In field use of water samples for genomic surveillance of infectious spleen and kidney necrosis virus (ISKNV) infecting tilapia fish in Lake Volta, Ghana.

Viral outbreaks are a constant threat to aquaculture, limiting production for better global food security. A lack of diagnostic testing and monitoring in resource-limited areas hinders the capacity to respond rapidly to disease outbreaks and to prevent viral pathogens becoming endemic in fisheries productive waters. Recent developments in diagnostic testing for emerging viruses, however, offers a solution for rapid in situ monitoring of viral outbreaks. Genomic epidemiology has furthermore proven highly effective in detecting viral mutations involved in pathogenesis and assisting in resolving chains of transmission. Here, we demonstrate the application of an in-field epidemiological tool kit to track viral outbreaks in aquaculture on farms with reduced access to diagnostic labs, and with non-destructive sampling. Inspired by the "lab in a suitcase" approach used for genomic surveillance of human viral pathogens and wastewater monitoring of COVID19, we evaluated the feasibility of real-time genome sequencing surveillance of the fish pathogen, Infectious spleen and kidney necrosis virus (ISKNV) in Lake Volta. Viral fractions from water samples collected from cages holding Nile tilapia (Oreochromis niloticus) with suspected ongoing ISKNV infections were concentrated and used as a template for whole genome sequencing, using a previously developed tiled PCR method for ISKNV. Mutations in ISKNV in samples collected from the water surrounding the cages matched those collected from infected caged fish, illustrating that water samples can be used for detecting predominant ISKNV variants in an ongoing outbreak. This approach allows for the detection of ISKNV and tracking of the dynamics of variant frequencies, and may thus assist in guiding control measures for the rapid isolation and quarantine of infected farms and facilities.

Combining Phage Display Technology with In Silico-Designed Epitope Vaccine to Elicit Robust Antibody Responses against Emerging Pathogen Tilapia Lake Virus.

Antigen epitope identification is a critical step in the vaccine development process and is a momentous cornerstone for the development of safe and efficient epitope vaccines. In particular, vaccine design is difficult when the function of the protein encoded by the pathogen is unknown. The genome of Tilapia lake virus (TiLV), an emerging virus from fish, encodes protein functions that have not been elucidated, resulting in a lag and uncertainty in vaccine development. Here, we propose a feasible strategy for emerging viral disease epitope vaccine development using TiLV. We determined the targets of specific antibodies in serum from a TiLV survivor by panning a Ph.D.-12 phage library, and we identified a mimotope, TYTTRMHITLPI, referred to as Pep3, which provided protection against TiLV after prime-boost vaccination; its immune protection rate was 57.6%. Based on amino acid sequence alignment and structure analysis of the target protein from TiLV, we further identified a protective antigenic site (399TYTTRNEDFLPT410) which is located on TiLV segment 1 (S1). The epitope vaccine with keyhole limpet hemocyanin (KLH-S1399-410) corresponding to the mimotope induced the tilapia to produce a durable and effective antibody response after immunization, and the antibody depletion test confirmed that the specific antibody against S1399-410 was necessary to neutralize TiLV. Surprisingly, the challenge studies in tilapia demonstrated that the epitope vaccine elicited a robust protective response against TiLV challenge, and the survival rate reached 81.8%. In conclusion, this study revealed a concept for screening antigen epitopes of emerging viral diseases, providing promising approaches for development and evaluation of protective epitope vaccines against viral diseases. IMPORTANCE Antigen epitope determination is an important cornerstone for developing efficient vaccines. In this study, we attempted to explore a novel approach for epitope discovery of TiLV, which is a new virus in fish. We investigated the immunogenicity and protective efficacy of all antigenic sites (mimotopes) identified in serum of primary TiLV survivors by using a Ph.D.-12 phage library. We also recognized and identified the natural epitope of TiLV by bioinformatics, evaluated the immunogenicity and protective effect of this antigenic site by immunization, and revealed 2 amino acid residues that play important roles in this epitope. Both Pep3 and S1399-410 (a natural epitope identified by Pep3) elicited antibody titers in tilapia, but S1399-410 was more prominent. Antibody depletion studies showed that anti-S1399-410-specific antibodies were essential for neutralizing TiLV. Our study demonstrated a model for combining experimental and computational screens to identify antigen epitopes, which is attractive for epitope-based vaccine development.

Infection of Tilapia tilapinevirus in Mozambique Tilapia (Oreochromis mossambicus), a Globally Vulnerable Fish Species.

Tilapia tilapinevirus, or tilapia lake virus (TiLV), is a highly contagious virus found in tilapia and its hybrid species that has been reported worldwide, including in Asia, the Americas, and Africa. In this study, we experimentally challenged Mozambique tilapia (Oreochromis mossambicus) with a virulent TiLV strain, VETKU-TV01, at both low (1 × 103 TCID50/mL) and high (1 × 105 TCID50/mL) concentration. After the challenge, the Mozambique tilapia showed pale skin with some hemorrhage and erosion, lethargy, abdominal swelling, congestion around the eye, and exophthalmos; there was a cumulative mortality rate at 48.89% and 77.78% in the groups that received the low and high concentration, respectively. Quantitative PCR and in situ hybridization confirmed the presence of TiLV in the internal organs of moribund fish. Notably, severe histopathological changes, including glycogen depletion, syncytial hepatic cells containing multiple nuclei and intracytoplasmic inclusion bodies, and infiltration of melanomacrophage into the spleen, were frequently found in the Mozambique tilapia challenged with high TiLV concentration. Comparatively, the infectivity and pathology of the TiLV infection in Mozambique tilapia and red hybrid tilapia (Oreochromis spp.) were found to be similar. Our results confirmed the susceptibility of Mozambique tilapia, which has recently been determined to be a vulnerable species, to TiLV infection, expanding knowledge that the virus can cause disease in this fish species.

Tilapia lake virus (TiLV): Genomic epidemiology and its early origin.

Tilapia lake virus (TiLV) is an emerging virus that is rapidly spreading across the world. Over the past 6 years (2014-2020), TiLV outbreaks had been reported in at least 16 countries, spanning three continents, including Asia, Africa, and America. Despite its enormous economic impact, its origin, evolution and epidemiology are still largely poorly characterized. Here, we report eight TiLV whole-genome sequences from Thailand sampled between 2014 and 2019. Together with publicly available sequences from various regions of the world, we estimated the origin of TiLV to be between 2003 and 2009, 5-10 years before the first report of the virus in Israel in 2014. Our analyses consistently showed that TiLV started to spread in 2000s, and reached its peak in 2014-2016, matching well with the timing of its first report. From 2016 onwards, the global TiLV population declined steadily. This could be a result of herd immunity building up in the fish population, and/or a reflection of a better awareness of the virus coupled with a better and more cautious protocol of Tilapia importation. Despite the fact that we included all publicly available sequences, our analyses revealed long unsampled histories of TiLVs in many countries, especially towards its basal diversification. This result highlights the lack and the need for systematic surveillance of TiLV in fish.

Meta-Transcriptomic Identification of Divergent Amnoonviridae in Fish.

Tilapia lake virus (TiLV) has caused mass mortalities in farmed and wild tilapia with serious economic and ecological consequences. Until recently, this virus was the sole member of the Amnoonviridae, a family within the order Articulavirales comprising segmented negative-sense RNA viruses. We sought to identify additional viruses within the Amnoonviridae through total RNA sequencing (meta-transcriptomics) and data mining of published transcriptomes. Accordingly, we sampled marine fish species from both Australia and China and discovered several segments of two new viruses within the Amnoonviridae, tentatively called Flavolineata virus and Piscibus virus, respectively. In addition, by mining vertebrate transcriptome data, we identified nine additional virus transcripts matching to multiple genomic segments of TiLV in both marine and freshwater fish. These new viruses retained sequence conservation with the distantly related Orthomyxoviridae in the RdRp subunit PB1, but formed a distinct and diverse phylogenetic group. These data suggest that the Amnoonviridae have a broad host range within fish and that greater animal sampling will identify additional divergent members of the Articulavirales.

Determination of a novel parvovirus pathogen associated with massive mortality in adult tilapia.

Tilapia is one of the most important economic and fastest-growing species in aquaculture worldwide. In 2015, an epidemic associated with severe mortality occurred in adult tilapia in Hubei, China. The causative pathogen was identified as Tilapia parvovirus (TiPV) by virus isolation, electron microscopy, experimental challenge, In situ hybridization (ISH), indirect immunofluorescence (IFA), and viral gene sequencing. Electron microscopy revealed large numbers of parvovirus particles in the organs of diseased fish, including kidney, spleen, liver, heart, brain, gill, intestine, etc. The virions were spherical in shape, non-enveloped and approximately 30nm in diameter. The TiPV was isolated and propagated in tilapia brain cells (TiB) and induced a typical cytopathic effect (CPE) after 3 days post-infection (dpi). This virus was used to experimentally infect adult tilapia and clinical disease symptoms similar to those observed naturally were replicated. Additionally, the results of ISH and IFA showed positive signals in kidney and spleen tissues from TiPV-infected fish. To identify TiPV-specific sequences, the near complete genome of TiPV was obtained and determined to be 4269 bp in size. Phylogenetic analysis of the NS1 sequence revealed that TiPV is a novel parvovirus, forms a separate branch in proposed genus Chapparvovirus of Parvoviridae. Results presented here confirm that TiPV is a novel parvovirus pathogen that can cause massive mortality in adult tilapia. This provides a basis for the further studies to define the epidemiology, pathology, diagnosis, prevention and treatment of this emerging viral disease.

Tilapia lake virus: The story so far.

Tilapia lake virus (TiLV) is a highly contagious pathogen that has detrimental effects on tilapia farming. This virus was discovered in 2014 and has received tremendous global attention from the aquaculture sector due to its association with high fish mortalities and its strong economic impact on the tilapia aquaculture industry. Currently, TiLV has been reported in 16 countries, and this number is continuing to rise due to improved diagnostic assays and surveillance activities around the world. In this review, we summarize the up-to-date knowledge of TiLV with regard to TiLV host species, the clinical signs of a TiLV infection, the affected tissues, pathogenesis and potential disease risk factors. We also describe the reported information concerning the virus itself: its morphology, genetic make-up and transmission pathways. We review the current methods for virus detection and potential control measures. We close the review of the TiLV story so far, by offering a commentary on the major TiLV research gaps, why these are delaying future TiLV research and why the TiLV field needs to come together and proceed as a more collaborative scientific community if there is any hope limiting the impact of this serious virus.

Reverse Transcription Loop-Mediated Isothermal Amplification (RT-LAMP) Assay for the Specific and Rapid Detection of Tilapia Lake Virus.

Tilapia lake virus disease (TiLVD), an emerging viral disease in tilapia caused by the tilapia lake virus (TiLV), is a persistent challenge in the aquaculture industry that has resulted in the mass morbidity and mortality of tilapia in many parts of the world. An effective, rapid, and accurate diagnostic assay for TiLV infection is therefore necessary to detect the initial infection and to prevent the spread of the disease in aquaculture farming. In this study, a highly sensitive and practical reverse transcription loop-mediated isothermal amplification (RT-LAMP) method is presented to detect tilapia lake virus in fish tissue. A comparison of the RT-qPCR and RT-LAMP assays of infected samples revealed positive results in 63 (100%) and 51 (80.95%) samples, respectively. Moreover, an analysis of uninfected samples showed that all 63 uninfected tissues yielded negative results for both the RT-qPCR and RT-LAMP assays. The cross-reactivity with five pathogens in tilapia was evaluated using RT-LAMP, and all the tests showed negative results. Both the liver and mucus samples obtained from infected fish showed comparable results using the RT-LAMP method, suggesting that mucus can be used in RT-LAMP as a nonlethal assay to avoid killing fish. In conclusion, the results demonstrated that the presented RT-LAMP assay provides an effective method for TiLV detection in tilapia tissue within 1 h. The method is therefore recommended as a screening tool on farms for the rapid diagnosis of TiLV.

The Segment Matters: Probable Reassortment of Tilapia Lake Virus (TiLV) Complicates Phylogenetic Analysis and Inference of Geographical Origin of New Isolate from Bangladesh.

Tilapia lake virus (TiLV), a negative sense RNA virus with a 10 segment genome, is an emerging threat to tilapia aquaculture worldwide, with outbreaks causing over 90% mortality reported on several continents since 2014. Following a severe tilapia mortality event in July 2017, we confirmed the presence of TiLV in Bangladesh and obtained the near-complete genome of this isolate, BD-2017. Phylogenetic analysis of the concatenated 10 segment coding regions placed BD-2017 in a clade with the two isolates from Thailand, separate from the Israeli and South American isolates. However, phylogenetic analysis of individual segments gave conflicting results, sometimes clustering BD-2017 with one of the Israeli isolates, and splitting pairs of isolates from the same region. By comparing patterns of topological difference among segments of quartets of isolates, we showed that TiLV likely has a history of reassortment. Segments 5 and 6, in particular, appear to have undergone a relatively recent reassortment event involving Ecuador isolate EC-2012 and Israel isolate Til-4-2011. The phylogeny of TiLV isolates therefore depends on the segment sequenced. Our findings illustrate the need to exercise caution when using phylogenetic analysis to infer geographic origin and track the movement of TiLV, and we recommend using whole genomes wherever possible.

Integrated analysis of mRNA-miRNA expression in Tilapia infected with Tilapia lake virus (TiLV) and identifies primarily immuneresponse genes.

We investigated differential gene expression in Tilapia infected with the Tilapia Lake virus (TiLV).We used high-throughput sequencing to identify mRNAs and miRNAs involved in TiLV infection progression We identified 25,359 differentially expressed genes that included 863 new genes. We identified 1770, 4142 and 4947 differently expressed genes comparing non-infected controls with 24 and 120 h infections and between the infected groups, respectively. These genes were enriched to 291 GO terms and 62 KEGG pathways and included immune system progress and virion genes. High-throughput miRNA sequencing identified 316 conserved miRNAs, 525 known miRNAs and 592 novel miRNAs. Furthermore, 138, 198 and 153 differently expressed miRNAs were found between the 3 groups listed above, respectively. Target prediction revealed numerous genes including erythropoietin isoform X2, double-stranded RNA-specific adenosine deaminase isoform X1, bone morphogenetic protein 4 and tapasin-related protein that are involved in immune responsiveness. Moreover, these target genes overlapped with differentially expressed mRNAs obtained from RNA-seq. These target genes were significantly enriched to GO terms and KEGG pathways including immune system progress, virion and Wnt signaling pathways. Expression patterns of differentially expressed mRNA and miRNAs were validated in 20 mRNA and 19 miRNAs by qRT-PCR. We also were able to construct a miRNA-mRNA target network that can further understand the molecular mechanisms on the pathogenesis of TiLV and guide future research in developing effective agents and strategies to combat TiLV infections in Tilapia.

Establishment of a brain cell line obtained from hybrids of Channa argus ×Channa maculata for the detection of tilapia lake virus.

A brain cell line (CAMB) derived from hybrid snakehead (Channa argus (♂) × Channa maculata (♀)) was established by trypsin and collagenase combined digestion. The culturing conditions and cell biological characteristics were systematically studied. For growth of the cells, M199 medium containing 10% fetal bovine serum was used and at 27 °C incubated. Based on morphological analysis, CAMB cells were confirmed to be epithelial. The cell line has been subcultured more than 80 times since its initial primary culture. Chromosome analysis revealed that CAMB cells had an abnormal chromosome number 2n = 64, whereas the chromosome number in the hybrid snakehead was 45. The suitability of CAMB for tilapia lake virus (TiLV) was demonstrated. A CPE was observed after infection with TiLV-2017A. The highest TiLV titer was observed after 12 days post infection (dpi) and reached 107.2 TCID50/mL. The virus replication was confirmed by electron microscopic observations. Additionally, immunofluorescence assay confirmed the presence of TiLV-2017A after infection of CAMB. Therefore, CAMB cells can be a useful tool for the investigation of the pathogenesis of the TiLV induced disease in tilapia.

Tilapia lake virus disease: Phylogenetic analysis reveals that two distinct clades are circulating in Israel simultaneously.

Tilapia lake virus (TiLV) is an emerging viral disease that affects several tilapia species in different countries since 2014. In 2017-2018, 129 samples were collected from 14 tilapia farms in Israel. Ninety samples represented TiLV-suspected cases (TSC), and 39 were used as control samples (CS). RT-qPCR was performed on 89 and 39 duplicate brain and liver tissue samples from TSC samples and CS, respectively. TiLV was diagnosed in 37 (40.1%) of TSC, and two of the CS samples (5%) were also positive for TiLV. Additional validation RT-PCR was performed on positive samples, and amplified products were sequenced. Maximum-likelihood phylogenetic analysis of segment-3 of 25 selected sequences revealed two distinct clades: one virtually identical to sequences from India and the second closely related to isolates from Ecuador, Thailand, Egypt and Peru, apparently imported to Israel from Thailand. Thus, our results indicate that at least two distinct clades of TiLV are circulating in Israel simultaneously. As of today, the number of TiLV sequences available in free publicly accessible databases is limited. Nevertheless, our study provides new molecular epidemiology baseline for further epidemiological studies of TiLV.

Structural Characterization of Open Reading Frame-Encoded Functional Genes from Tilapia Lake Virus (TiLV).

In recent years, large-scale mortalities are observed in tilapia due to infection with a novel orthomyxo-like virus named, tilapia lake virus (TiLV) which is marked to be a severe threat to universal tilapia industry. Currently, there are knowledge gaps relating to the antiviral peptide as well as there are no affordable vaccines or drugs available against TiLV yet. To understand the spreading of infection of TiLV in different organs of Oreochromis niloticus, RT-PCR analysis has been carried out. The gene segments of TiLV were retrieved from the NCBI database for computational biology analysis. The 14 functional genes were predicted from the 10 gene segments of TiLV. Phylogenetic analysis was employed to find out a better understanding for the evolution of tilapia lake virus genes. Out of 14 proteins, only six proteins show transmembrane helix region. Moreover, molecular modeling and molecular dynamics simulations of the predicted proteins revealed structural stability of the protein stabilized after 10-ns simulation. Overall, our study provided a basic bioinformatics on functional proteome of TiLV. Further, this study could be useful for development of novel peptide-based therapeutics to control TiLV infection.

Gray (Oreochromis niloticus x O. aureus) and Red (Oreochromis spp.) Tilapia Show Equal Susceptibility and Proinflammatory Cytokine Responses to Experimental Tilapia Lake Virus Infection.

Tilapia is the second most farmed fish species after carp in the world. However, the production has come under threat due to emerging diseases such as tilapia lake virus (TiLV) that causes massive mortalities with high economic losses. It is largely unknown whether different tilapia strains are equally susceptible to TiLV infection. In the present study we compared the susceptibility of gray (Oreochromis niloticus x O. aureus) and red tilapia (Oreochromis spp.) to experimental TiLV infection. Virus was injected intraperitoneally at a concentration of 104 TCID50/mL. Our findings show that gray tilapia had a lower mortality, 86.44%, but statistically not significantly different (p = 0.068) from red tilapia (100%). The duration of the mortality period from onset to cessation was similar for the two species, starting at 2-3 days post challenge (dpc) with a median at 10-11 dpi and ending on 20-22 dpi. In addition, there was no difference between species in mean viral loads in brain, liver and headkidney from fish collected soon after death. As for host response, expression levels of IL-1ß and TNFα were equally high in brain and headkidney samples while levels in liver samples were low for both red and gray tilapia, which coincides with lower viral loads in liver compared to brain and headkidney for both species. We find that red and gray tilapia were equally susceptible to TiLV infection with similar post challenge mortality levels, equal virus concentration in target organs and similar proinflammatory cytokine responses in target and lymphoid organs at time of death. Nonetheless, we advocate that the search for less susceptible tilapia strains should continue with the view to reduce losses from TiLV infection in aquaculture.

Development of a simple and rapid reverse transcription-loopmediated isothermal amplification (RT-LAMP) assay for sensitive detection of tilapia lake virus.

Recently, substantial mortality of farmed and wild tilapia caused by tilapia lake virus (TiLV) infection has been observed worldwide. However, sensitive and reliable diagnostic method is limited. A reverse transcription-loopmediated isothermal amplification (RT-LAMP) assay has been applied for the detection of TiLV nucleotide sequence. Six primers targeting two locations on the target gene based on a highly conserved sequence in the segment 1 (S1) region of the TiLV genome have been designed. The optimized RT-LAMP reaction was maintained at the isothermal condition of 63°C for 45 min. And the amplifications could be verified by turbidity or a colour change with the addition of SYBR Green I. Subsequently, RT-LAMP products could be observed by a ladder pattern following gel electrophoresis. The species-specific assay showed that the method was sensitive enough to detect as low as 1.6 copies of viral particle, and the assay was highly specific because no cross-reactivity was observed with other pathogens, and had a diagnostic sensitivity and specificity of 100% when TiLV-positive samples and non-target virus were tested. In summary, all the results demonstrate that this RT-LAMP is a rapid, effective and sensitive method for TiLV detection in tilapia aquaculture.

Minimal risk of tilapia lake virus transmission via frozen tilapia fillets.

Recent outbreaks of a novel tilapia lake virus (TiLV) have raised concerns regarding the international spread of TiLV in frozen tilapia products. This study investigated the potential risks of frozen tilapia fillet as a source of TiLV transmission. It revealed that TiLV genomic RNA could be detected in tilapia fillet and the virus isolated from non-frozen and frozen fillets with clinical TiLV infection stored up to 28 days caused a cytopathic effect (CPE) formation in the susceptible cell line in vitro. However, frozen fillets from clinical TiLV infection stored for 90 and 120 days did not cause CPE in the susceptible cell line. Similarly, CPE was not observed in TiLV isolated from subclinically TiLV-infected fish fillets. In addition, in vivo bioassay revealed that despite the presence of TiLV isolated from subclinically TiLV-infected fillet stored at -20°C for 14 days, there was no evidence of TiLV disease in naïve red hybrid tilapia based on the absence of clinical signs and mortality and without the detection of TiLV genomic RNA using reverse transcription-quantitative polymerase chain reaction assay. Collectively, these findings suggested minimal risk of transmission of TiLV via frozen tilapia fillets.

Detection of Tilapia Lake Virus Using Conventional RT-PCR and SYBR Green RT-qPCR.

The aim of this method is to facilitate the fast, sensitive and specific detection of Tilapia Lake Virus (TiLV) in tilapia tissues. This protocol can be used as part of surveillance programs, biosecurity measures and in TiLV basic research laboratories. The gold standard of virus diagnostics typically involves virus isolation followed by complementary techniques such as reverse-transcription polymerase chain reaction (RT-PCR) for further verification. This can be cumbersome, time-consuming and typically requires tissue samples heavily infected with virus. The use of RT-quantitative (q)PCR in the detection of viruses is advantageous because of its quantitative nature, high sensitivity, specificity, scalability and its rapid time to result. Here, the entire method of PCR based approaches for TiLV detection is described, from tilapia organ sectioning, total ribonucleic acid (RNA) extraction using a guanidium thiocyanate-phenol-chloroform solution, RNA quantification, followed by a two-step PCR protocol entailing, complementary deoxyribonucleic acid (cDNA) synthesis and detection of TiLV by either conventional PCR or quantitative identification via qPCR using SYBR green I dye. Conventional PCR requires post-PCR steps and will simply inform about the presence of the virus. The latter approach will allow for absolute quantification of TiLV down to as little as 2 copies and thus is exceptionally useful for TiLV diagnosis in sub-clinical cases. A detailed description of the two PCR approaches, representative results from two laboratories and a thorough discussion of the critical parameters of both have been included to ensure that researchers and diagnosticians find their most suitable and applicable method of TiLV detection.

Assessing the population transmission dynamics of tilapia lake virus in farmed tilapia.

A novel virus, tilapia lake virus (TiLV), has been identified as a key pathogen responsible for disease outbreak and mass mortality of farmed tilapia. We used a deterministic susceptible-infectious-mortality (SIM) model to derive key disease information appraised with published TiLV-induced cumulative mortality data. The relationship between tilapia mortality and TiLV exposure dosages was described by the Hill model. Furthermore, a disease control model was proposed to determine the status of controlled TiLV infection using a parsimonious control reproduction number (RC )-control line criterion. Results showed that the key disease determinants of transmission rate and basic reproduction number (R0 ) could be derived. The median R0 estimate was 2.59 in a cohabitation setting with 2.6 × 105  TCID50 fish-1 TiLV. The present RC -control model can be employed to determine whether TiLV containment is feasible in an outbreak farm by quantifying the current level of transmission. The SIM model can then be applied to predict what additional control is required to manage RC  < 1. We offer valuable tools for aquaculture engineers and public health scientists the mechanistic-based assessment that allows a more rigorous evaluation of different control strategies to reduce waterborne diseases in aquaculture farming systems.

Hordes of Phages in the Gut of the Tilapia Sarotherodon melanotheron.

Preliminary studies conducted on the human gastro-intestinal tract have revealed that enteric viral communities play a preponderant role in microbial homeostatis. However to date, such communities have never been investigated in the fish gut. Herein, we examined the main ecological traits of viruses in the digestive tract of a euryhaline fish, the tilapia Sarotherodon melanotheron. Individuals were collected at 8 different sites in Senegal covering a salinity gradient from 3 to 104‰, and showing large disparities in their organic pollutant concentrations. Results showed that the gut of S. melanotheron is home to a highly abundant viral community (0.2-10.7 × 109 viruses ml-1), distinct from the surrounding water, and essentially composed of phages of which a substantial proportion is temperate (the fraction of lysogenized cells-FLC ranging from 8.1 to 33.0%). Also, a positive and significant correlation was detected between FLC and the concentrations of polycyclic aromatic hydrocarbon in sediment, while no clear relationships were found between salinity and any of the microbial parameters considered. Finally, our data suggest that virus-bacteria interactions within the fish intestine are likely sensitive to the presence of particular xenobiotics, which may compromise the balance in the gut microbiota, and subsequently affect the health of their host.

Experimental infection of Tilapia Lake Virus (TiLV) in Nile tilapia (Oreochromis niloticus) and red tilapia (Oreochromis spp.).

Since 2015, a novel orthomyxo-like virus, tilapia lake virus (TiLV) has been associated with outbreaks of disease and massive mortality of cultured Nile and red tilapia (Oreochromis niloticus and Oreochromis spp., respectively) in Thailand. In this study, TiLV was isolated from field samples and propagated in the permissive E-11 cell line, with cytopathic effect (CPE) development within 3-5days post-inoculation. Electron micrographs of infected E-11 cells and fish tissues confirmed the rounded, enveloped virions of 60 to 80nm with characteristics very similar to those of Orthomyxoviridae. In vivo challenge studies showed that high mortality in Nile (86%) and red tilapia (66%) occurred within 4-12days post-infection. The virus was re-isolated from challenged fish tissues in the permissive cell line, and PCR analysis confirmed TiLV as a causative pathogen. The distinct histopathology of challenged fish included massive degeneration and inflammatory cell infiltration in the liver and brain as well as the presence of eosinophilic intracytoplasmic inclusions in hepatocytes and splenic cells. Our results fulfilled Koch's postulates and confirmed that TiLV is an etiologic agent of mass mortality of tilapia in Thailand. The emergence of this virus in many countries has helped increase awareness that it is a potential threat to tilapia aquacultured in Thailand, Asia, and worldwide.