Infectious salmon anaemia virus (ISAV) can cause disease and mortality in Atlantic salmon (Salmo salar). The virus is endemic in several regions including the Atlantic coast of Canada. Lumpfish (Cyclopterus lumpus) are increasingly utilised as cleaner fish to control sea lice levels in Atlantic salmon farms. With welfare concerns for both species, and the possibility of escaped fish, the objective of this study was to determine the susceptibility of lumpfish to ISAV. Lumpfish were exposed to two ISAV isolates (high and low virulence) through a cohabitation challenge (i.e., exposure to infected salmon) and through an intra-peritoneal challenge. Naïve salmon were exposed simultaneously to the same isolates tested to verify their virulence. In the cohabitation challenge, ISAV infection progressed as expected in the naïve salmon. ISAV was detected in a small number of lumpfish gill samples, but the lack of detection in other organs, e.g., heart, kidney, spleen, liver, and blood, suggests environmental contamination. Although two lumpfish blood samples tested positive in the later stages of the challenge, all other disease indicators, including a lack of anaemia, did not support potential for virus replication in lumpfish. Based on both exposure experiments, lumpfish do not appear to be susceptible to ISAV.

The objective of this study was to evaluate welfare and health effects following single and repeated non-lethal blood sampling from the caudal vasculature of Atlantic salmon. Two experiments were conducted at three different temperatures: a 6-week freshwater experiment with fish weighing 50-100 g, undergoing up to four blood withdrawals, and a 4-week seawater experiment with fish weighing 300-450 g, undergoing up to three blood withdrawals. Observed clinical signs included dark pigmentation of the skin and vascular disturbances posterior to the puncture site. Histological evaluations revealed injuries to the vasculature, vertebral column, nervous tissue and skeletal muscle. At 12°C and 15°C, fish displayed strong capacity for tissue regeneration and haematological recovery, but also an elevated risk of acute mortality. At 5°C, acute mortality was minimal, but recovery time was approximately twice as long compared with higher temperatures. Mortality rates and clinical findings decreased with increasing fish weight. These findings demonstrate that non-lethal caudal blood sampling poses a welfare risk in salmon weighing 450 g or less. The risk factors identified here may inform future risk assessments and support the development of welfare-compatible guidelines for non-lethal blood sampling in Atlantic salmon.

Flavobacterium spp. are the etiological agents of columnaris disease. Although there is evidence that columnaris poses a significant threat to the farming of tambaqui (Colossoma macropomum), there remains a large gap in understanding the genetic and phenotypic diversities of columnaris-causing bacteria (CCB). This study was designed to identify diversity traits of CCB from tambaqui reared in Brazil. From 26 CCB outbreaks previously identified as F. columnare with MALDI-TOF were prospected. A multiplex PCR was conducted to confirm the bacterial species, while REP-PCR was used to determine the differences in genetic patterns (GP), and putative virulence genes were accessed using PCR (norB, trx and gtf). All isolates were identified as F. oreochromis by multiplex PCR. REP-PCR identified 4 genetic variants (GP1, GP2, GP3 and GP4), with GP3 being the most frequent. All the putative virulence genes were detected in all analysed strains. One strain representing each GP was subjected to the description of phenotypic traits (growth patterns, morphology and pathogenicity). The pathogenicity and patterns of virulence were assessed and achieved for all GP, and AMFO14 (GP3) displayed the highest frequency. Based on recent taxonomic advances on Flavobacterium spp. from freshwater fish worldwide, this study provides unprecedented insights regarding CCB retrieved from tambaqui.

Edwardsiella anguillarum is a gram-negative bacterium, synonymous with previously described atypical, fish-pathogenic Edwardsiella tarda. Originally described from eels in 2015, E. anguillarum is an important global fish pathogen, particularly in tilapia. This study describes intraspecific phenotypic and genotypic variability among 17 E. anguillarum isolates from non-anguillid fish hosts and varied geographic origins. Isolates demonstrated similar biochemical characteristics, with slight variation in motility and hydrogen sulfide production. Genomic relatedness among isolates was analysed with repetitive extragenic palindromic sequence-based PCR (rep-PCR) and multilocus sequence analysis (MLSA). Rep-PCR with the ERIC II primer set revealed two distinct genetic clusters, while amplification strategies utilising the GTG5, BOX or ERIC I&II primer sets yielded more uniform profiles. While rep-PCR deemed the isolates largely clonal, MLSA schemes using reference genes from published Edwardsiella MLSA studies revealed E. anguillarum isolates formed five discrete phylogroups. A unique, ~91.5 kB plasmid was identified in Costa Rican and Colombian isolates, associated with conjugative and transposable elements, plasmid mobilisation, and adhesion; however, no plasmid mediated antibiotic resistance genes were identified. This study provides insight into genetic diversity among E. anguillarum isolates from different hosts and geographic regions, identifying an optimal MLSA scheme from previous reports applicable to E. anguillarum isolates.

Water temperature is a critical environmental factor influencing the severity, replication and transmission of infectious diseases in aquaculture. However, its role in the pathogenesis and shedding dynamics of tilapia lake virus (TiLV), which is an emerging viral pathogen threatening global tilapia production, remains to be fully characterised. We therefore evaluated the temperature-dependent effects of TiLV infection in Nile tilapia (Oreochromis niloticus) by simulating direct and indirect cohabitation models. The fish were challenged with TiLV and maintained at 24°C, 28°C or 32°C. The highest mortality and viral loads in water and mucus occurred at 28°C, while outcomes at 32°C showed lower mortality, indicating a non-monotonic temperature response. In contrast, infection at 24°C resulted in delayed infection onset, lower viral shedding and minimal mortality among the cohabitant fish. Notably, the indirect cohabitation system consistently showed reduced transmission and lower mortality among the cohabitant fish. The TiLV concentrations in the water and mucus peaked prior to the highest mortality events in both cohabitation experiments, which supports their applicability as noninvasive early warning indicators before outbreaks occur. Our findings demonstrate that tilapia mortality and shedding dynamics of TiLV are significantly influenced by fixed water temperature. These results provide a baseline for understanding how fixed thermal regimes impact TiLV outbreaks and highlight the importance of environmental monitoring in control strategies.

Chemokines are essential for vertebrate immune regulation, and teleosts possess diverse CXCL8 lineages. However, studies on CXCL8 in C. auratus remain limited. In the present study, four CXCL8 homologues (CaCXCL8-1 to -4) were successfully cloned and identified from C. auratus, and their expression patterns were analysed in the tissues of healthy and those naturally infected with H. doneci. Bioinformatic analyses revealed that all homologues possess the conserved structural features of CXC chemokines. However, the divergence was observed in the N-terminal ELR motif: CaCXCL8-1 and CaCXCL8-2 harboured a QLR motif, whereas CaCXCL8-3 and CaCXCL8-4 possessed a DPR motif. Phylogenetic analysis classified CaCXCL8-1 and CaCXCL8-2 into the CXCL8_L2 lineage, and CaCXCL8-3 and CaCXCL8-4 into the teleost-specific CXCL8_L1b lineage. Tertiary structure predictions indicated that CaCXCL8-1, -3, and -4 conserve key receptor-binding sites, whereas CaCXCL8-2 retained only GAG-binding sites, suggesting potential functional divergence. Under healthy conditions, the four homologues were all constitutively expressed but displayed divergent basal profiles. Following H. doneci infection, they exhibited distinct tissue-specific expression patterns: CaCXCL8-1 was significantly upregulated in the gills; CaCXCL8-2 was specifically induced in the spleen; CaCXCL8-3 was markedly upregulated in both the gills and head kidney; and CaCXCL8-4 was upregulated in the liver and head kidney but downregulated in the spleen. These divergent expression patterns suggest functional specialisation among the CaCXCL8 homologues, with different paralogs potentially coordinating localised versus systemic immune responses. Our findings provide evidence for the functional diversification of the CXCL8 chemokine in teleosts and contribute to understanding the evolution of complex immune systems in vertebrates.

Streptococcus agalactiae is a multi-host pathogen and a major threat to aquaculture, particularly tilapia production. This study characterised the serotype distribution and virulence gene profiles of 97 S. agalactiae isolates obtained from non-tilapia freshwater species in Vietnam, including carp species not previously recognised as hosts, namely black carp, grass carp and common carp. Three serotypes were identified among non-tilapia isolates, Ia (44.3%), III (39.2%) and Ib (16.5%). Virulence gene distributions varied substantially within and between serotypes; however, all isolates consistently carried the fbsA and cfb genes. Distinct serotype-associated gene patterns were observed. Serotype Ia isolates possessed fbsA, fbsB, bca, hylB and cfb but lacked lmb and spb1-F, whereas serotype III isolates harboured pavA, cylE, fbsA, lmb, scpB, cfb and cspA but lacked bac. Serotype Ib isolates carried pavA, cylE, fbsA, hylB, cfb and cspA, while spb1-F and bac were absent. Overall, this study identifies additional non-tilapia hosts of S. agalactiae in freshwater aquaculture provides the first evidence of infection in black carp, grass carp and common carp. These findings demonstrate the diversity of circulating serotypes and virulence gene profiles. These findings indicate increasing epidemiological complexity and highlight the need for integrated prevention and disease management strategies targeting this pathogen.

Infectious pancreatic necrosis virus (IPNV) can cause devastating disease in Atlantic salmon (Salmon salar). IPNV has a broad host range and may threaten other aquaculture species. Understanding interspecies transmission of IPNV is crucial for protecting the aquaculture industry. With the expansion of fish farming (in Norway), it is important to assess whether a pathogencan transmit from one fish species to another and cause disease. We investigated whether IPNV-infected Atlantic cod can shed IPNV, leading to infection in other fish important to Norwegian aquaculture: halibut, salmon and lumpfish, using the cohabitation experimental trial method. Virus shedding, transmission, fish mortality and pathology were assessed. We documented virus shedding in water and mortality in IPNV-injected Atlantic cod. No mortality was observed in the cohabitated fish species during the experimental period. We confirmed lesions consistent with IPN by histopathology and immunohistochemistry in IPNV-injected Atlantic cod and in IPNV-PCR positive cohabitant Atlantic halibut. Cohabitant Atlantic cod, Atlantic salmon, Atlantic halibut and lumpfish were also found positive for IPNV by PCR, suggesting that IPNV-infected Atlantic cod can transfer infection to other farmed fish species. These findings highlight the potential risk of pathogen spread among farmed fish species and demonstrate the importance of understanding infectious fish disease epidemiology.

Adult Chinook salmon (Oncorhynchus tshawytscha) trap-and-haul operations exist in rivers across the Pacific Northwest with the objective of re-establishing salmon populations above impoundments. These efforts are complicated by high prespawn mortality (PSM), which can hinder restoration by reducing reproductive success. Recent improvements at trapping facilities in the Willamette River Basin, Oregon, USA, may reduce PSM by streamlining trap-and-haul operations to reduce handling stress. However, studies suggest that PSM is also related to factors associated with haul operations, including fish density and exposure to external stressors such as pathogens. Environmental DNA (eDNA) can help detect pathogens in confined systems with high fish-to-water ratios. In this work, pathogen eDNA samples were collected from anaesthesia and transport tank waters at a trap-and-haul facility in 2017 and 2021 and analysed to assess pathogen presence. Pathogen DNA was detected in both tanks prior to fish addition, including Aeromonas salmonicida, Renibacterium salmoninarum, Salmincola californiensis, and Ceratonova shasta. After fish entered the anaesthesia tank, average pathogen concentrations increased in magnitude for A. salmonicida (73%), S. californiensis (3165%), C. shasta (1619%), and decreased for R. salmoninarum (-57%). Results suggest shedding of horizontally transmitted pathogens, particularly A. salmonicida, during transport.

The PSMB8 gene belongs to the immunoproteasome β-subunit (PSMB) family and has been implicated in diverse physiological processes in mammals, including tumorigenesis, autophagy and apoptosis. However, its role in antiviral immunity in groupers remains largely unexplored. In this study, we cloned and characterised EcPSMB8, a homologue of PSMB8, from the orange-spotted grouper (Epinephelus coioides), and investigated its potential involvement in antiviral immune responses. EcPSMB8 encodes a conserved 275-amino-acid protein that is ubiquitously expressed in grouper tissues and predominantly localised in the cytoplasm. Functional analyses showed that EcPSMB8 overexpression was associated with reduced replication of both Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV). This antiviral effect was accompanied by suppression of basal and virus-induced autophagy, as indicated by decreased LC3-II accumulation and reduced expression of autophagy-related genes. Meanwhile, EcPSMB8 overexpression correlated with enhanced major histocompatibility complex class I (MHC I)-associated immune signalling, including increased promoter activities of MHC I-A, ISRE and IFN3, as well as upregulation of related cytokines. Together, these findings suggest that EcPSMB8 may contribute to grouper antiviral defence through coordinated modulation of immune signalling and autophagy. Importantly, a specific polyclonal antibody against EcPSMB8 was generated, providing a useful tool for protein-level analyses and supporting future studies on antiviral immune mechanisms in teleost fish.