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

Knowns and unknowns of TiLV-associated neuronal disease

Tilapia Lake Virus (TiLV) is associated with pathological changes in the brain of infected fish, but the mechanisms driving the virus's neuropathogenesis remain poorly characterized. TiLV establishes a persistent infection in the brain of infected fish even when the virus is no longer detectable in the peripheral organs, rendering therapeutic interventions and disease management challenging. Moreover, the persistence of the virus in the brain may pose a risk for viral reinfection and spread and contribute to ongoing tissue damage and neuroinflammatory processes. In this review, we explore TiLV-associated neurological disease. We discuss the possible mechanism(s) used by TiLV to enter the central nervous system (CNS) and examine TiLV-induced neuroinflammation and brain immune responses. Lastly, we discuss future research questions and knowledge gaps to be addressed to significantly advance this field.

In vitro analysis of the expression of inflammasome, antiviral, and immune genes in an Oreochromis niloticus liver cell line following stimulation with bacterial ligands and infection with tilapia lake virus

The inflammasome is a multimeric protein complex that plays a vital role in the defence against pathogens and is therefore considered an essential component of the innate immune system. In this study, the expression patterns of inflammasome genes (NLRC3, ASC, and CAS-1), antiviral genes (IFNγ and MX), and immune genes (IL-1β and IL-18) were analysed in Oreochromis niloticus liver (ONIL) cells following stimulation with the bacterial ligands peptidoglycan (PGN) and lipopolysaccharide (LPS) and infection with TiLV. The cells were stimulated with PGN and LPS at concentrations of 10, 25, and 50 µg/ml. For viral infection, 106 TCID50 of TiLV per ml was used. After LPS stimulation, all seven genes were found to be expressed at specific time points at each of the three doses tested. However, at even higher doses of LPS, NLRC3 levels decreased. Following TiLV infection, all of the genes showed significant upregulation, especially at early time points. However, the gene expression pattern was found to be unique in PGN-treated cells. For instance, NLRC3 and ASC did not show any response to PGN stimulation, and the expression of IFNγ was downregulated at 25 and 50 µg of PGN per ml. CAS-1 and IL-18 expression was downregulated at 25 µg of PGN per ml. At a higher dose (50 µg/ml), IL-1β showed downregulation. Overall, our results indicate that these genes are involved in the immune response to viral and bacterial infection and that the degree of response is ligand- and dose-dependent.

Chromosome level genome assembly and transcriptome analysis of E11 cells infected by tilapia lake virus

The E11 cell line, derived from striped snakehead fish (Channa striata), possesses a distinctive feature: it is persistently infected with a C-type retrovirus. Notably, it exhibits high permissiveness to piscine nodavirus and the emerging tilapia lake virus (TiLV). Despite its popularity in TiLV research, the absence of genome assembly for the E11 cell line and Channa striata has constrained research on host-virus interactions. This study aimed to fill this gap by sequencing, assembling, and annotating the E11 cell line genome. Our efforts yielded a 600.5 Mb genome including 24 chromosomes with a BUSCO score of 98.8%. In addition, the complete proviral DNA sequence of snakehead retrovirus (SnRV) was identified in the E11 cell genome. Comparative genomic analysis between the E11 cell line and another snakehead species Channa argus revealed the loss of many immune-related gene families in the E11 cell genome, indicating a compromised immune response. We also conducted transcriptome analysis of mock- and TiLV-infected E11 cells, unveiling new perspectives on virus-virus and host-virus interactions. The TiLV infection suppressed the high expression of SnRV in E11 cells, and activated some other endogenous retroviruses. The protein-coding gene comparison revealed a pronounced up-regulation of genes involved in immune response, alongside a down-regulation of genes associated with specific metabolic processes. In summary, the genome assembly and annotation of the E11 cell line provide valuable resources to understand the SnRV and facilitate further studies on nodavirus and TiLV. The RNA-seq profiles shed light on the cellular mechanisms employed by fish cells in response to viral challenges, potentially guiding the development of therapeutic strategies against TiLV in aquaculture. This study also provides the first insights into the viral transcriptome profiles of endogenous SnRV and evading TiLV, enhancing our understanding of host-virus interactions in fish.

Widespread occurrence of Tilapia parvovirus in farmed Nile tilapia Oreochromis niloticus from India

Tilapia parvovirus (TiPV) has been associated with heavy mortalities in tilapia as a single infection or in co-infection with Tilapia lake virus (TiLV). In this study, TiPV was detected in farmed Nile tilapia, Oreochromis niloticus, from two geographical regions of India, Maharashtra and Uttar Pradesh. TiPV-specific polymerase chain reaction (PCR) reported earlier was used in the screening. Tilapia collected from Maharashtra showed characteristic clinical signs, and TiPV was detected along with TiLV and/or Aeromonas spp. However, fish from Uttar Pradesh were apparently healthy and only TiPV could be detected in these samples. A high prevalence of TiPV was recorded from both the geographical locations, Maharashtra and Uttar Pradesh (59.6% and 95.0% respectively). The virus could be detected in tissues such as the spleen, liver, kidney, brain and mucus. The spleen appeared to be the best tissue for detecting TiPV in apparently healthy tilapia. The presence of TiPV was further confirmed through sequencing the PCR products, isolation of the virus in the cell line and electron microscopy. Sequences of the NS1 gene of the two TiPV isolates showed similarity to the earlier reported TiPV isolates. The virus could be successfully propagated in O. niloticus Liver (OnL) cell line, and cytopathic effect was observed as early as 3 days post-infection. Furthermore, the presence of non-enveloped icosahedral to round virus particles measuring about 26-35 nm could be demonstrated in the cytoplasm and nucleus of infected OnL cells in transmission electron microscopy. With this confirmation of the presence of the virus, India is the third country to report TiPV after China and Thailand. The detection of TiPV in co-infection cases with TiLV and in apparently healthy Nile tilapia suggests its wide distribution and potential synergistic effect in co-infection cases. Therefore, this emerging virus needs holistic attention to understand its virulence, host-specificity and epidemiological risk factors.

Immune responses to Tilapia lake virus infection: what we know and what we don't know

Tilapia lake virus (TiLV) is a novel contagious pathogen associated with a lethal disease affecting and decimating tilapia populations on several continents across the globe. Fish viral diseases, such as Tilapia lake virus disease (TiLVD), represent a serious threat to tilapia aquaculture. Therefore, a better understanding of the innate immune responses involved in establishing an antiviral state can help shed light on TiLV disease pathogenesis. Moreover, understanding the adaptive immune mechanisms involved in mounting protection against TiLV could greatly assist in the development of vaccination strategies aimed at controlling TiLVD. This review summarizes the current state of knowledge on the immune responses following TiLV infection. After describing the main pathological findings associated with TiLVD, both the innate and adaptive immune responses and mechanisms to TiLV infection are discussed, in both disease infection models and in vitro studies. In addition, our work, highlights research questions, knowledge gaps and research areas in the immunology of TiLV infection where further studies are needed to better understand how disease protection against TiLV is established.

Development of a TaqMan quantitative reverse transcription PCR assay to detect tilapia lake virus

Tilapia lake virus disease (TiLVD) is an emerging viral disease associated with high morbidity and mortality in cultured tilapia worldwide. In this study, we have developed and validated a TaqMan quantitative reverse transcription PCR (RT-qPCR) assay for TiLV, targeting a conserved region within segment 10 of the genome. The RT-qPCR assay was efficient (mean ± SD: 96.71 ± 3.20%), sensitive with a limit of detection of 10 RNA viral copies per reaction, and detected TiLV strains from different geographic regions including North America, South America, Africa, and Asia. The intra- and inter-assay variability ranged over 0.18-1.41% and 0.21-2.21%, respectively. The TaqMan RT-qPCR assay did not cross-react with other RNA viruses of fish, including an orthomyxovirus, a betanodavirus, a picornavirus, and a rhabdovirus. Analysis of 91 proven-positive and 185 proven-negative samples yielded a diagnostic sensitivity of 96.7% and a diagnostic specificity of 100%. The TaqMan RT-qPCR assay also detected TiLV RNA in infected Nile tilapia liver tissue extracts following an experimental challenge study, and it successfully detected TiLV RNA in SSN-1 (E-11 clone) cell cultures displaying cytopathic effects following their inoculation with TiLV-infected tissue homogenates. Thus, the validated TaqMan RT-qPCR assay should be useful for both research and diagnostic purposes. Additionally, the TiLV qPCR assay returns the clinically relevant viral load of a sample which can assist health professionals in determining the role of TiLV during disease investigations. This RT-qPCR assay could be integrated into surveillance programs aimed at mitigating the effects of TiLVD on global tilapia production.

Chitosan nanoparticle immersion vaccine offers protection against tilapia lake virus in laboratory and field studies

Tilapia lake virus (TiLV), an enveloped negative-sense single-stranded RNA virus, causes tilapia lake virus disease (TiLVD), which is associated with mass mortality and severe economic impacts in wild and farmed tilapia industries worldwide. In this study, we developed a chitosan nanoparticle TiLV immersion vaccine and assessed the efficacy of the vaccine in laboratory and field trials. Transmission electron microscopy showed that the inactivated vaccine had a particle size of 210.3 nm, while the nano inactivated vaccine had a spherical shape with a diameter of 120.4 nm. Further analysis using fluorescent staining and immunohistochemistry analysis revealed the mucoadhesive properties of the nanovaccine (CN-KV) through fish gills. We assessed the efficacy of an immersion-based TiLV nanovaccine using a cohabitation challenge model. The fish that received the nanovaccine showed better relative percent survival (RPS) at 68.17% compared with the RPS of the inactivated virus vaccine (KV) group at 25.01%. The CN-KV group also showed a higher TiLV-specific antibody response than the control and KV groups (p < 0.05). Importantly, under field conditions, the fish receiving the CN-KV nanovaccine had better RPS at 52.2% than the nonvaccinated control group. Taken together, the CN-KV nanovaccinated fish showed better survival and antibody response than the control and KV groups both under laboratory control challenge conditions and field trials. The newly developed immersion-based nanovaccine is easy to administer in small fish, is less labor-intensive, and allows for mass vaccination to protect fish from TiLV infection.

Combining segments 9 and 10 in DNA and recombinant protein vaccines conferred superior protection against tilapia lake virus in hybrid red tilapia (oreochromis sp.) compared to single segment vaccines

Tilapia lake virus (TiLV) now affects Nile tilapia culture worldwide, with no available commercial vaccine for disease prevention. DNA and recombinant protein-based vaccines were developed and tested following viral isolation and characterization. The viral strain isolated from diseased hybrid red tilapia (Oreochromis sp.) shared high levels of morphological and genomic similarity (95.49-99.52%) with other TiLV isolates in the GenBank database. TiLV segment 9 (Tis9) and segment 10 (Tis10) DNA vaccines (pcDNA-Tis9 and pcDNA-Tis10) and recombinant protein vaccines (Tis9 and Tis10) were prepared and tested for their efficacy in juvenile hybrid red tilapia. Fish were immunized with either single vaccines (pcDNA-Tis9, pcDNA-Tis10, Tis9 and Tis10) or combined vaccines (pcDNA-Tis9 + pcDNA-Tis10 and Tis9 + Tis10) by intramuscular injection and intraperitoneal injection for DNA and protein vaccines, respectively. Negative controls were injected with PBS or a naked pcDNA3.1 vector in the same manner. An experimental challenge with TiLV was carried out at 4 weeks post-vaccination (wpv) by intraperitoneal injection with a dose of 1 × 10⁵ TCID₅₀ per fish. Relative percent survival (RPS) ranged from 16.67 ± 00.00 to 61.11 ± 9.62%. The Tis10 and pcDNA-Tis10 vaccines conferred better protection compared to Tis9 and pcDNA-Tis9. Highest levels of protection were observed in pcDNA-Tis9 + pcDNA-Tis10 (61.11 ± 9.62%) and Tis9 + Tis10 (55.56 ± 9.62%) groups. Specific antibody was detected in all vaccinated groups at 1-4 wpv by Dot Blot method, with the highest integrated density at 2 and 3 wpv. In silico analysis of Tis9 and Tis10 revealed a number of B-cell epitopes in their coil structure, possibly reflecting their immunogenicity. Findings suggested that the combination of Tis9 and Tis10 in DNA and recombinant protein vaccine showed high efficacy for the prevention of TiLV disease in hybrid red tilapia.

Carps, Catla catla, Cirrhinus mrigala and Hypophthalmichthys molitrix Are Resistant to Experimental Infection with Tilapia Lake Virus (TiLV)

Tilapia tilapinevirus, also known as tilapia lake virus (TiLV), is an emerging fish virus that primarily affects tilapines. However, the virus has also been detected in a few non-tilapines. As tilapia is generally farmed in polyculture systems along with carps in South Asian countries, there is a likelihood that TiLV-infected tilapia can transmit the virus to the co-cultured species. In view of the above, the susceptibility of three carp species, namely catla (Catla catla), mrigal (Cirrhinus mrigala) and silver carp (Hypophthalmichthys molitrix) was evaluated vis-à-vis tilapia, following experimental infection with TiLV. No clinical signs and histopathological alterations could be observed in carps. RT-qPCR revealed that TiLV copy numbers in liver and brain of all the three carps were almost negligible and did not show any increase with time, suggesting that the virus did not replicate in liver and brain, the target organs of TiLV. Further, TiLV could not be isolated from pooled liver and brain tissues of carps using permissive CFF cell line. On the contrary, in tilapia, typical clinical signs and histopathological lesions were observed and there was significant increase in TiLV copy number up to 6 days post-injection. Furthermore, the virus was successfully isolated from pooled liver and brain tissue of infected tilapia. From the above findings, it could be concluded that C. catla, C. mrigala and H. molitrix are resistant to TiLV infection and unlikely to be carriers for this virus.

Tilapia Lake Virus-Induced Neuroinflammation in Zebrafish: Microglia Activation and Sickness Behavior

In mammals, the relationship between the immune system and behavior is widely studied. In fish, however, the knowledge concerning the brain immune response and behavioral changes during brain viral infection is very limited. To further investigate this subject, we used the model of tilapia lake virus (TiLV) infection of zebrafish (Danio rerio), which was previously developed in our laboratory. We demonstrated that TiLV persists in the brain of adult zebrafish for at least 90 days, even when the virus is not detectable in other peripheral organs. The virions were found in the whole brain. During TiLV infection, zebrafish displayed a clear sickness behavior: decreased locomotor activity, reduced food intake, and primarily localizes near the bottom zone of aquaria. Moreover, during swimming, individual fish exhibited also unusual spiral movement patterns. Gene expression study revealed that TiLV induces in the brain of adult fish strong antiviral and inflammatory response and upregulates expression of genes encoding microglia/macrophage markers. Finally, using zebrafish larvae, we showed that TiLV infection induces histopathological abnormalities in the brain and causes activation of the microglia which is manifested by changes in cell shape from a resting ramified state in mock-infected to a highly ameboid active state in TiLV-infected larvae. This is the first study presenting a comprehensive analysis of the brain immune response associated with microglia activation and subsequent sickness behavior during systemic viral infection in zebrafish.

Weight-dependent susceptibility of tilapia to tilapia lake virus infection

The emergence of tilapia lake virus (TiLV) has had a severely negative impact on global tilapia aquaculture. TiLV infection has been reported at different life stages of tilapia, with more emphasis on fry and fingerlings; however, the virulence and pathology of TiLV at different tilapia size remains unexplored. In this study, tilapias from a single spawning were raised to 5 g, 25 g, and 65 g, and subsequently challenged by the intraperitoneal injection and cohabitation of a virulent strain of TiLV. The cumulative mortality, viral load, and histopathology of the fish were determined until 22 days post-infection (dpi). The cumulative mortality of the 5 g, 25 g, and 65 g fish was 85% (±1.67), 55% (±2.89), and 51.67% (±7.49), respectively. At 14 dpi, the mean TiLV load in the liver of the 5 g fish was significantly higher than in the 25 g and 65 g fish. All the weight groups showed severe pathological changes in the liver, spleen, and intestine after TiLV infection, but no particular difference was otherwise noted during the study with the exception of higher pathological scores in the liver of the small fish at 14 dpi. Overall, this study indicated that small fish are more susceptible to TiLV infection than large fish. Although multiple factors, including environmental factors, farm management practices, strains of virus could contribute to different susceptibility of fish to viral infection, the present study provides the evidence to support that fish weight affects the mortality and clinical outcome during TiLV infection. More intensive measures such as strict biosecurity and disease surveillance during the susceptible weight should therefore be emphasized to reduce the impact of this virus.

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.

Transcriptome analysis of liver elucidates key immune-related pathways in Nile tilapia Oreochromis niloticus following infection with tilapia lake virus

Nile tilapia (Oreochromis niloticus) is one of the most important aquaculture species farmed worldwide. However, the recent emergence of tilapia lake virus (TiLV) disease, also known as syncytial hepatitis of tilapia, has threatened the global tilapia industry. To gain more insight regarding the host response against the disease, the transcriptional profiles of liver in experimentally-infected and control tilapia were compared. Analysis of RNA-Seq data identified 4640 differentially expressed genes (DEGs), which were involved among others in antigen processing and presentation, MAPK, apoptosis, necroptosis, chemokine signaling, interferon, NF-kB, acute phase response and JAK-STAT pathways. Enhanced expression of most of the DEGs in the above pathways suggests an attempt by tilapia to resist TiLV infection. However, upregulation of some of the key genes such as BCL2L1 in apoptosis pathway; NFKBIA in NF-kB pathway; TRFC in acute phase response; and SOCS, EPOR, PI3K and AKT in JAK-STAT pathway and downregulation of the genes, namely MAP3K7 in MAPK pathway; IFIT1 in interferon; and TRIM25 in NF-kB pathway suggested that TiLV was able to subvert the host immune response to successfully establish the infection. The study offers novel insights into the cellular functions that are affected following TiLV infection and will serve as a valuable genomic resource towards our understanding of susceptibility of tilapia to TiLV infection.

Type I interferon-dependent response of zebrafish larvae during tilapia lake virus (TiLV) infection

Tilapia lake virus (TiLV; genus: Tilapinevirus, family: Amnoonviridae) is a recently characterised enveloped virus with a linear, negative-sense single-stranded RNA genome, which causes high mortality in tilapia species. In the present study, we demonstrated that zebrafish (Danio rerio) larvae are susceptible to TiLV infection upon systemic injection. TiLV replicated in zebrafish larvae and caused their high mortality (of about 70%). Histopathological examination revealed that TiLV infection caused pathological abnormalities in zebrafish larvae that were well visible within the brain. Moreover, gene expression analysis revealed that TiLV infection induced up-regulation of the expression of the immune-related genes encoding pathogen recognition receptors involved in sensing of viral dsRNA (rig-I (ddx58), tlr3, tlr22), transcription factors (irf3, irf7), type I interferon (infϕ1), antiviral protein (mxa), and pro-inflammatory cytokine (il-1β). We also demonstrated the protective role of the recombinant zebrafish IFNϕ1 on the survival of zebrafish larvae during TiLV infection. Our results show the importance of type I IFN response during TiLV infection in zebrafish larvae and demonstrate that zebrafish is a good model organism to study interactions between TiLV - a newly emerging in aquaculture virus, and fish host.

Pathogenesis and immune response of Nile tilapia (Oreochromis niloticus) exposed to Tilapia lake virus by intragastric route

Tilapia lake virus (TiLV) is regarded as one of the most important pathogens in tilapia aquaculture worldwide. Despite this, little is known regarding disease pathogenesis and immune responses to infection. The main objective of this study was to investigate the tissue distribution, histopathological changes, and immune response of fish exposed to TiLV. Nile tilapia (Oreochromis niloticus) maintained at 25 ± 2 °C were challenged with TiLV via intragastric-gavage. At 0.5, 1, 3, 5, 7, 10 and 15 days post-challenge (dpc), six fish per treatment were euthanized and subjected to complete necropsy. TiLV exposed fish presented 45% cumulative mortality at the end of the study. Gross lesions included cutaneous petechiae and ecchymoses, scale losses, skin ulcers, and exophthalmia. Mild multifocal hepatocellular degeneration and necrosis was observed as early as 3 dpc occasionally accompanied by syncytial formation, intracytoplasmic inclusion bodies, and inflammatory infiltrates of lymphocytes at subsequent time points. Necrosis of epithelial cells of the gastric glands and intestinal glands was also observed as early as 5 dpc. Intestinal samples showed reactive in situ hybridization signals as early as 1 dpc. No other lesions were observed in the brain or other organs. Histological changes were associated with viral dissemination and disease progression, as evidenced by increased TiLV detection in the intestine, gills, liver and spleen. Highest TiLV abundance was detected 7 dpc in gills, intestine, and liver showing an average of 6 LOG genome equivalent per ng of total RNA. Different transcript abundance was detected for the pro-inflammatory cytokine interleukin-1β and interferon-induced myxovirus resistance protein gene in the mucosal sites (gills and intestine). Interferon regulatory transcription factor 3 transcript was more abundant in systemic organs (liver and spleen) while the expression in gills and intestine showed mixed expression at different time points. On the other hand, transforming growth factor β expression patterns differed amongst the tissues with a trend towards downregulation of the gene in liver and gills, and a trend towards upregulation in the spleen and intestine. Overall, these results demonstrate the intestinal routes as a main port of entry for TiLV, which subsequently spreads systematically throughout the fish body.

Tilapia lake virus downplays innate immune responses during early stage of infection in Nile tilapia (Oreochromis niloticus)

Tilapia lake virus (TiLV) causes high mortality and high economic losses in tilapines. We describe an experimental challenge study focusing on early post challenge innate immune responses. Nile tilapia (Oreochromis niloticus) were infected with 10⁵ TCID₅₀/mL TiLV intraperitoneally, followed by virus quantification, histopathology and gene expression analysis in target (brain/liver) and lymphoid (spleen/headkidney) organs at 3, 7, 12, 17, and 34 days post challenge (dpc). Onset of mortality was from 21 dpc, and cumulative mortality was 38.5% by 34 dpc. Liver and kidney histopathology developed over the period 3–17 dpc, characterized by anisocytosis, anisokaryocytosis, and formation of multinucleated hepatocytes. Viral loads were highest at early time (3 dpc) in liver, spleen and kidney, declining towards 34 dpc. In brain, viral titer peaked 17 dpc. Innate sensors, TLRs 3/7 were inversely correlated with virus titer in brain and headkidney, and IFN-ß and Mx showed a similar pattern. All organs showed increased mRNA IgM expression over the course of infection. Overall, high virus titers downplay innate responses, and an increase is seen when viral titers decline. In silico modeling found that TiLV segments 4, 5 and 10 carry nucleolar localization signals. Anti-viral effects of TiLV facilitate production of virus at early stage of infection.

Tilapia develop protective immunity including a humoral response following exposure to tilapia lake virus

Tilapia lake virus (TiLV) is an emerging virus associated with high mortality in cultured tilapia. Since the first report of tilapia lake virus, it has been detected in diseased tilapia in sixteen countries around the world. Thus, there is an urgent need to develop an efficacious vaccine to prevent TiLV disease (TiLVD) and reduce its global economic impact. Understanding the role of the adaptive immune response following exposure of tilapia to TiLV is a critical step in the development of such a vaccine. In this study, we challenged red hybrid tilapia by cohabitation or intraperitoneal injection and demonstrated that surviving fish develop a protective immunity. We also demonstrated that tilapia that survived experimental infections possess significant antibodies against the protein encoded by the TiLV segment 4. We then developed a TiLV indirect ELISA to determine the antibody response in tilapia. The ELISA revealed high antibody levels in survivors of experimental challenges and following outbreaks on farms. The ELISA effectively distinguished TiLV-exposed from unexposed tilapia and was used to monitor anti-TiLV antibody kinetics following infection. During the primary infection, tilapia developed an antibody response as early as 7 days post TiLV challenge (dpc), peaked at 15 dpc, showed a gradual decline up until about 42 dpc, but persisted in some fish up until day 110 dpc. Upon re-infection, an increased antibody response occurred within 7-14 days, demonstrating that tilapia that survive TiLV infections develop humoral memory. In conclusion, our results demonstrated that tilapia mount antibody responses against TiLV that supports protective immunity to subsequent TiLV disease. The persistence of anti-TiLV antibodies in survivors following a single exposure suggests a single vaccination might be adequate to protect tilapia during the entire grow-out period. This study provides important information about the immune response of tilapia following exposure to TiLV as a first step in the development of an efficacious vaccine against this emerging and economically important 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.

Antiviral response of adult zebrafish (Danio rerio) during tilapia lake virus (TiLV) infection

Tilapia lake virus (TiLV) is a novel enveloped orthomyxo-like virus with a genome of 10 segments of linear negative-sense single-stranded RNA. It causes massive mortality of wild and farmed tilapia species and because of its spread in Asia, Africa, South and North America, it is considered a threat to tilapia aquaculture. Here, we have evaluated the possible use of zebrafish (Danio rerio) to study immune response and host-pathogen interactions during an infection with TiLV. Adult zebrafish were infected with TiLV by intraperitoneal (i.p) injection or by cohabitation. Increased viral load was observed in liver, spleen and kidney of i.p. injected fish at 1, 3, 6, and 14 days post infection (dpi) but not in fish from the cohabitation group (only liver was tested). We also demonstrated that in spleen and kidney i.p. injection of TiLV induced up-regulation of the expression of the immune-related genes encoding pathogen recognition receptors involved in sensing of viral dsRNA (rig-I, tlr3, tlr22), transcription factors (irf3, irf7), type I interferon (infϕ1), antiviral protein (mxa), pro-inflammatory (il-1β, tnf-α, il-8, ifnγ1-2) and anti-inflammatory (il-10) cytokines, CD4 markers (cd4-1, cd4-2), and IgM (igm). Moreover, tissue tropism of TiLV and histopathological changes were analyzed in selected organs of i.p. injected zebrafish. Our results indicate that zebrafish is a good model to study mechanisms of the TiLV infection and to follow antiviral responses.

Tilapia Lake Virus Does Not Hemagglutinate Avian and Piscine Erythrocytes and NH4Cl Does Not Inhibit Viral Replication In Vitro

Tilapia lake virus (TiLV) is a negative-sense single-stranded RNA (-ssRNA) icosahedral virus classified to be the only member in the family Amnoonviridae. Although TiLV segment-1 shares homology with the influenza C virus PB1 and has four conserved motifs similar to influenza A, B, and C polymerases, it is unknown whether there are other properties shared between TiLV and orthomyxovirus. In the present study, we wanted to determine whether TiLV agglutinated avian and piscine erythrocytes, and whether its replication was inhibited by lysosomotropic agents, such as ammonium chloride (NH₄Cl), as seen for orthomyxoviruses. Our findings showed that influenza virus strain A/Puerto Rico/8 (PR8) was able to hemagglutinate turkey (Meleagris gallopavo), Atlantic salmon (Salmo salar L), and Nile tilapia (Oreochromis niloticus) red blood cells (RBCs), while infectious salmon anemia virus (ISAV) only agglutinated Atlantic salmon, but not turkey or tilapia, RBCs. In contrast to PR8 and ISAV, TiLV did not agglutinate turkey, Atlantic salmon, or tilapia RBCs. qRT-PCR analysis showed that 30 mM NH₄Cl, a basic lysosomotropic agent, neither inhibited nor enhanced TiLV replication in E-11 cells. There was no difference in viral quantities in the infected cells with or without NH₄Cl treatment during virus adsorption or at 1, 2, and 3 h post-infection. Given that hemagglutinin proteins that bind RBCs also serve as ligands that bind host cells during virus entry leading to endocytosis in orthomyxoviruses, the data presented here suggest that TiLV may use mechanisms that are different from orthomyxoviruses for entry and replication in host cells. Therefore, future studies should seek to elucidate the mechanisms used by TiLV for entry into host cells and to determine its mode of replication in infected cells.