogenicity, YHV7-specific TaqMan real-time qPCR and conventional nested PCR primer sets were designed to ORF1b gene sequences divergent from the other YHV genotypes. Using high (≥10⁸) copies of plasmid (p)DNA controls containing ORF1b gene inserts of representative strains of YHV genotypes 1-7, both PCR tests displayed specificity for YHV7. Amplifications of serial 10-fold dilutions of quantified YHV7 pDNA and synthetic ssRNA showed that both tests could reliably detect 10 genome copies. Pleopods/gills from wild P. monodon sourced from locations in geographically disparate regions across northern Australia as well as 96 juveniles (48 either appearing normal or displaying signs of morbidity) from a commercial pond experiencing mortalities were screened to partially validate the diagnostic capacity of the qPCR test. Based on these data and PCR primer/probe sequence mismatches with other newly identified YHV genotypes, both YHV7-specific PCR tests should prove useful in the sensitive detection and discrimination of this genotype from YHV 2 (gill-associated virus) and YHV6 that can occur in Australian P. monodon, as well as from YHV genotypes currently listed as exotic to Australia.

Identification and Characterization of a Ribose 2'-O-Methyltransferase Encoded by the Ronivirus Branch of Nidovirales

The order Nidovirales currently comprises four virus families: Arteriviridae, Coronaviridae (divided into the subfamilies Coronavirinae and Torovirinae), Roniviridae, and the recently recognized Mesoniviridae RNA cap formation and methylation have been best studied for coronaviruses, with emphasis on the identification and characterization of two virus-encoded methyltransferases (MTases) involved in RNA capping, a guanine-N7-MTase and a ribose-2'-O-MTase. Although bioinformatics analyses suggest that these MTases may also be encoded by other nidoviruses with large genomes, such as toroviruses and roniviruses, no experimental evidence has been reported thus far. In this study, we show that a ronivirus, gill-associated virus (GAV), encodes the 2'-O-MTase activity, although we could not detect 2'-O-MTase activity for the homologous protein of a torovirus, equine torovirus, which is more closely related to coronaviruses. Like the coronavirus 2'-O-MTase, the roniviral 2'-O-MTase harbors a catalytic K-D-K-E tetrad that is conserved among 2'-O-MTases and can target only the N7-methylated cap structure of adenylate-primed RNA substrates. However, in contrast with the coronavirus protein, roniviral 2'-O-MTase does not require a protein cofactor for stimulation of its activity and differs in its preference for several biochemical parameters, such as reaction temperature and pH. Furthermore, the ronivirus 2'-O-MTase can be targeted by MTase inhibitors. These results extend our current understanding of nidovirus RNA cap formation and methylation beyond the coronavirus family.

IMPORTANCE

Methylation of the 5'-cap structure of viral RNAs plays important roles in genome replication and evasion of innate recognition of viral RNAs by cellular sensors. It is known that coronavirus nsp14 acts as an N7-(guanine)-methyltransferase (MTase) and nsp16 as a 2'-O-MTase, which are involved in the modification of RNA cap structure. However, these enzymatic activities have not been shown for any other nidoviruses beyond coronaviruses in the order Nidovirales In this study, we identified a 2'-O-methyltransferase encoded by ronivirus that shows common and unique features in comparison with that of coronaviruses. Ronivirus 2'-O-MTase does not need a protein cofactor for MTase activity, whereas coronavirus nsp16 needs the stimulating factor nsp10 for its full activity. The conserved K-D-K-E catalytic tetrad is identified in ronivirus 2'-O-MTase. These results extend our understanding of nidovirus RNA capping and methylation beyond coronaviruses and also strengthen the evolutionary and functional links between roniviruses and coronaviruses.

Reverse transcription-PCR assays for the differentiation of various US porcine epidemic diarrhea virus strains

Concurrently, several porcine epidemic diarrhea virus (PEDV) variants are circulating in US swine farms, including the original US and the spike insertion-deletion (S-INDEL) strains. In this study, reverse transcription (RT)-PCR assays for the detection and differentiation of different US PEDV variants were developed based on the differences in the S1 domain of the spike (S) gene. This assay successfully differentiated three PEDV strains: PC22A (the original US virulent), Iowa106 (S-INDEL), and PC177 (S-197DEL) that was derived from cell culture adaptation and has a 197 amino acid-deletion in the S1 domain. The assays did not amplify the porcine deltacoronavirus OH-FD22 strain or transmissible gastroenteritis virus Miller strain. It is the first report on the development of RT-PCR assays allowing the detection and differentiation of all major types of US PEDV variants.

Nidoviruses of Fish and Crustaceans

Viruses with diverse virion architectures demarcated into four families in the order Nidovirales have been discovered in vertebrate mammalian and fish species, as well as in invertebrate crustacean and mosquito species. The order is unified by nidoviruses sharing intermediate (12.7 kb) to very long (31.7 kb) (+) ssRNA genomes, each possessing a long 5′-terminal gene encoding overlapping ORF1a and ORF1b reading frames that contain a diversity of functionally related enzymes and that are translated in toto using a −1 ribosomal frameshift mechanism, as well as by semiconserved strategies for transcribing a nested set of 3′-coterminal subgenomic mRNAs that translate the viral proteins. The nidovirus that is most important to an aquaculture species is yellow head virus (YHV), which causes disease in shrimp farmed throughout the Eastern Hemisphere and is classified in the genus Okavirus, family Roniviridae. Fathead minnow nidovirus, genus Bafinivirus, subfamily Torovirinae, family Coronaviridae, also causes disease in minnows grown for the baitfish industry in the United States. Virions similar in morphology to okaviruses and bafiniviruses have also been detected in several crab species. Of these, however, only Eriocheir sinensis ronivirus, which causes disease in the Chinese mitten crab, an important freshwater aquaculture species in China, has been shown to possess a ~22 kb ssRNA genome that supports its being a nidovirus, but its taxonomic classification awaits genome sequence analysis. This chapter provides an overview of the structure, replication and biology of these viruses with a particular focus on YHV disease characteristics, diagnostic methods and disease prevention strategies.

New yellow head virus genotype (YHV7) in giant tiger shrimp Penaeus monodon indigenous to northern Australia

In 2012, giant tiger shrimp Penaeus monodon originally sourced from Joseph Bonaparte Gulf in northern Australia were examined in an attempt to identify the cause of elevated mortalities among broodstock at a Queensland hatchery. Nucleic acid extracted from ethanol-fixed gills of 3 individual shrimp tested positive using the OIE YHV Protocol 2 RT-PCR designed to differentiate yellow head virus (YHV1) from gill-associated virus (GAV, synonymous with YHV2) and the OIE YHV Protocol 3 RT-nested PCR designed for consensus detection of YHV genotypes. Sequence analysis of the 794 bp (Protocol 2) and 359 bp (Protocol 3) amplicons from 2 distinct regions of ORF1b showed that the yellow-head-complex virus detected was novel when compared with Genotypes 1 to 6. Nucleotide identity on the Protocol 2 and Protocol 3 ORF1b sequences was highest with the highly pathogenic YHV1 genotype (81 and 87%, respectively) that emerged in P. monodon in Thailand and lower with GAV (78 and 82%, respectively) that is enzootic to P. monodon inhabiting eastern Australia. Comparison of a longer (725 bp) ORF1b sequence, spanning the Protocol 3 region and amplified using a modified YH30/31 RT-nPCR, provided further phylogenetic evidence for the virus being distinct from the 6 described YHV genotypes. The virus represents a unique seventh YHV genotype (YHV7). Despite the mortalities observed, the role of YHV7 remains unknown.

Exploiting formalin-preserved fish specimens for resources of DNA barcoding

With the development of the DNA barcoding project, a large number of specimens are required to establish the library of reference barcode. Formalin-fixed samples from museums provide a potential resource for it. However, recovery of DNA and amplification of the target gene from formalin-fixed samples are challenging. In this study, a hot alkali pre-treatment accompanied by the use of cetyltrimethylammonium bromide (CTAB) method was employed for DNA recovery from formalin-preserved samples, with the purpose of pursuing the optimal condition for high quantity and quality of DNA and minimizing PCR inhibition. Meanwhile, a semi-nested PCR-based method was developed to enhance the efficacy of amplification. This advanced protocol was demonstrated to be reliable and effective. Even for 23-year-old samples, genomic DNA could be extracted, and COI gene was correctly sequenced.

RNA-binding domain in the nucleocapsid protein of gill-associated nidovirus of penaeid shrimp

Gill-associated virus (GAV) infects Penaeus monodon shrimp and is the type species okavirus in the Roniviridae, the only invertebrate nidoviruses known currently. Electrophoretic mobility shift assays (EMSAs) using His₆-tagged full-length and truncated proteins were employed to examine the nucleic acid binding properties of the GAV nucleocapsid (N) protein in vitro. The EMSAs showed full-length N protein to bind to all synthetic single-stranded (ss)RNAs tested independent of their sequence. The ssRNAs included (+) and (−) sense regions of the GAV genome as well as a (+) sense region of the M RNA segment of Mourilyan virus, a crustacean bunya-like virus. GAV N protein also bound to double-stranded (ds)RNAs prepared to GAV ORF1b gene regions and to bacteriophage M13 genomic ssDNA. EMSAs using the five N protein constructs with variable-length N-terminal and/or C-terminal truncations localized the RNA binding domain to a 50 amino acid (aa) N-terminal sequence spanning Met¹¹ to Arg⁶⁰. Similarly to other RNA binding proteins, the first 16 aa portion of this sequence was proline/arginine rich. To examine this domain in more detail, the 18 aa peptide (M¹¹PVRRPLPPQPPRNARLI²⁹) encompassing this sequence was synthesized and found to bind nucleic acids similarly to the full-length N protein in EMSAs. The data indicate a fundamental role for the GAV N protein proline/arginine-rich domain in nucleating genomic ssRNA to form nucleocapsids. Moreover, as the synthetic peptide formed higher-order complexes in the presence of RNA, the domain might also play some role in protein/protein interactions stabilizing the helical structure of GAV nucleocapsids.

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.

A consensus real-time RT-PCR for detection of all genotypic variants of yellow head virus of penaeid shrimp

A real-time quantitative (q)RT-PCR employing consensus degenerate PCR primers was developed to detect all six genotypes known currently to comprise the yellow head virus (YHV) complex and found commonly in Penaeus monodon shrimp. The test primers possess only limited (eight-fold) degeneracy and target ORF1b gene sequences identified to be highly conserved amongst 57 strains of the six genotypes detected in P. monodon sourced from various regions of the Indo-Pacific. The qRT-PCR amplifies a 147bp sequence and analysis of dilutions of synthetic genotype 2 RNA showed it to be 99.8% efficient and capable of detecting as few as 2.5 RNA copies reliably. As the test detects all six YH-complex genotypes, is extremely sensitive, capable of quantifying infection loads, and amenable to high-throughput application, it should prove useful for managing infections in P. monodon broodstock and seedstock used for aquaculture.

Real-time reverse transcription loop-mediated isothermal amplification for rapid detection of yellow head virus in shrimp

A real-time reverse transcription loop-mediated isothermal amplification (real-time RT-LAMP) method was applied for detecting the replicase polyprotein-encoding gene of yellow head virus (YHV) in shrimp, Penaeus monodon. It is a novel, gene-specific assay that amplifies nucleic acid with high specificity, sensitivity and rapidity under isothermal conditions using a set of six specially designed primers that recognize eight distinct sequences of the target gene. This method works with even low copies of DNA and is based on magnesium pyrophosphate turbidity detection by an inexpensive photometer for quantitative analysis. A user-friendly protocol was developed with optimal conditions standardized at 63 °C for 60 min. With this protocol, the assay sensitivity was 10 times higher than the widely used YHV nested RT-PCR system. Cross-reactivity analysis using other shrimp virus DNA/cDNA and YHV-negative shrimp demonstrated high specificity of the assay. The real-time RT-LAMP method was performed also for an internal control gene, EF-1α, to compare with the expressions of the YHV gene in different organs of infected shrimp, and the resulting standard curves showed high correlation coefficient values. These results suggest that this assay is applicable widely as a new quantitative detection method in the pursuit of YHV-free shrimp culture.

Homologous genetic recombination in the yellow head complex of nidoviruses infecting Penaeus monodon shrimp

Yellow head virus (YHV) is a highly virulent pathogen of Penaeus monodon shrimp. It is one of six known genotypes in the yellow head complex of nidoviruses which also includes mildly pathogenic gill-associated virus (GAV, genotype 2) and four other genotypes (genotypes 3-6) that have been detected only in healthy shrimp. In this study, comparative phylogenetic analyses conducted on replicase- (ORF1b) and glycoprotein- (ORF3) gene amplicons identified 10 putative natural recombinants amongst 28 viruses representing all six genotypes from across the Indo-Pacific region. The approximately 4.6 kb genomic region spanning the two amplicons was sequenced for three putative recombinant viruses from Vietnam (genotype 3/5), the Philippines (genotype 5/2) and Indonesia (genotype 3/2). SimPlot analysis using these and representative parental virus sequences confirmed that each was a recombinant genotype and identified a recombination hotspot in a region just upstream of the ORF1b C-terminus. Maximum-likelihood breakpoint analysis predicted identical crossover positions in the Vietnamese and Indonesian recombinants, and a crossover position 12 nt upstream in the Philippine recombinant. Homologous genetic recombination in the same genome region was also demonstrated in recombinants generated experimentally in shrimp co-infected with YHV and GAV. The high frequency with which natural recombinants were identified indicates that genetic exchange amongst genotypes is occurring commonly in Asia and playing a significant role in expanding the genetic diversity in the yellow head complex. This is the first evidence of genetic recombination in viruses infecting crustaceans and has significant implications for the pathogenesis of infection and diagnosis of these newly emerging invertebrate pathogens.

Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies

Shrimp aquaculture has grown rapidly over several decades to become a major global industry that serves the increasing consumer demand for seafood and has contributed significantly to socio-economic development in many poor coastal communities. However, the ecological disturbances and changes in patterns of trade associated with the development of shrimp farming have presented many of the pre-conditions for the emergence and spread of disease. Shrimp are displaced from their natural environments, provided artificial or alternative feeds, stocked in high density, exposed to stress through changes in water quality and are transported nationally and internationally, either live or as frozen product. These practices have provided opportunities for increased pathogenicity of existing infections, exposure to new pathogens, and the rapid transmission and transboundary spread of disease. Not surprisingly, a succession of new viral diseases has devastated the production and livelihoods of farmers and their sustaining communities. This review examines the major viral pathogens of farmed shrimp, the likely reasons for their emergence and spread, and the consequences for the structure and operation of the shrimp farming industry. In addition, this review discusses the health management strategies that have been introduced to combat the major pathogens and the reasons that disease continues to have an impact, particularly on poor, small-holder farmers in Asia.

Consensus RT-nested PCR detection of yellow head complex genotypes in penaeid shrimp

A consensus RT-nested (n)PCR is described that detects the six distinct genotypic variants in the yellow head virus (YHV) complex. The PCR primers targeted ORF1b gene regions more highly conserved amongst the reference strains of YHV (genotype 1) and gill-associated virus (GAV, genotype 2) and a set of 57 field isolates containing multiple representatives of each genotype. The test employed short PCR (359 bp) and nPCR (147 bp) amplicons to minimise the effects of RNA degradation. To ensure < or = 8-primer degeneracy, two primers were designed to each site, one accommodating sequence variations amongst genotype 1 isolates and the other variations amongst isolates of the other genotypes. The analytical sensitivity limits of the PCR and nPCR were estimated to be approximately 1250 and approximately 1.25 RNA copies, respectively. The superior group-specificity of the consensus RT-nPCR compared to other OIE-recommended PCR tests for YHV/GAV was demonstrated using RNA from 17 Penaeus monodon shrimp infected with representatives of each of the six genotypes. Phylogenetic analysis using the 94 nt ORF1b gene sequence spanned by the nPCR primers generated genotype assignments that were consistent with those obtained using the extended 671 nt sequence used for the initial identification of genotypes.

Multiplex RT-PCR assay for simultaneous detection of six viruses of penaeid shrimp

In the present study, multiplex reverse transcription-polymerase chain reaction (mRT-PCR) was developed for simultaneously detection of six major shrimp viruses including yellow-head virus (YHV), white spot syndrome virus (WSSV), Taura syndrome virus (TSV), hepatopancreatic parvovirus (HPV), infectious hypodermal and hematopoietic necrosis virus (IHHNV) and monodon baculovirus (MBV). The six primer sets could amplify viral nucleic acids resulting in PCR products with different sizes. They were highly specific and did not cross-hybridize with other viral or shrimp nucleic acids. The sensitivity of the multiplex RT-PCR was 0.15pg for IHHNV, 0.15pg for TSV, 1.00pg for HPV, 1.5pg for MBV, 5.00pg for WSSV and 10.00pg for YHV. In the field application, 42 samples including whole tissue of post-larvae and hepatopancreas of Penaeus monodon collected from ponds in the central and southern parts of Thailand during 2002-2005 were examined by multiplex RT-PCR. The results revealed that a single infection was dominant and WSSV was the highest prevalence at that time. Dual infection was found in one sample. This developed multiplex RT-PCR will be useful for simultaneous detection of six major viruses of penaeid shrimp and benefit to shrimp cultured industry.

Yellow head-like viruses affecting the penaeid aquaculture industry: a review

This review focuses on relevant scientific information regarding the current knowledge of the yellow head complex viruses, yellow head virus and gill-associated virus. The yellow head complex viruses have been problematic within the aquaculture industry for over 10 years and still retain their research topicality. Presently, there are numerous research papers from different journals covering the identification, disease expression and spread, pathogenesis, detection, morphology, genomic sequence and protein profiles of the yellow head complex viruses. Indeed, there has been no extensive review to compare these studies, and as a corollary, to assess flaws in contemporary research and knowledge. Additionally, the yellow head complex viruses rank within the top four prawn viruses with respect to disease impact and economic loss. This review collectively reports on all the findings and current methods of research and aims to identify weak areas of research where conclusions have been unjustifiably drawn and furthermore to elucidate areas that have a gap of knowledge.

Development of a rapid, sensitive and non-lethal diagnostic assay for the detection of viral haemorrhagic septicaemia virus

A non-lethal diagnostic procedure based on polymerase chain reaction (PCR) technology was developed to detect viral haemorrhagic septicaemia virus (VHSV). Sensitivity of the assay was tested using purified viral RNA and seeded tissues. Detection limits of the reverse transcriptase-polymerase chain reaction (RT-PCR) assay were estimated to be 10 fg of purified RNA and 0.97 x 10(3) or 10(0) TCID(50)/g of seeded tissue, depending on the experimental approach employed (viral adsorption allowed for 1 or 24h). Addition of nested PCR increased sensitivity up to 100-fold when cDNA excised from the agarose gel was used as template. Both, RT-PCR and nested RT-PCR, as well as Southern blot were applied to RNA extracted from blood of experimentally infected brown trout and the results were compared with those obtained by applying the same techniques to tissues and also with those of conventional viral isolation in cell culture. The superiority of the nested RT-PCR applied to blood samples has been clearly demonstrated in terms of sensitivity, obtaining positive results in 85% of fish tested, as against 40% obtained by RT-PCR and Southern blot, and only 5% viral isolations in cell culture. This procedure could turn into an important tool for screening of wild stocks as well as valuable individuals in commercial fish farms, since it makes to kill the fish unnecessary.