Detection and characterization of a novel marine birnavirus isolated from Asian seabass in Singapore

Diversity, evolution, and emergence of fish viruses

The production of aquatic animals has more than doubled over the last 50 years and is anticipated to continually increase. While fish are recognized as a valuable and sustainable source of nutrition, particularly in the context of human population growth and climate change, the rapid expansion of aquaculture coincides with the emergence of highly pathogenic viruses that often spread globally through aquacultural practices. Here, we provide an overview of the fish virome and its relevance for disease emergence, with a focus on the insights gained through metagenomic sequencing, noting potential areas for future study. In particular, we describe the diversity and evolution of fish viruses, for which the majority have no known disease associations, and demonstrate how viruses emerge in fish populations, most notably at an expanding domestic-wild interface. We also show how wild fish are a powerful and tractable model system to study virus ecology and evolution more broadly and can be used to identify the major factors that shape vertebrate viromes. Central to this is a process of virus-host co-divergence that proceeds over many millions of years, combined with ongoing cross-species virus transmission.

Discovery, Pathogenesis, and Complete Genome Characterization of Lates calcarifer Herpesvirus

In 2015 and 2016, two Barramundi (Lates calcarifer) farms in Singapore reported a disease outbreak characterized by lethargic behavior, pronounced inappetence, generalized skin lesions, erosions of the fins and tail, and ultimately high mortality in their fish. Next-generation sequencing and PCR confirmed presence of a novel virus belonging to the Alloherpesviridae family, Lates calcarifer herpesvirus (LCHV), which was subsequently isolated and cultured. We characterize, for the first time, the complete genome of two cultured LCHV isolates. The genome contains a long unique region of approximately 105,000 bp flanked by terminal repeats of approximately 24,800 bp, of which the first 8.2 kb do not show any similarity to described genomes in the Alloherpesviridae family. The two cultured isolates share 89% nucleotide identity, and their closest relatives are the viruses belonging to the genus Ictalurivirus. Experimental infections using one of the cultured LCHV isolates resulted in identical clinical signs as originally described in the index farm, both in intraperitoneal-injection infected fish and cohabitant fish, with mortality in both groups. Histopathological analysis showed pronounced abnormalities in the gills. Virus culture and PCR analysis confirmed the replication of LCHV in the infected fish, and thus Koch's postulates were fulfilled.

Riding the wave of innovation: immunoinformatics in fish disease control

The spread of infectious illnesses has been a significant factor restricting aquaculture production. To maximise aquatic animal health, vaccination tactics are very successful and cost-efficient for protecting fish and aquaculture animals against many disease pathogens. However, due to the increasing number of immunological cases and their complexity, it is impossible to manage, analyse, visualise, and interpret such data without the assistance of advanced computational techniques. Hence, the use of immunoinformatics tools is crucial, as they not only facilitate the management of massive amounts of data but also greatly contribute to the creation of fresh hypotheses regarding immune responses. In recent years, advances in biotechnology and immunoinformatics have opened up new research avenues for generating novel vaccines and enhancing existing vaccinations against outbreaks of infectious illnesses, thereby reducing aquaculture losses. This review focuses on understanding in silico epitope-based vaccine design, the creation of multi-epitope vaccines, the molecular interaction of immunogenic vaccines, and the application of immunoinformatics in fish disease based on the frequency of their application and reliable results. It is believed that it can bridge the gap between experimental and computational approaches and reduce the need for experimental research, so that only wet laboratory testing integrated with in silico techniques may yield highly promising results and be useful for the development of vaccines for fish.

Molecular Characterization of MHC Class I Alpha 1 and 2 Domains in Asian Seabass (Lates calcarifer)

The Asian seabass is of importance both as a farmed and wild animal. With the emergence of infectious diseases, there is a need to understand and characterize the immune system. In humans, the highly polymorphic MHC class I (MHC-I) molecules play an important role in antigen presentation for the adaptive immune system. In the present study, we characterized a single MHC-I gene in Asian seabass (Lates calcarifer) by amplifying and sequencing the MHC-I alpha 1 and alpha 2 domains, followed by multi-sequence alignment analyses. The results indicated that the Asian seabass MHC-I α1 and α2 domain sequences showed an overall similarity within Asian seabass and retained the majority of the conserved binding residues of human leukocyte antigen-A2 (HLA-A2). Phylogenetic tree analysis revealed that the sequences belonged to the U lineage. Mapping the conserved binding residue positions on human HLA-A2 and grass carp crystal structure showed a high degree of similarity. In conclusion, the availability of MHC-I α1 and α2 sequences enhances the quality of MHC class I genetic information in Asian seabass, providing new tools to analyze fish immune responses to pathogen infections, and will be applicable in the study of the phylogeny and the evolution of antigen-specific receptors.

Determination and Characterization of a Novel Birnavirus Associated with Massive Mortality in Largemouth Bass

Largemouth bass (Micropterus salmoides) is an important and fast-growing aquaculture species in China. In 2017, an epidemic associated with severe mortality occurred in fingerlings of largemouth bass in Guangdong, China. The causative pathogen was identified and named as largemouth bass Birnavirus (LBBV) by virome analysis, viral isolation, electron microscopy, genome sequencing, Western blot, indirect immunofluorescence, experimental challenge, and so on. Virome sequencing results showed that the relative abundance reads related to the family Birnaviridae were the highest, occupied ∼25% of the total viral reads. Electron microscopy revealed large numbers of nonenveloped virus particles in the spleen of diseased fish with a diameter of about 53 nm. LBBV was isolated and propagated in Chinese perch brain cells and induced a typical cytopathic effect. LBBV was stable to chloroform, heat, and 5-bromo-2'-deoxyuridine, but sensitive to acid (pH 3.0). The complete genome of LBBV was comprised of segment A with a size 3525 bp and segment B with a size 2737 bp. Phylogenetic analysis basing on RdRp and VP2 protein sequences revealed that LBBV were clustered into one clade with Lates calcarifer Birnavirus (LCBV), sharing 98.7% or 91.9% sequence identity with LCBV, respectively, but only sharing 59.7% and 52.7% sequence identity with Blosnavirus, suggesting that LBBV and LCBV probably belonged to a new genus. Challenge experiments results indicated that clinical disease symptoms similar to those observed naturally were replicated and the cumulative mortality reached 100% at 3 dpi by i.p. injection. The investigation of prevalence of LBBV infection showed that 41.5% (17/41) sample pools collected from diseased ponds was positive during 2017-2020, indicating that an emerging outbreak of this disease may be spreading within the largemouth bass in China. Above results confirmed that LBBV is a novel Birnavirus associated with massive mortality for fingerlings of largemouth bass. This provides a basis for prevention and control of this emerging viral disease. IMPORTANCE Pathogen isolation and identification are vital for emerging infectious outbreaks. Here we report the isolation, determination and characterization of a novel largemouth bass Birnavirus (LBBV) associated with massive mortality in largemouth bass. And genome of LBBV is determined and analyzed. Based on phylogenetic and alignment analysis of genome, we suggest LBBV belongs to a new genus (designated as Perbirnavirus genus) in Birnaviridae family. Our findings will provide a basis for the further study on prevention and control of this emerging viral disease.

Human Land-Use Impacts Viral Diversity and Abundance in a New Zealand River

Although water-borne viruses have important implications for the health of humans and other animals, little is known about the impact of human land use on viral diversity and evolution in water systems such as rivers. We used metatranscriptomic sequencing to compare the diversity and abundance of viruses at sampling sites along a single river in New Zealand that differed in human land-use impacts, ranging from pristine to urban. From this, we identified 504 putative virus species, of which 97 per cent were novel. Many of the novel viruses were highly divergent and likely included a new subfamily within the Parvoviridae. We identified at least sixty-three virus species that may infect vertebrates—most likely fish and water birds—from the Astroviridae, Birnaviridae, Parvoviridae, and Picornaviridae. No putative human viruses were detected. Importantly, we observed differences in the composition of viral communities at sites impacted by human land use (farming and urban) compared to native forest sites (pristine). At the viral species level, the urban sites had higher diversity (327 virus species) than the farming (n = 150) and pristine sites (n = 119), and more viruses were shared between the urban and farming sites (n = 76) than between the pristine and farming or urban sites (n = 24). The two farming sites had a lower viral abundance across all host types, while the pristine sites had a higher abundance of viruses associated with animals, plants, and fungi. We also identified viruses linked to agriculture and human impact at the river sampling sites in farming and urban areas that were not present at the native forest sites. Although based on a small sample size, our study suggests that human land use can impact viral communities in rivers, such that further work is needed to reduce the impact of intensive farming and urbanisation on water systems.

Molecular evidence for homologous strains of infectious spleen and kidney necrosis virus (ISKNV) genotype I infecting inland freshwater cultured Asian sea bass (Lates calcarifer) in Thailand

Infectious spleen and kidney necrosis virus (ISKNV) is a fish-pathogenic virus belonging to the genus Megalocytivirus of the family Iridoviridae. In 2018, disease occurrences (40-50% cumulative mortality) associated with ISKNV infection were reported in grown-out Asian sea bass (Lates calcarifer) cultured in an inland freshwater system in Thailand. Clinical samples were collected from seven distinct farms located in the eastern and central regions of Thailand. The moribund fish showed various abnormal signs, including lethargy, pale gills, darkened body, and skin hemorrhage, while hypertrophied basophilic cells were observed microscopically in gill, liver, and kidney tissue. ISKNV infection was confirmed on six out of seven farms using virus-specific semi-nested PCR. The MCP and ATPase genes showed 100% sequence identity among the virus isolates, and the virus was found to belong to the ISKNV genotype I clade. Koch's postulates were later confirmed by challenge assay, and the mortality of the experimentally infected fish at 21 days post-challenge was 50-90%, depending on the challenge dose. The complete genome of two ISKNV isolates, namely KU1 and KU2, was recovered directly from the infected specimens using a shotgun metagenomics approach. The genome length of ISKNV KU1 and KU2 was 111,487 and 111,610 bp, respectively. In comparison to closely related ISKNV strains, KU1 and KU2 contained nine unique genes, including a caspase-recruitment-domain-containing protein that is potentially involved in inhibition of apoptosis. Collectively, this study indicated that inland cultured Asian sea bass are infected by homologous ISKNV strains. This indicates that ISKNV genotype I should be prioritized for future vaccine research.

Scale Drop Disease Virus (SDDV) and Lates calcarifer Herpes Virus (LCHV) Coinfection Downregulate Immune-Relevant Pathways and Cause Splenic and Kidney Necrosis in Barramundi Under Commercial Farming Conditions

Marine farming of barramundi (Lates calcarifer) in Southeast Asia is currently severely affected by viral diseases. To better understand the biological implications and gene expression response of barramundi in commercial farming conditions during a disease outbreak, the presence of pathogens, comparative RNAseq, and histopathology targeting multiple organs of clinically “sick” and “healthy” juveniles were investigated. Coinfection of scale drop disease virus (SDDV) and L. calcarifer herpes virus (LCHV) were detected in all sampled fish, with higher SDDV viral loads in sick than in healthy fish. Histopathology showed that livers in sick fish often had moderate to severe abnormal fat accumulation (hepatic lipidosis), whereas the predominant pathology in the kidneys shows moderate to severe inflammation and glomerular necrosis. The spleen was the most severely affected organ, with sick fish presenting severe multifocal and coalescing necrosis. Principal component analysis (PC1 and PC2) explained 70.3% of the observed variance and strongly associated the above histopathological findings with SDDV loads and with the sick phenotypes, supporting a primary diagnosis of the fish being impacted by scale drop disease (SDD). Extracted RNA from kidney and spleen of the sick fish were also severely degraded likely due to severe inflammation and tissue necrosis, indicating failure of these organs in advanced stages of SDD. RNAseq of sick vs. healthy barramundi identified 2,810 and 556 differentially expressed genes (DEGs) in the liver and muscle, respectively. Eleven significantly enriched pathways (e.g., phagosome, cytokine-cytokine-receptor interaction, ECM-receptor interaction, neuroactive ligand-receptor interaction, calcium signaling, MAPK, CAMs, etc.) and gene families (e.g., tool-like receptor, TNF, lectin, complement, interleukin, chemokine, MHC, B and T cells, CD molecules, etc.) relevant to homeostasis and innate and adaptive immunity were mostly downregulated in sick fish. These DEGs and pathways, also previously identified in L. calcarifer as general immune responses to other pathogens and environmental stressors, suggest a failure of the clinically sick fish to cope and overcome the systemic inflammatory responses and tissue degeneration caused by SDD.

Draft genome sequence of scale drop disease virus (SDDV) retrieved from metagenomic investigation of infected barramundi, Lates calcarifer (Bloch, 1790)

Scale drop disease virus (SDDV) is a novel viral pathogen considered to be distributed in farmed barramundi (Lates calcarifer) in South-East Asia. Despite the severity of the disease, only limited genomic information related to SDDV is available. In this study, samples of SDDV-infected fish collected in 2019 were used. The microbiome of brain tissue was investigated using Illumina HiSeq DNA sequencing. Taxonomic analysis showed that SDDV was the main pathogen contained in the affected barramundi. De novo metagenome assembly recovered the SDDV genome, named isolate TH2019, 131 kb in length, and comprised of 135 ORFs. Comparison between this genome and the Singaporean SDDV reference genome revealed that the nucleotide identity within the aligned region was 99.97%. Missense, frameshift, insertion and deletion mutations were identified in 26 ORFs. Deletion of four deduced amino acid sequence in ORF_030L, identical to the SDDV isolate previously identified in Thailand, would be a potential biomarker for future strain classification. Interestingly, the genome of SDDV TH2019 harboured a unique 7,695-bp-long genomic region containing six hypothetical protein-encoded genes. Collectively, this study demonstrated that the SDDV genome can be sequenced directly, although with limited coverage depth, using metagenomic analysis of barramundi sample with severe infection.

Development of a SYBR Green quantitative PCR assay for detection of Lates calcarifer herpesvirus (LCHV) in farmed barramundi

Lates calcarifer herpes virus (LCHV) is a novel virus of farmed barramundi in Southeast Asia. However, a rapid detection method is yet to be available for LCHV. This study, therefore, aimed to develop a rapid quantitative PCR (qPCR) detection method for LCHV and made it timely available to public for disease diagnostics and surveillance in barramundi farming countries. A newly designed primer set targeting a 93-bp fragment of the LCHV putative major envelope protein encoding gene (MEP) was used for developing and optimizing a SYBR Green based qPCR assay. The established protocol could detect as low as 10 viral copies per μl of DNA template in a reaction containing spiked host DNA. No cross-amplification with genomic DNA extracted from host as well as common aquatic pathogens (12 bacteria and 4 viruses) were observed. Validation test of the method with clinical samples revealed that the virus was detected in multiple organs of the clinically sick fish but not in the healthy fish. We thus recommend that barramundi farming countries should promptly initiate active surveillance for LCHV in order to understand their circulation for preventing possibly negative impact to the industry.