Histopathology of cultured shrimp showing gross signs of yellow head syndrome and white spot syndrome during 1994 Indian epizootics

Insight into molecular interaction between shrimp and white spot syndrome virus through MjsvCL-VP28 complex: an in-silico approach

White Spot disease is a devastating disease of shrimps caused by White Spot Syndrome Virus in multifarious shrimp species. At present there is no absolute medication to suppress the disease hence, there is an urgent need for development of drug against the virus. Molecular interaction between viral envelope protein VP28 and shrimp receptor protein especially chitins play a pivotal role in ingression of WSSV. In the present study, we have tried to shed light on structural aspects of lectin protein in Marsupenaeus japonicus (MjsvCL). A structural insight to the CTLD-domain of MjsvCL has facilitated the understanding of the binding mechanism between the two proteins that is responsible for entry of WSSV into shrimps. Further, incorporation of molecular dynamics simulation and MMPBSA studies revealed the affinity of binding and certain hotspot residues, which are critical for association of both the proteins. For the first time we have proposed that these amino acids are quintessential for formation of VP28-MjsvCL complex and play crucial role in entry of WSSV into shrimps. Targeting the interaction between VP28 and CTLD of MjsvCL may possibly serve as a potential drug target. The current study provides information for better understanding the interaction between VP28 and MjsvCL that could be a plausible site for future inhibitors against WSSV in shrimps.Communicated by Ramaswamy H. Sarma.

Toll receptor 2 (Toll2) positively regulates antibacterial immunity but promotes white spot syndrome virus (WSSV) infection in shrimp

The Toll family of receptors are a group of conserved pattern recognition receptors (PRRs) essentially controlling the initiation of innate immune responses. The white spot syndrome virus (WSSV) and Vibrio parahaemolyticus are major pathogens of aquaculture shrimp. Previous study has suggested that expression of the Toll2 receptor in Pacific white shrimp Penaeus vannamei was up-regulated by white spot syndrome virus (WSSV) infection but did not significantly changed upon infection with the bacterial pathogen Vibrio parahaemolyticus. The current study intends to investigate the role of P. vannamei Toll2 in antibacterial and antiviral immunity. We demonstrated that compared with the control, the Toll2-silenced shrimp was more susceptible to V. parahaemolyticus infection, suggesting that Toll2 may play a positive role in antibacterial immunity. However, silencing of Toll2 significantly enhanced survivorship of shrimp infected with WSSV and reduced the viral load in shrimp tissues. The expression of WSSV structural protein VP28 was also inhibited in Toll2-silenced shrimp. Histologic pathology analysis further showed that the WSSV infection was attenuated in stomach tissues from Toll2-silenced shrimp. These suggested that Toll2 could promote WSSV infection in shrimp. In Toll2-silenced shrimp, expression of antimicrobial peptides ALFs and PENs was significantly changed, which may contribute to the role of Toll2 in antibacterial immunity and WSSV infection.

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

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

Dietary Hizikia fusiforme enhance survival of white spot syndrome virus infected crayfish Procambarus clarkii

The sea vegetable Hizikia fusiforme is not only a good source of dietary fiber but also enhances immunity. In this study, we investigated the effects of H. fusiforme on innate immunity in invertebrates, using white spot syndrome virus (WSSV) challenge in the crayfish, Procambarus clarkii. Supplementation with H. fusiforme significantly reduced mortality caused by WSSV infection and also reduced copy numbers of the WSSV protein VP28. Quantitative reverse transcription-polymerase chain reaction showed that supplementation of feed with H. fusiforme increased the expression of immune-related genes, including NF-κB and crustin 1. Further analysis showed that supplementation with H. fusiforme also affected three immune parameters, total hemocyte count, and phenoloxidase and superoxide dismutase activity. H. fusiforme treatment significantly increased hemocyte apoptosis rates in both WSSV-infected and uninfected crayfish. H. fusiforme thus regulates the innate immunity of crayfish, and both delays and reduces mortality after WSSV challenge. Our study demonstrates the potential for the commercial use of H. fusiforme, either therapeutically or prophylactically, to regulate the innate immunity and protect crayfish against WSSV infection.

A multi-target dsRNA for simultaneous inhibition of yellow head virus and white spot syndrome virus in shrimp

Outbreaks of diseases caused by yellow head virus (YHV) and white spot syndrome virus (WSSV) infection in shrimp have resulted in economic losses worldwide. DsRNA-mediated RNAi has been used to control these viruses, and the best target genes for efficient inhibition of YHV and WSSV are the protease and ribonuleotide reductase small subunit (rr2), respectively. However, one dsRNA can suppress only one virus, and therefore the production of multi-target dsRNA to effectively inhibit both YHV and WSSV is needed. In this study, plasmids pETpro-rr2_one stem and pETpro-rr2_two stems were constructed to produce two different forms of multi-target dsRNA in E. coli, which were designed specifically to both YHV protease and WSSV rr2 genes. The potency of each dsRNA in inhibiting YHV and WSSV and reducing shrimp death were investigated in L. vannamei. Shrimp were injected with the dsRNAs into the hemolymph before challenge with YHV or WSSV. The results showed that both dsRNAs could inhibit the viruses, however the one stem construct was more effective than the two stems construct when shrimp were infected with WSSV. This study establishes a potential strategy for dual inhibition of YHV and WSSV for further application in shrimp aquaculture.

The Complete Genome of an Endogenous Nimavirus (Nimav-1_LVa) From the Pacific Whiteleg Shrimp Penaeus (Litopenaeus) Vannamei

White spot syndrome virus (WSSV), the lone virus of the genus Whispovirus under the family Nimaviridae, is one of the most devastating viruses affecting the shrimp farming industry. Knowledge about this virus, in particular, its evolution history, has been limited, partly due to its large genome and the lack of other closely related free-living viruses for comparative studies. In this study, we reconstructed a full-length endogenous nimavirus consensus genome, Nimav-1_LVa (279,905 bp), in the genome sequence of Penaeus (Litopenaeus) vannamei breed Kehai No. 1 (ASM378908v1). This endogenous virus seemed to insert exclusively into the telomeric pentanucleotide microsatellite (TAACC/GGTTA)n. It encoded 117 putative genes, with some containing introns, such as g012 (inhibitor of apoptosis, IAP), g046 (crustacean hyperglycemic hormone, CHH), g155 (innexin), g158 (Bax inhibitor 1 like). More than a dozen Nimav-1_LVa genes are involved in the pathogen-host interactions. We hypothesized that g046, g155, g158, and g227 (semaphorin 1A like) were recruited host genes for their roles in immune regulation. Sequence analysis indicated that a total of 43 WSSV genes belonged to the ancestral/core nimavirus gene set, including four genes reported in this study: wsv112 (dUTPase), wsv206, wsv226, and wsv308 (nucleocapsid protein). The availability of the Nimav-1_LVa sequence would help understand the genetic diversity, epidemiology, evolution, and virulence of WSSV.

A novel cuticle protein involved in WSSV infection to the Pacific white shrimp Litopenaeus vannamei

As the most productive crustacean species in aquaculture, Litopenaeus vannamei is seriously threatened by white spot syndrome virus (WSSV), which has caused huge economic damage in the past decades. Shrimp cuticle proteins are the important components in the frontier target tissues, including cuticle and the chitinous lining of the digestive tract. In present study, a novel cuticle protein gene, named LvCPAP1, was isolated and demonstrated to play an important role in WSSV infection. The deduced amino acid sequence of LvCPAP1 contained a signal peptide and a conserved chitin-binding domain type 2 (ChBD2). Tissue distribution analysis revealed that LvCPAP1 was predominantly expressed in epidermis and stomach. The transcription levels of LvCPAP1 in epidermis and stomach were significantly regulated upon WSSV challenge. DsRNA silencing of LvCPAP1 decreased the in vivo WSSV copy numbers and the death rate of shrimp after WSSV infection, indicating that LvCPAP1 might facilitate WSSV invasion. In addition, the interaction between LvCPAP1 and the major envelop protein VP24 of WSSV was revealed by yeast two-hybrid system and further confirmed by dot blot and pull-down assays. The present study implied that cuticle protein LvCPAP1 might favor the entry process of WSSV, which provided new clues for understanding the role of cuticle proteins during virus infection.

Hesperetin protects crayfish Procambarus clarkii against white spot syndrome virus infection

Hesperetin is a natural flavanone compound, which mainly exists in lemons and oranges, and has potential antiviral and anticancer activities. In this study, hesperetin was used in a crayfish pathogen challenge to discover its effects on the innate immune system of invertebrates. The crayfish Procambarus clarkii was used as an experimental model and challenged with white spot syndrome virus (WSSV). Pathogen challenge experiments showed that hesperetin treatment significantly reduced the mortality caused by WSSV infection, while the VP28 copies of WSSV were also reduced. Quantitative reverse transcriptase polymerase chain reaction revealed that hesperetin increased the expression of several innate immune-related genes, including NF-kappaB and C-type lectin. Further analysis showed that hesperetin treatment plays a positive effects on three immune parameters like total hemocyte count, phenoloxidase and superoxide dismutase activity. Nevertheless, whether or not infected with WSSV, hesperetin treatment would significantly increase the hemocyte apoptosis rates in crayfish. These results indicated that hesperetin could regulate the innate immunity of crayfish, and delaying and reducing the mortality after WSSV challenge. Therefore, the present study provided novel insights into the potential therapeutic or preventive functions associated with hesperetin to regulate crayfish immunity and protect crayfish against WSSV infection, provide certain theoretical basis for production practice.

Global distribution of white spot syndrome virus genotypes determined using a novel genotyping assay

White spot disease, caused by infection with white spot syndrome virus (WSSV), is a serious panzootic affecting prawn aquaculture. The disease has spread rapidly around the prawn-culturing regions of the world through a number of previously identified mechanisms. The ability to distinguish and trace strains of WSSV is of great benefit to identify, and then limit, the translocation routes of the disease. Here, we describe a novel genotyping method using 34 short tandem repeat regions of the viral genome concurrently. This technique is highly sensitive to strain differences when compared to previous methods. The efficacy of the described method is demonstrated by testing WSSV isolates from around the globe, showing regional genotypic differences. The differences in the genotypes were used to create a global minimum spanning network, and in most cases the observed relationships were substantiated with verification of transboundary movement. This novel panel of STR markers will provide a valuable epidemiological tool for white spot disease. We have applied this to an outbreak of the disease in Queensland, Australia, that occurred in 2016. While the results indicate that the source of this outbreak currently remains cryptic, the analyses have provided valuable insights with which to further study the origins of the strains involved.

A cuticle protein from the Pacific white shrimp Litopenaeus vannamei involved in WSSV infection

White spot syndrome virus (WSSV) is a major viral pathogen in global shrimp farming, causing huge economic damage. Through penetrating the outer surface of the target tissues, WSSV enters into the cells of the target tissue to complete the replication process in the host. In the present study, a cuticle protein gene from Litopenaeus vannamei, designated as LvAMP13.4, was identified and proved to be involved in WSSV invasion. The deduced amino acid sequence of LvAMP13.4 contained a signal peptide and a conserved chitin-binding domain type 4 (ChBD4). This cuticle protein gene was mainly expressed in stomach, gill and epidermis. The expression level of LvAMP13.4 was significantly changed during WSSV infection. Silencing of LvAMP13.4 by dsRNA interference apparently reduced the mortality rate and the WSSV copy number in shrimp upon WSSV infection. Furthermore, yeast two-hybrid system and Co-IP assay were performed to confirm that LvAMP13.4 could interact with the major envelop protein VP24 of WSSV. These data indicated that LvAMP13.4 was involved in the invasion process of WSSV through interaction with VP24. The present results could provide new insights for us in understanding the role of host cuticle proteins during virus invasion.

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

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

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.

Regulation of the immediate-early genes of white spot syndrome virus by Litopenaeus vannamei kruppel-like factor (LvKLF)

Kruppel-like factors (KLFs) belong to a subclass of Cys2/His2 zinc-finger DNA-binding proteins, and act as important regulators with diverse roles in cell growth, proliferation, differentiation, apoptosis and tumorigenesis. Our previous research showed that PmKLF from Penaeus monodon is crucial for white spot syndrome virus (WSSV) infection, yet the mechanisms by which PmKLF influences WSSV infection remain unclear. This study cloned KLF from Litopenaeus vannamei (LvKLF), which had 93% similarity with PmKLF. LvKLF formed a dimer via the C-terminal zinc-finger motif. Knockdown of LvKLF expression by dsRNA injection in WSSV-challenged shrimps was found to significantly inhibit the transcription of two important immediate-early (IE) genes, IE1 and WSSV304, and also reduced WSSV copy numbers. Moreover, reporter assays revealed that the promoter activities of these two WSSV IE genes were substantially enhanced by LvKLF. Mutations introduced in the promoter sequences of IE1 and WSSV304 were shown to abolish LvKLF activation of promoter activities; and an electrophoretic mobility shift assay demonstrated that LvKLF binds to putative KLF-response elements (KRE) in the promoters. Taken together, these results indicate that LvKLF transcriptional regulation of key IE genes is critical to WSSV replication.

A new shrimp peritrophin-like gene from Exopalaemon carinicauda involved in white spot syndrome virus (WSSV) infection

Peritrophin was first separated from insect peritrophic membrane (PM), and it played an important role in stimulating the digestion of food and protecting insects from invasion by microorganisms. In this study, a full-length cDNA of a new peritrophin-like protein (EcPT) was cloned from the ridgetail white shrimp Exopalaemon carinicauda, which was an excellent experimental animal for shrimp. The full length cDNA comprised 1235 bp including an 873 bp open reading frame encoding 291 amino acids. The deduced amino acid sequence contained a segment of signal peptide and three conserved chitin binding type 2 domains (ChtBD2) characterized by having a 6-cysteine motif. Tissue expression analysis revealed that EcPT was mainly expressed in stomach and gills, which were also the two main target tissues of WSSV infection. The transcription levels of EcPT in both stomach and gills were found to have significantly changed upon WSSV infection by real-time PCR. Silencing EcPT by dsRNA interference led to higher survival rate of shrimp against WSSV challenge, which suggested that EcPT might be involved in WSSV infection.

Development of a monoclonal antibody-based flow-through immunoassay (FTA) for detection of white spot syndrome virus (WSSV) in black tiger shrimp Penaeus monodon

A flow-through immunoassay (FTA), an improved version of immunodot, was developed using a nitrocellulose membrane baked onto adsorbent pads enclosed in a plastic cassette to detect white spot syndrome virus (WSSV) in shrimp. Sharp purple dots developed with WSSV against the white background of the nitrocellulose membrane. The detection limits of WSSV by the FTA and immunodot were 0.312 and 1.2 μg mL(-1) crude WSSV protein, respectively. The FTA could be completed in 8-10 min compared with 90 min for immunodot. The FTA was 100 times more sensitive than 1-step polymerase chain reaction (PCR) and in between that of the 1- and 2-step PCR protocol recommended by the Office of International Epizootics (OIE). In experimental, orally infected shrimp post-larvae, WSSV was first detected 14, 16 and 18 h post-infection (hpi) by FTA, immunodot and one-step PCR, respectively. The FTA detected WSSV 2 and 4 h earlier than immunodot and one-step PCR, respectively. The FTA was more sensitive (25/27) than one-step PCR (23/27) and immunodot (23/27) for the detection of WSSV from white spot disease outbreak ponds. The reagent components of the FTA were stable giving expected results for 6 m at 4-8 °C. The FTA is available as a rapid test kit called 'RapiDot' for the early detection of WSSV under field conditions.

Protection from yellow head virus (YHV) infection in Penaeus vannamei pre-infected with Taura syndrome virus (TSV)

Pacific white shrimp Penaeus vannamei that were pre-exposed to Taura syndrome virus (TSV) and then challenged with yellow head virus (YHV) acquired partial protection from yellow head disease (YHD). Experimental infections were carried out using specific-pathogen-free (SPF) shrimp which were first exposed per os to TSV; at 27, 37 and 47 d post infection they were then challenged by injection with 1 × 104 copies of YHV per shrimp (designated the TSV-YHV group). Shrimp not infected with TSV were injected with YHV as a positive control. Survival analyses comparing the TSV-YHV and YHV (positive control) groups were conducted, and significant survival rates were found for all the time groups (p < 0.001). A higher final survival was found in the TSV-YHV group (mean 55%) than in the positive control (0%) (p < 0.05). Duplex reverse transcription quantitative PCR was used to quantify both TSV and YHV. Lower YHV copy numbers were found in the TSV-YHV group than in the positive control in pleopods (3.52 × 109 vs. 1.88 × 1010 copies µg RNA-1) (p < 0.001) and lymphoid organ (LO) samples (3.52 × 109 vs. 1.88 × 1010 copies µg RNA-1) (p < 0.01). In situ hybridization assays were conducted, and differences in the distribution of the 2 viruses in the target tissues were found. The foci of LO were infected with TSV but were not infected with YHV. This study suggests that a viral interference effect exists between TSV and YHV, which could, in part, explain the absence of YHD in the Americas, where P. vannamei are often raised in farms where TSV is present.

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

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

Indian isolates of white spot syndrome virus exhibit variations in their pathogenicity and genomic tandem repeats

To detect genomic variation of white spot syndrome virus (WSSV) isolates from different geographical regions of India, the variable number of the tandem repeat (VNTR) region of the ORF 94 (Thailand WSSV isolate - GeneBank Accession No. AF369029) was analysed using five specific sets of primers. Analysis of 70 WSSV-positive samples showed the presence of 14 different genotypes of WSSV with VNTRs ranging from 2 to 16 tandem repeats with the majority (85.47%) having 6-12 tandem repeats. Occurrence of different genotypes of WSSV was found to be neither correlated to any specific geographical region nor to the different growth stage of the tiger shrimp, Penaeus monodon. Pathogenicity studies conducted with 25 isolates of WSSV revealed the presence of virulent and avirulent strains of WSSV in Indian shrimp farms. However, an unambiguous link could not be established between the different genotypes and their virulence.

Gene expression kinetics of the yellow head virus in experimentally infected Litopenaeus vannamei

The yellow head virus (YHV) has been reported to be one of most pathogenic viruses for cultivated shrimp; however, serious problems have only been reported in farms in south and southeastern Asian. Recently, a YHV strain was detected in Litopenaeus vannamei cultivated in Mexican farms that lacked virus-associated mortalities or epizooties, and the animals were apparently healthy. The identity of the virus was confirmed by sequencing replicative and structural protein-encoding regions and comparing with homologous virus sequences. Phylogenic relationships and genetic distances were also determined and, although some differences were observed, an influence on virulence was uncertain. In addition, the expression levels of several transcripts (3CLPRO, POL, GP64 and GP116) were evaluated by quantitative real-time polymerase chain reaction during an experimental infection. Although the transcript showed varying kinetics, viral genes were expressed in infected L. vannamei, demonstrating the replicative capability of this YHV strain.

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.

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.

Presumptive detection of yellow head virus by reverse transcriptase-polymerase chain reaction and dot-blot hybridization in Litopenaeus vannamei and L. stylirostris cultured on the Northwest coast of Mexico

In order to assess the presence of yellow head virus (YHV) in shrimp farms along the Pacific coast of Mexico, 39 samples from 26 randomly chosen farms were analysed by means of reverse transcriptase-polymerase chain reaction (RT-PCR) and dot-blot hybridization. Eleven samples were positive for YHV. The disease was reproduced by means of an infectivity bioassay performed with an extract of pleopods from the positive samples. Cumulative mortality reached 50% in 14 days. Four pairs of primers which amplified several YHV genome regions were designed and used to test dead and surviving shrimp from the bioassay by RT-PCR, resulting in positive results for every expected amplicon. The results of this study provide presumptive evidence of the presence of YHV in Mexican shrimp farms at least during 1999-2000.

Effect of water temperature on the immune response and infectivity pattern of white spot syndrome virus (WSSV) in freshwater crayfish

The susceptibility of two species of freshwater crayfish, Pacifastacus leniusculus and Astacus astacus, to white spot syndrome virus (WSSV) by intramuscular injection was compared and the results show that both species are susceptible to WSSV. The effect of water temperature on the development of white spot disease in crayfish was also studied. Crayfish were exposed to different temperatures after WSSV injection or oral exposure and the mortalities were recorded over a period of 45 days. No mortality was observed when crayfish were held at 4+/-2 degrees C or 12+/-2 degrees C and reached 100% when these crayfish transferred to 22+/-2 degrees C. The mortalities of nearly moribund crayfish at 22+/-2 degrees C with WSSV could be delayed after transfer to temperature below 16 degrees C. These results clearly show that low temperature affects the WSSV pathogenicity in crayfish. Moreover, haemocyte counts, phenoloxidase activity, mRNA levels of prophenoloxidase (proPO) and the lipopolysaccharide and beta-1,3-glucan binding protein (LGBP) in crayfish exposed to various water temperatures were studied. Total haemocyte and granular cell counts of crayfish held at different temperatures were not significantly (P>0.05) different, except for the total haemocyte number at 18 degrees C was significantly (P<0.05) higher than in crayfish at 4 degrees C. The percentage of granular cells in crayfish held at 4 degrees C was the highest compared to crayfish maintained at other temperatures. The phenoloxidase activities in haemocyte lysate supernatant (HLS) of crayfish at all temperature groups remained similar. The amount of proPO-mRNAs in haemocytes was much higher than the amount of LGBP-m RNAs in all the experimental groups. However, there was no change in the level of pro PO-mRNA at the tested temperatures. Interestingly, the level of LGBP-mRNA of crayfish kept at 22 degrees C was much lower than in those held at lower temperatures. Proliferation of the haematopoietic tissues was higher at high temperatures which may support replication of WSSV, and explain the high mortality of crayfish with WSSV infection at high temperature. Based on these studies it is concluded that crayfish might act as a carrier of WSSV at low water temperature and could develop white spot disease if the water temperature is increased.

A review of potential pathogens of sea lice and the application of cleaner fish in biological control

There are many examples of successful biological control of pest populations in aquatic environments. This approach to sea louse control has environmental benefits and is cost-effective. The range of possible pathogens of lice is reviewed and epibionts recorded from sea lice, including the monogenean Udonella caligorum and ciliates, are examined. Baculoviruses when ingested by insects form occlusion bodies resulting in severe damage to the digestive system and subsequent death, and this may be a promising approach. Cleaner wrasse (Labridae) have been stocked commercially with farmed salmon since 1989, and recent work on improving the method is reviewed. Wrasse are sourced from a wild fishery and stocked at ratios of 1 to 25-150 salmon. Over 5 million wrasse are stocked annually in Norway and c 30% of smolts in Scotland were stocked with wrasse until 1998, when an outbreak of infections salmon anaemia (ISA) deterred many farmers from transferring wild fish to cages. A case study is given showing that salmon in cages stocked with wrasse had a burden of one to eight lice through the first year compared with up to 40 lice per fish on unprotected and untreated fish. Electivity indices were used to compare the relative composition of lice developmental stages on salmon in stocked and unstocked cages, and adult male and female lice were found to comprise only 6% of the population in cages with wrasse, compared with 49% adults on fish in control cages. Measures to improve the efficacy of wrasse as a way of cleaning salmon in the second production year include the use of refuges to assist over-wintering survival, and stocking ballan wrasse. Health hygiene includes sourcing wrasse in the farm locality, testing for pathogens, vaccination of wrasse and ultimately rearing wrasse for stocking. The role of wrasse in an IPM strategy is described.