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

Effects of light and circadian clock on the antiviral immune response in zebrafish

The circadian clock mechanism, which is evolutionarily conserved across various organisms, plays a crucial role in synchronizing physiological responses to external conditions, primarily in response to light availability. By maintaining homeostasis of biological processes and behavior, the circadian clock serves as a key regulator. This biological mechanism also coordinates diurnal oscillations of the immune response during infections. However there is limited information available regarding the influence of circadian oscillation on immune regulation, especially in lower vertebrates like teleost fish. Therefore, the present study aimed to investigate the effects of light and the timing of infection induction on the antiviral immune response in zebrafish. To explore the relationship between the timing of infection and the response activated by viral pathogens, we used a zebrafish model infected with tilapia lake virus (TiLV). Our findings demonstrated that light availability significantly affects the antiviral immune response and the functioning of the molecular clock mechanism during TiLV infection. This is evident through alterations in the expression of major core clock genes and the regulation of TiLV replication and type I IFN pathway genes in the kidney of fish maintained under LD (light-dark) conditions compared to constant darkness (DD) conditions. Moreover, infection induced during the light phase of the LD cycle, in contrast to nocturnal infection, also exhibited similar effects on the expression of genes associated with the antiviral response. This study indicates a more effective mechanism of the zebrafish antiviral response during light exposure, which inherently involves modification of the expression of key components of the molecular circadian clock. It suggests that the zebrafish antiviral response to infection is regulated by both light and the circadian clock.

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.

Tilapia lake virus (TiLV) causes severe anaemia and systemic disease in tilapia

Tilapia lake virus disease (TiLVD) is an emerging disease in tilapia that is associated with mass mortality affecting global tilapia aquaculture. In this study, red hybrid tilapias (Oreochromis spp.) were experimentally infected by intracoelomic injection with Tilapia lake virus (TiLV) to gain a better understanding of the clinicopathological changes during infection. Pale bodies and gill were observed in infected fish after 7 days of post-challenge (dpc) associated with severe anaemia. Further haematological analysis in TiLV-infected fish revealed decreased levels of haemoglobin and haematocrit at 3 dpc. Common pathological findings included pale and friable liver, pale intestine with catarrhal content, and dark and shrunken spleen in TiLV-infected fish at 7 dpc and 14 dpc. Histologically, reduced numbers of red blood cells and accumulation of melano-macrophage centre in the spleen were found in infected fish at 3 dpc, and severe lesions were more commonly observed at 7 and 14 dpc. Lymphocyte infiltration, syncytial cell formation and multifocal necrotic hepatitis were the prominent pathological findings in the liver of infected fish. The severity of pathological changes was associated with TiLV-infection with higher viral loads and with the expression pattern of pro-inflammatory cytokines and antiviral genes, including interferon regulatory factor 1 (irf1), interleukin (il-8), radical s-adenosyl methionine domain containing 2 (rsad2) and mx. Our study provides a comprehensive analysis of the haematological profile and pathological changes in tilapia during TiLV infection. Overall, lesions present in various organs, together with alteration of host immune response in TiLV-infected fish, indicate the systemic infection of this virus. The knowledge gained from this study improves our understanding of how TiLV causes pathological and haematological changes in tilapia.

The Modulation of Immune Responses in Tilapinevirus tilapiae-Infected Fish Cells through MAPK/ERK Signalling

Tilapia lake virus (TiLV) is a novel RNA virus that has been causing substantial economic losses across the global tilapia industry. Despite extensive research on potential vaccines and disease control methods, the understanding of this viral infection and the associated host cell responses remains incomplete. In this study, the involvement of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway in the early stages of TiLV infection was investigated. The results showed a distinct pattern of ERK phosphorylation (p-ERK) upon TiLV infection in two fish cell lines, E-11 and TiB. Specifically, the p-ERK levels in the TiB cells decreased substantially, while the p-ERK levels in the E-11 cells remained constant. Interestingly, a large number of cytopathic effects were observed in the infected E-11 cells but none in the infected TiB cells. Furthermore, when p-ERK was suppressed using the inhibitor PD0325901, a significant reduction in the TiLV load and decrease in the mx and rsad2 gene expression levels were observed in the TiB cells in days 1–7 following infection. These findings highlight the role of the MAPK/ERK signalling pathway and provide new insights into the cellular mechanisms during TiLV infection that could be useful in developing new strategies to control this virus.

Susceptibility and Permissivity of Zebrafish (Danio rerio) Larvae to Cypriniviruses

The zebrafish (Danio rerio) represents an increasingly important model organism in virology. We evaluated its utility in the study of economically important viruses from the genus Cyprinivirus (anguillid herpesvirus 1, cyprinid herpesvirus 2 and cyprinid herpesvirus 3 (CyHV-3)). This revealed that zebrafish larvae were not susceptible to these viruses after immersion in contaminated water, but that infections could be established using artificial infection models in vitro (zebrafish cell lines) and in vivo (microinjection of larvae). However, infections were transient, with rapid viral clearance associated with apoptosis-like death of infected cells. Transcriptomic analysis of CyHV-3-infected larvae revealed upregulation of interferon-stimulated genes, in particular those encoding nucleic acid sensors, mediators of programmed cell death and related genes. It was notable that uncharacterized non-coding RNA genes and retrotransposons were also among those most upregulated. CRISPR/Cas9 knockout of the zebrafish gene encoding protein kinase R (PKR) and a related gene encoding a protein kinase containing Z-DNA binding domains (PKZ) had no impact on CyHV-3 clearance in larvae. Our study strongly supports the importance of innate immunity-virus interactions in the adaptation of cypriniviruses to their natural hosts. It also highlights the potential of the CyHV-3-zebrafish model, versus the CyHV-3-carp model, for study of these interactions.

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.

Persistent detection of Tilapia lake virus in wild tilapia and tinfoil barbs

Background and Aim:

One of the emerging viral diseases in freshwater fish is Tilapia lake virus (TiLV), which infects all stages of fish and results in mass mortalities. Previously, a TiLV case was detected in the wild environment in Malaysia that involved tilapia and tinfoil barb. Hence, this study aimed to determine the presence of TiLV in wild tilapia (Oreochromis niloticus) as well as tinfoil barbs (Barbonymus schwanenfeldii) at the similar lake after the initial outbreak in year 2017.

Materials and Methods:

Both fish species were sampled from this lake at a month interval for two years and subjected to TiLV detection using reverse transcriptase-polymerase chain reaction and cell culture isolation. Concurrently, bacterial isolation and water quality measurements were performed to deduce their correlation with TiLV occurrence. Other wild fish species and mollusk were also occasionally sampled during the fish inventory activity at this lake. The fish’s weight, length, and associated clinical signs were noted throughout the entire study period.

Results:

Mortality was not observed throughout the whole study period, and results indicated a moderate to high prevalence of TiLV infection in both tilapia and tinfoil barbs. There was no correlation between TiLV infection with the isolation rate of opportunistic bacteria such as Aeromonas spp., Plesiomonas spp., and Edwardsiella spp. in the study site. At the same time, the Pearson correlation test revealed a moderate negative correlation between the water pH with the presence of TiLV (R=−0.4472; p<0.05) and a moderate positive correlation between the water iron content with the monthly detection of Aeromonas spp. in wild tilapia. This is contrary to tinfoil barbs, where there was a moderate negative correlation between the water iron content with the monthly isolation of Aeromonas spp. (R=−0.5190; p<0.05). Furthermore, isolation of TiLV on cell culture-induced viral invasion was resulted in the cytopathic effects.

Conclusions:

Our results suggest that the wild fish may harbor TiLV for an extended period following a massive die-off event in 2017 without any obvious clinical signs and mortality. The persistency of viruses in the wild may need continuous and effective control as well as prevention strategies.

Tilapia Lake Virus was not detected in non-tilapine species within tilapia polyculture systems of Bangladesh

Sixteen countries, including Bangladesh, have reported the presence of tilapia lake virus (TiLV), an emerging tilapia pathogen. Fish polyculture is a common farming practice in Bangladesh. Some unusual mortalities reported in species co-cultivated with TiLV-infected tilapia led us to investigate whether any of the co-cultivated species would also test positive for TiLV and whether they were susceptible to TiLV infection under controlled laboratory experiments. Using 183 samples obtained from 15 farms in six districts across Bangladesh, we determined that 20% of the farms tested positive for TiLV in tilapia, while 15 co-cultivated fish species and seven other invertebrates (e.g. insects and crustaceans) considered potential carriers all tested negative. Of the six representative fish species experimentally infected with TiLV, only Nile tilapia showed the typical clinical signs of the disease, with 70% mortality within 12 days. By contrast, four carp species and one catfish species challenged with TiLV showed no signs of TiLV infection. Challenged tilapia were confirmed as TiLV-positive by RT-qPCR, while challenged carp and walking catfish all tested negative. Overall, our field and laboratory findings indicate that species used in polycultures are not susceptible to TiLV. Although current evidence suggests that TiLV is likely host-specific to tilapia, targeted surveillance for TiLV in other fish species in polyculture systems should continue, in order to prepare for a possible future scenario where TiLV mutates and/or adapts to new host(s).

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.

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.

Modeling Virus-Induced Inflammation in Zebrafish: A Balance Between Infection Control and Excessive Inflammation

The inflammatory response to viral infection in humans is a dynamic process with complex cell interactions that are governed by the immune system and influenced by both host and viral factors. Due to this complexity, the relative contributions of the virus and host factors are best studied in vivo using animal models. In this review, we describe how the zebrafish (Danio rerio) has been used as a powerful model to study host-virus interactions and inflammation by combining robust forward and reverse genetic tools with in vivo imaging of transparent embryos and larvae. The innate immune system has an essential role in the initial inflammatory response to viral infection. Focused studies of the innate immune response to viral infection are possible using the zebrafish model as there is a 4-6 week timeframe during development where they have a functional innate immune system dominated by neutrophils and macrophages. During this timeframe, zebrafish lack a functional adaptive immune system, so it is possible to study the innate immune response in isolation. Sequencing of the zebrafish genome has revealed significant genetic conservation with the human genome, and multiple studies have revealed both functional conservation of genes, including those critical to host cell infection and host cell inflammatory response. In addition to studying several fish viruses, zebrafish infection models have been developed for several human viruses, including influenza A, noroviruses, chikungunya, Zika, dengue, herpes simplex virus type 1, Sindbis, and hepatitis C virus. The development of these diverse viral infection models, coupled with the inherent strengths of the zebrafish model, particularly as it relates to our understanding of macrophage and neutrophil biology, offers opportunities for far more intensive studies aimed at understanding conserved host responses to viral infection. In this context, we review aspects relating to the evolution of innate immunity, including the evolution of viral pattern recognition receptors, interferons and interferon receptors, and non-coding RNAs.

Sickness behaviors across vertebrate taxa: proximate and ultimate mechanisms

There is nothing like a pandemic to get the world thinking about how infectious diseases affect individual behavior. In this respect, sick animals can behave in ways that are dramatically different from healthy animals: altered social interactions and changes to patterns of eating and drinking are all hallmarks of sickness. As a result, behavioral changes associated with inflammatory responses (i.e. sickness behaviors) have important implications for disease spread by affecting contacts with others and with common resources, including water and/or sleeping sites. In this Review, we summarize the behavioral modifications, including changes to thermoregulatory behaviors, known to occur in vertebrates during infection, with an emphasis on non-mammalian taxa, which have historically received less attention. We then outline and discuss our current understanding of the changes in physiology associated with the production of these behaviors and highlight areas where more research is needed, including an exploration of individual and sex differences in the acute phase response and a greater understanding of the ecophysiological implications of sickness behaviors for disease at the population level.

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.

Cytosolic Sensors for Pathogenic Viral and Bacterial Nucleic Acids in Fish

Recognition of the non-self signature of invading pathogens is a crucial step for the initiation of the innate immune mechanisms of the host. The host response to viral and bacterial infection involves sets of pattern recognition receptors (PRRs), which bind evolutionarily conserved pathogen structures, known as pathogen-associated molecular patterns (PAMPs). Recent advances in the identification of different types of PRRs in teleost fish revealed a number of cytosolic sensors for recognition of viral and bacterial nucleic acids. These are DExD/H-box RNA helicases including a group of well-characterized retinoic acid inducible gene I (RIG-I)-like receptors (RLRs) and non-RLR DExD/H-box RNA helicases (e.g., DDX1, DDX3, DHX9, DDX21, DHX36 and DDX41) both involved in recognition of viral RNAs. Another group of PRRs includes cytosolic DNA sensors (CDSs), such as cGAS and LSm14A involved in recognition of viral and intracellular bacterial dsDNAs. Moreover, dsRNA-sensing protein kinase R (PKR), which has a role in antiviral immune responses in higher vertebrates, has been identified in fish. Additionally, fish possess a novel PKR-like protein kinase containing Z-DNA binding domain, known as PKZ. Here, we review the current knowledge concerning cytosolic sensors for recognition of viral and bacterial nucleic acids in teleosts.

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.