Fish Iridoviridae: infection, vaccination and immune response

Chitosan-selenium nanoparticles suppress infectious spleen and kidney necrosis virus through immune modulation and viral replication inhibition in zebrafish (Danio rerio)

Infectious spleen and kidney necrosis virus (ISKNV) is one of the most challenging diseases causing substantial financial losses in global aquaculture. However, effective strategies for controlling ISKNV infections in ornamental fish are lacking. In this study, chitosan-selenium nanoparticles (CTS-SeNPs) were synthesized and evaluated as potential strategies for controlling ISKNV infection. The characterization of CTS-SeNPs confirmed the conjugation of chitosan with SeNPs, resulting in the formation of spherical particles with an average size of 51.73 nm and exhibiting moderate stability. Furthermore, the morphology and stability were retained for 24 h in the freshwater environment. The CTS-SeNPs exhibited viral suppression efficacy at 40 μg/mL, downregulating major capsid protein gene expression in ISKNV-infected dwarf gourami fin (DGF) cells. The inhibition rates at 48 and 72 hpi in the DGF cells were 94.02 ± 0.39 % and 91.82 ± 3.84 %, respectively. During the ISKNV replication cycle, CTS-SeNPs were found to affect the attachment and replication stage. In zebrafish, the viral suppression efficacy of CTS-SeNPs was evaluated by two different administration methods: intraperitoneal injection (IP) and immersion administration (IM). At safe concentrations (1 μg/mL for IP and 40 μg/mL for IM), CTS-SeNPs demonstrated improved survival rates of 53.33 ± 9.42 % (IP) and 50.00 ± 8.16 % (IM). Additionally, viral loads in survivors were lower than those in ISKNV-infected fish. In terms of immune gene expression, CTS-SeNPs upregulated interferon-inducible genes. Moreover, after ISKNV infection following CTS-SeNPs administration, type I interferon induction (interferon φ2 and interferon φ3) was identified, suggesting that CTS-SeNPs enhance viral suppression through immune modulation, indicating their potential as effective anti-ISKNV agents.

Finfish infectious diseases in the Mediterranean basin: A systematic review with insights on vaccination possibilities

Official data on the distribution, prevalence and impact of infectious diseases in Mediterranean finfish aquaculture are lacking. The absence of data on diseases has an undeniable negative impact on the efficacy of control strategies. Therefore, the aim of our systematic review was to gather data from the available literature referring to years 2010-2024 to identify the most important diseases affecting finfish of the Mediterranean Sea. Out of 9021 articles screened, 84 papers were selected, quality assessed and summarized. The most frequently reported viral diseases were caused by Betanodaviridae followed by Iridoviridae, which are responsible for Infectious Spleen and Kidney Necrosis disease (ISKN) and Lymphocystis diseases (LCVD), respectively. Amongst bacterial pathogens, Vibrio alginolyticus, Mycobacterium marinum and Vibrio harveyi ranked at the top three with Aeromonas hydrophila, Photobacterium damselae piscicida and Vibrio vulnificus closely following. For parasitic agents, Amiloodinium ocellatum and Lernanthropus kroyeri were most reported followed by Cryptocarion irritans, Enterospora nucleophila, Ceratothoa ostreoides, Ergasilus sieboldi and Livoneca redmanii (equal merit). Many authors highlighted the increasing impact of climate change on disease outbreaks and parasite life cycles. This review also revealed significant discrepancies in the number and quality of reports from different parts of the Mediterranean, with clear evidences of underreporting in several countries, likely exacerbated by the limited availability of trained diagnostic laboratories. Consequences of diseases epidemiology on the efficacy of vaccination is also discussed. Finally, the obtained data highlight the urgent need to standardize the reporting of aquaculture-based epizootics.

Multiple Horizontal Transfers of Immune Genes Between Distantly Related Teleost Fishes

Horizontal gene transfer (HGT) is less frequent in eukaryotes than in prokaryotes, yet can have strong functional implications and was proposed as a causal factor for major adaptations in several eukaryotic lineages. Most cases of eukaryote HGT reported to date are inter-domain transfers, and few studies have investigated eukaryote-to-eukaryote HGTs. Here, we performed a large-scale survey of HGT among 242 species of ray-finned fishes. We found multiple lines of evidence supporting 19 teleost-to-teleost HGT events that involve 17 different genes in 11 teleost fish orders. The genes involved in these transfers show lower synonymous divergence than expected under vertical transmission, their phylogeny is inconsistent with that of teleost fishes, and they occur at non-syntenic positions in donor and recipient lineages. The distribution of HGT events in the teleost tree is heterogenous, with 8 of the 19 transfers occurring between the same two orders (Osmeriformes and Clupeiformes). Though we favor a scenario involving multiple HGT events, future work should evaluate whether hybridization between species belonging to different teleost orders may generate HGT-like patterns. Besides the previously reported transfer of an antifreeze protein, most transferred genes play roles in immunity or are pore-forming proteins, suggesting that such genes may be more likely than others to confer a strong selective advantage to the recipient species. Overall, our work shows that teleost-to-teleost HGT has occurred on multiple occasions, and it will be worth further quantifying these transfers and evaluating their impact on teleost evolution as more genomes are sequenced.

The Protective Effects and Immunological Responses Induced by a Carboxymethyl Cellulose Microcapsule-Coated Inactivated Vaccine Against Largemouth Bass Ranavirus (LMBRaV) in Largemouth Bass (Micropterus salmoides)

BACKGROUND

Epizootics of largemouth bass ranavirus (LMBRaV) in largemouth bass (Micropterus salmoides) populations are associated with elevated mortality and significant financial losses. Given the lack of effective and safe medication to treat this disease, oral vaccination, which directly targets the intestinal mucosal immune system, is crucial for disease resistance.

METHODS

This study utilized carboxymethyl cellulose (CMC) to coat LMBRaV inactivated vaccine (LIV) (micro-CMC@LIV). The morphology and characteristics of the CMC microcapsules were determined. In vitro simulated gastric and intestinal conditions were used to validate that the microcapsules could tolerate gastric conditions and subsequently release their contents in the intestinal tract. This was confirmed using CMC-coated coumarin 6 (C6) fluorescence microcapsules.

RESULTS

After the oral administration of micro-CMC@LIV, the detection of LMBRaV major capsid protein confirmed effective antigen release and absorption in the midgut and hindgut. Neutralizing antibody titers were significantly higher (1:81.71) in the micro-CMC@LIV group compared to the uncoated vaccine group (1:21.69). The expression of genes linked to the innate and adaptive immune systems was upregulated post-micro-CMC@LIV treatment. Following the LMBRaV challenge, the micro-CMC@LIV group exhibited a relative percent survival (RPS) of 82.14%, significantly higher than the uncoated vaccine group (61.61%). Droplet digital PCR analysis revealed significantly lower viral loads in the liver, spleen, and head kidney of the micro-CMC@LIV group compared to the control group and the uncoated vaccine group.

CONCLUSIONS

These results collectively suggest that the CMC-coated LIV can be effectively delivered to the intestinal tract and induce robust antibody and immune responses, providing a reliable method for preventing and controlling LMBRaV disease in the largemouth bass industry.

Rapid, sensitive, and visual detection of mandarin fish ranavirus and infectious spleen and kidney necrosis virus using an RPA-CRISPR/Cas12a system

Iridoviruses are large cytoplasmic icosahedral viruses that contain dsDNA. Among them, mandarin fish ranavirus (MRV) and infectious spleen and kidney necrosis virus (ISKNV) are particularly notable due to their high contagiousness and pathogenicity. These viruses pose a significant threat to fish aquaculture, resulting in substantial annual economic losses for the fish farming industry. Therefore, the development of novel, rapid virus detection technologies is essential for the prevention and control of ISKNV and MRV diseases. In this study, we developed a rapid, sensitive, and visual detection method for MRV and ISKNV using the recombinase polymerase amplification (RPA)-CRISPR/Cas12a system. This method can detect as low as 1 copy/μL of MRV and 0.1 copy/μL of ISKNV, demonstrating excellent specificity and reproducibility. The detection can be performed at a constant temperature of 37-39°C, eliminating the need for complex equipment. A 30-min RPA amplification followed by a 15-min CRISPR/Cas reaction is sufficient for detecting most samples. For low-concentration samples, extending the CRISPR/Cas reaction time to 60 min improves result visibility. The designed RPA reaction system is capable of performing reverse transcription of RNA, allowing for the detection of mRNA transcribed from the MCP gene of MRV and ISKNV in the sample. Furthermore, two probes were identified that can be observed without the need for excitation light. In conclusion, a field-suitable detection method for ISKNV and MRV has been established, providing a powerful tool for the prompt diagnosis of these aquatic pathogens and aiding in the prevention and control of ISKNV and MRV diseases.

scTRIM44 Positively Regulated Siniperca Chuatsi Rhabdovirus Through RIG-I- and MDA5-Mediated Interferon Signaling

Tripartite Motif-Containing 44 (TRIM44) is responsible for cancers, neurodegenerative diseases, and viral infections. However, the role of Siniperca chuatsi TRIM44 (scTRIM44) during viral infection remains unclear. In the present study, we analyzed the molecular characteristics of scTRIM44 and its role in infectious spleen and kidney necrosis virus (ISKNV), largemouth bass virus (LMBV), and Siniperca chuatsi rhabdovirus (SCRV) infection. ScTRIM44 contained one B-box domain (B, 166-207 aa) and a coiled-coil domain (CC, 279-309 aa), but lacked the canonical RING domain of E3 ubiquitin ligases. The scTRIM44 mRNA was expressed relatively high in immune-related tissues. The mRNA expression of scTRIM44 significantly decreased in vivo and vitro post-ISKNV and -LMBV infection. However, the expression of scTRIM44 mRNA showed significant up-regulation post-SCRV infection. ScTRIM44 positively regulated SCRV infection in CPB cells, but copies of ISKNV and LMBV showed no significant alteration in over-expressed or knocked-down scTRIM44 cells. Moreover, scTRIM44 positively regulated RIG-I- and MDA5-mediated interferon molecule signaling. These data suggested that scTRIM44 promoted SCRV infection by positively regulating RIG-I- and MDA5-mediated interferon molecule signaling, but didn't regulate ISKNV and LMBV infection. This research provided a comprehensive insight into the antiviral activity of scTRIM44.

Isolation, Identification and Genomic Analysis of Orange-Spotted Grouper Iridovirus Hainan Strain in China

The orange-spotted grouper (Epinephelus coioides) is an important mariculture fish in China. However, in recent years, with the rapid development of aquaculture activities, outbreaks of viral diseases have affected the grouper aquaculture industry, causing severe economic losses. In the present study, we isolated and identified a virus from diseased, orange-spotted groupers from an aquaculture farm in Hainan Province, China. The isolated virus was identified as orange-spotted grouper iridovirus, hence named the orange-spotted grouper iridovirus Hainan strain (OSGIV-HN-2018-001). OSGIV-HN-2018-001 induces a cytopathic effect after the infection of mandarin fish (Siniperca chuatsi) brain clonal passage (SBC) cells. In addition, the cytoplasm of the OSGIV-HN-2018-001-infected SBC cells was found to contain a large number of hexagonal virus particles with a diameter of approximately 134 nm. Using the Illumina NovaSeq system, we assembled the sequence data and annotated the complete genome of OSGIV-HN-2018-001 (GenBank accession number: PP974677), which consisted of 110,699 bp and contained 122 open reading frames (ORFs). Phylogenetic tree analysis showed that OSGIV-HN-2018-001 was most closely related to ISKNV-ASB-23. The cumulative mortality rate of groupers infected with OSGIV-HN-2018-001 reached 100% on day 8. The spleens were enlarged and blackened after the dissection of the dying groupers. These results contribute to the understanding of the molecular regulatory mechanism of the iridovirus infection and provide a basis for iridovirus prevention.

Current Challenges of Vaccination in Fish Health Management

Vaccination is an essential method of immunological preventive care required for the health management of all animals, including fish. More particularly, immunization is necessary for in-land aquaculture to manage diseases in fish broodstocks and healthy seed production. According to the latest statistics in 2020, 90.3 million tons of capture fishery production was achieved from the aquaculture sector. Out of the above, 78.8 million tons were from marine water aquaculture sectors, and 11.5 million tons were from inland water aquaculture sectors. About a 4% decline in fish production was achieved in 2020 in comparison to 2018 from inland aquaculture sectors. On the other hand, the digestive protein content, healthy fats, and nutritional values of fish products are comparatively more affordable than in other meat sources. In 2014, about 10% of aquatic cultured animals were lost (costing global annual losses > USD 10 billion) due to infectious diseases. Therefore, vaccination in fish, especially in broodstocks, is one of the essential approaches to stop such losses in the aquaculture sector. Fish vaccines consist of whole-killed pathogens, protein subunits, recombinant proteins, DNA, or live-attenuated vaccines. Challenges persist in the adaption of vaccination in the aquaculture sector, the route of administration, the use of effective adjuvants, and, most importantly, the lack of effective results. The use of autogenous vaccines; vaccination via intramuscular, intraperitoneal, or oral routes; and, most importantly, adding vaccines in feed using top dressing methods or as a constituent in fish feed are now emerging. These methods will lower the risk of using antibiotics in cultured water by reducing environmental contamination.