Journal of fish diseases最新文献

Thirty lactic acid bacteria (LAB) isolates were obtained from the digestive tracts of whiteleg shrimp (Penaeus vannamei) and screened for antagonistic activity against Vibrio parahaemolyticus, the causative agent of Acute Hepatopancreatic Necrosis Disease (AHPND). Among these, four isolates demonstrated strong inhibitory effects, producing inhibition zones of 16-20 mm in agar well diffusion assays. These four LAB isolates were reported to cause no shrimp mortality during the feeding trials. Notably, shrimp fed with the isolate CG02 for 4 weeks showed a significantly improved survival (p < 0.05) after the immersion challenge with V. parahaemolyticus strain pVPA3-1. Based on 16S rDNA sequencing, the isolate CG02 was identified as Lactobacillus plantarum. To improve its viability, the isolate CG02 was encapsulated into spray-dried microcapsules. The resulting probiotic powder exhibited high cell viability, with counts of 9 ± 1.45 × 10⁸ CFU g^-1 after 4 months at room temperature, and 4.11 ± 0.38 × 10⁸ CFU g^-1 after 8 months at 4°C. When incorporated into shrimp feed, the CG02 powder did not cause any shrimp mortality. Moreover, it provided a relative percent survival (RPS) of 34.78% compared to the control group following the immersion challenge with V. parahaemolyticus. These results highlight the potential of spray-dried L. plantarum CG02 as a probiotic feed additive for enhancing shrimp health and supporting sustainable aquaculture practices.

Red-spotted masu trout (Oncorhynchus masou ishikawae; hereinafter, referred as amago trout), is a freshwater salmonid fish species endemic to Japan. From September to October 2023, chronic mortality was observed on an amago trout farm in eastern Japan. To identify the cause of death, bacterial isolation, predominant bacterial species identification, histopathology and experimental challenge test were performed. As a result, bacterial colonies with characteristic features were isolated from all moribund fish. The 16S rRNA and rpoB gene sequences of the isolates showed high similarity with those of Bacillus sp. Identical 16S rRNA and rpoB gene sequences were also detected in moribund fish tissues. The histopathology revealed bacteria in multiple organs, including the spleen, in five of six fish examined. In the experimental challenge test, the fish that were infected with a high dose of bacteria (1.26E+07 cfu per fish) started to die at 3 days post-infection (dpi) and all fish died by 7 dpi. The cumulative mortality rate was lower in the group infected with a lower number of bacteria (1.26E+06 cfu per fish). Our findings identify Bacillus sp. as the disease agent. This is the first report of Bacillus infection in salmonids.

Largemouth bass ranavirus (LMBV) poses a significant threat to the aquaculture industry by causing severe systemic disease and substantial economic losses. This study investigates the tissue distribution, pathological damage, and host response following LMBV-GD1909 infection using two infection models: experimental injection and simulated natural immersion. In the injection model, infected fish exhibited clinical signs, including skin ulcers, haemorrhaging at fin bases, body darkening and organ swelling. Histopathological analysis revealed extensive multi-organ damage, particularly in the liver, spleen, and kidney, characterised by necrosis, haemorrhage, vacuolar degeneration and inflammatory infiltration. Haematological and serum biochemical analyses indicated significant alterations in key parameters, including elevated WBC, RBC and GLU levels, and reduced PLT, TP and LDH levels, reflecting systemic physiological disruption. Viral dynamics analysis demonstrated the pantropic nature of LMBV, with rapid dissemination to multiple tissues post-injection. The liver and spleen were identified as primary target organs, showing the highest viral loads and most severe pathology, indicating a direct correlation between viral load and organ dysfunction. In the immersion model, simulating natural infection, the gills and skin displayed the highest viral loads initially, identifying them as the major entry portals. The gills also served as a key site for viral replication, with a clear gradient of viral spread from external to internal tissues observed over time. This study comprehensively elucidates the tissue tropism, dissemination pathways, and pathological impact of LMBV, providing critical insights for developing targeted prevention and control strategies against LMBV.

While nephrocalcinosis has received considerable attention in Atlantic salmon aquaculture, mineralisation of the urinary bladder remains largely overlooked. Obstruction of the urinary bladder, well recognised in terrestrials as a potentially fatal condition, is often missed in fish due to its exclusion from routine histological assessments. An increasing number of field cases, colloquially referred to as 'urinary plugs', are now being observed, frequently occurring without or with only slight, nephron mineralisation. This study describes histopathological changes associated with bladder mineral deposits, urocystolithiasis, in farmed Atlantic salmon, including obstruction, urothelial degeneration and intra-renal lesions. The findings highlight urocystolithiasis as an underdiagnosed condition with potential fish health and welfare implications. Functional and histopathological studies targeting the entire urinary system are warranted to elucidate its pathophysiological nature and to support the development of preventative and therapeutic approaches.

Piscirickettsia salmonis , the causal agent of salmonid rickettsial septicemia (SRS), is the main pathogen affecting farmed salmonids in Chile. Outbreaks of SRS lead to substantial economic losses for producers. Many determinants related to SRS outcomes are still poorly understood. Here, we conducted a retrospective cohort study from 2014 to 2021 to investigate the epidemiology of SRS at the farm level in southern Chile, employing historical monitoring data. Using time series analysis of weekly SRS mortality risk and sea lice (Caligus rogercresseyi ) infestation levels, we found that SRS mortality risk had a strong seasonal component, with mortalities being significantly higher in the warmer seasons. While Caligus infestation levels have increased significantly over the years, SRS mortality risk has remained constant. Using mixed effects regression models, we identified that a key predictor for both increased weekly SRS mortality risk and higher hazard of reporting the first SRS outbreak of a production cycle was the level of female egg-laying sea lice. We hypothesise that the interaction between sea lice, P. salmonis and rising water temperatures may produce synergistic stress on salmon that accelerates disease progression and prompts overuse of antimicrobials. This calls for an urgent integrated pest management approach in aquaculture practice.

A spleen-derived cell line was established from the spleen of hybrid snakehead (♀Channa argus × ♂Channa maculata) (abbreviated as CAMsp), a species of considerable economic importance in China's freshwater aquaculture, which is severely impacted by the hybrid snakehead rhabdovirus (HSHRV), largemouth bass ranavirus (LMBV), and infectious spleen and kidney necrosis virus (ISKNV). The establishment of a stable in vitro culture system is imperative for the effective isolation, identification, and study of fish viruses. CAMsp, generated through trypsin digestion, had successfully undergone over 80 passages since its initial culture. This cell line exhibited rapid proliferation in Leibovitz's-15 medium (L-15) supplemented with 10% fetal bovine serum at 28°C, achieving monolayer formation within 24 h at a passage ratio of 1:3. Chromosomal analysis of CAMsp at the 60th passage identified a chromosome count of 42; the chromosome number in hybrid snakehead somatic cells is 44, 45, or 46, suggesting chromosomal alterations. Inoculation of CAMsp monolayers with HSHRV, LMBV, and ISKNV resulted in characteristic cytopathic effects (CPE), including cell rounding, aggregation, and eventual detachment. Transmission electron microscopy (TEM) confirmed viral replication and revealed extensive cytopathological changes within the infected cells, demonstrating the susceptibility of the CAMsp cell line to all three viruses. Viral titers, determined by TCID₅₀ assay at 7 days post-infection (dpi), reached 10^9.25 ± 10^0.36 TCID₅₀/mL for LMBV, 10⁶·¹⁵ ± 10^0.25 TCID₅₀/mL for ISKNV, and 10⁸·³³ ± 10^0.12 TCID₅₀/mL for HSHRV, indicating efficient viral propagation in this cell line. The CAMsp cell line serves as a valuable model for studying certain fish viruses, virus-host interactions, and disease prevention strategies.

Tilapia parvovirus (TiPV) is an emerging pathogen associated with high mortality rates in farmed tilapia, highlighting the urgent need for rapid and accurate diagnostic tools. In this study, we established an RPA-CRISPR/Cas12a detection system targeting the TiPV NS1 gene. The assay conditions were systematically optimised, including 15-min RPA amplification at 39°C, with reagent concentrations of 200 nM Cas12a, 250 nM crRNA and 200 nM ssDNA reporter. Specificity tests showed no cross-reactivity with other tilapia pathogens (TiLV, S. agalactiae) and other aquatic pathogens (LMBRaV, YcCV, GCRV II, WSSV, CyHV-2, SVCV). Sensitivity evaluation revealed a limit of detection (LoD) of 1.97 × 10¹copies/μL, which was 100-fold more sensitive than PCR (1.97 × 10³copies/μL). Clinical validation with 20 tilapia samples demonstrated a 50% positive detection rate for RPA-CRISPR/Cas12a, 15% higher than PCR (35%). This integrated method combines the advantages of RPA and CRISPR-based signal transduction, offering a field-applicable solution for TiPV monitoring in resource-limited aquaculture environments.

The formation of extracellular traps (ETosis) is an innate immune mechanism in shrimp against pathogens. Microorganisms are entrapped in extruded DNA fibres co-localised with antimicrobial peptides and eventually killed. However, as a cell death mechanism, its strict regulation is essential as excessive formation of ETs may cause detrimental effects to the host by contributing to disease pathophysiology. Here, we investigated the ability of freeze-dried Lactiplantibacillus plantarum (FD-LAB), previously reported to enhance shrimp immunity against pathogenic infections, to regulate ETosis. In an ex vivo setup, gill cells and circulating hemocytes were pre-exposed to FD-LAB and were then stimulated with Vibrio parahaemolyticus. Results showed that V. parahaemolyticus alone induced ETosis in both gill cells and circulating hemocytes, while FD-LAB alone did not. However, when cells were pre-exposed to FD-LAB prior to stimulation with V. parahaemolyticus, no ETosis occurred. Similarly, changes in reactive oxygen species (ROS) production coincided with the formation of ETs, thus signifying that FD-LAB may regulate ETosis in shrimp gill cells and circulating hemocytes possibly by dampening ROS production. These results present a novel means to regulate ETosis and indicate that FD-LAB may enhance shrimp immunity while also acting on immune regulation.

Tilapia Lake Virus (TiLV) is a significant threat to global tilapia aquaculture, highlighting the need for rapid and accurate diagnostic methods to manage outbreaks and minimise economic losses. This study presents the development and partial validation of a one-pot assay integrating RT-LAMP with the CRISPR/Cas12b system for sensitive and specific TiLV detection. This assay amplifies viral RNA using RT-LAMP, while CRISPR/Cas12b enables a real-time detectable signal. Targeting a conserved region in TiLV segment four, the assay achieves results within 75 min at 62°C, with easy visualisation using a portable fluorescence viewer. It demonstrated high sensitivity, with a 95% limit of detection of 79.6 copies (95% CI: 48-132 copies), and high specificity, with no cross-reaction to other fish RNA or DNA viruses. Based on a validation panel of 261 samples from 9 source populations, the assay exhibited 92% diagnostic sensitivity (95% CI: 87%-96%) and 100% diagnostic specificity (95% CI: 97%-100%). When assessed as a non-lethal sample, gills provided a reliable and less invasive alternative despite lower viral loads compared to internal organs. Therefore, this partially validated one-pot assay is potentially practical for enhancing TiLV detection and disease management in aquaculture systems, especially in field settings and resource-limited laboratories.