Journal of fish diseases最新文献

Formalin-inactivated vaccines are widely employed as a primary preventive strategy against red sea bream iridoviral disease (RSIVD), which poses a substantial economic threat to the aquaculture of rock bream (Oplegnathus fasciatus). However, conventional quantitative PCR (qPCR) cannot differentiate infectious virions from noninfectious vaccine residues or viral debris, limiting accurate assessment of vaccine efficacy. Therefore, the present study aimed to evaluate the protective efficacy and viral shedding dynamics of a formalin-inactivated vaccine using propidium monoazide (PMAxx)-based viability qPCR (vqPCR). Vaccination demonstrated robust protection, achieving a relative percent survival of approximately 80% in immersion challenges and significantly reducing viral shedding into seawater. Notably, comparative analysis revealed that conventional qPCR significantly overestimated viral risk in vaccinated fish by detecting noninfectious DNA artifacts. Although vaccination did not confer sterilizing immunity, it suppressed infectious viral replication to sublethal levels, effectively preventing horizontal transmission to naïve cohabitants. Furthermore, disease progression and shedding kinetics were temperature-dependent, occurring more rapidly at 25°C than at 20°C. Overall, these findings highlight that vaccination induces functional sterilization of shedding and underscore the need to adopt vqPCR for accurate epidemiological risk assessment in aquaculture populations.

Nocardiosis is a chronic bacterial disease causing significant losses in marine aquaculture worldwide. This study reports the first confirmed outbreak of nocardiosis caused by Nocardia seriolae in farmed spotted scat (Scatophagus argus) in Vietnam, with cumulative mortalities reaching up to 70%. Diseased fish exhibited skin ulceration and disseminated white nodules in internal organs and musculature. Histopathological examination revealed typical granulomatous lesions containing abundant Gram-positive, branching filamentous bacteria. A bacterium with consistent morphological characteristics was isolated from affected organs, and molecular identification based on 16S rRNA gene sequencing and species-specific PCR confirmed the pathogen as N. seriolae. The experimental challenge reproduced the clinical signs and pathological features of natural infection, with an estimated LD50 of 5.3 × 10³ CFU/fish, confirming high pathogenicity in this host. Antimicrobial susceptibility testing showed reduced susceptibility to amoxicillin and universal reduced susceptibility to trimethoprim-sulfamethoxazole, while all isolates remained wild type or susceptible to florfenicol, oxytetracycline, erythromycin, and doxycycline based on previously reported cut-off values. This study extends the known host range of N. seriolae and highlights nocardiosis as an emerging disease threat to spotted scat aquaculture, emphasising the need for early diagnosis and targeted health management strategies.

Edwardsiella anguillarum causes Edwardsiellosis in fish, resulting in severe internal organ lesions and substantial economic losses in aquaculture. The limited efficacy of antibiotics in treating this disease, coupled with concerns regarding resistance and side effects, has driven interest in exploring fish vaccines. Various vaccine candidates derived from E. anguillarum, including inactivated cells and protein subunits, have shown potential. Here, we developed a vaccine against E. anguillarum in milkfish, and its efficacy was evaluated through vaccination challenge trials using milkfish as the target species. The formalin-killed vaccine (FKC) in combination with an adjuvant exhibited superior efficacy, achieving a relative percent survival (RPS) of 83.4%. Furthermore, the agglutination titers indicated a robust immune response in the FKC + Adjuvant group, suggesting a potent and sustained antibody response. Additionally, serum lysozyme activity was significantly higher in vaccinated fish than in controls. Immune-related gene expression analysis in spleen and head kidney revealed that IL-1β peaked at 14 days post-vaccination, indicating early pro-inflammatory response, while MHC-II showed progressive elevation, suggesting sustained antigen presentation and humoral immunity. Overall, this study underscores the effectiveness of the FKC + Adjuvant combination in enhancing immune response and protecting against bacterial pathogens in aquaculture, presenting a promising strategy for disease control.

Cyprinid herpesvirus 2 (CyHV-2) is a major pathogen causing high mortality in farmed goldfish and crucian carp, for which effective prophylactic or therapeutic treatments are currently limited. Lauric acid (LA) and glycerol monolaurate (GML), representative of a medium-chain fatty acid (MCFA) and its corresponding monoglyceride, respectively, have been reported to possess antiviral and immunomodulatory properties. This study systematically evaluated the anti-CyHV-2 effects of LA and GML through in vitro assays, transcriptomic profiling, and in vivo experiments. In vitro, both LA and GML significantly reduced viral copy numbers in gibel carp caudal fin (GiCF) cells, attenuated virus-induced cytopathic effects (CPE), and suppressed intracellular viral replication. Transcriptomic analysis revealed that LA and GML induced widespread alterations in host signalling pathways, with significant enrichment of pathways related to steroid biosynthesis, ECM-receptor interaction, protein digestion and absorption, and cell survival-associated signalling. Quantitative PCR analysis further demonstrated that the expression levels of insr, akt2, pdk1, lamtor3, and hsp90b were significantly upregulated, whereas inpp4b expression was downregulated. These results further validate that host cell metabolic processes, stress responses, and signal transduction pathways are substantially modulated following LA and GML treatment. The in vivo protective efficacy of GML was further assessed. In goldfish infected with CyHV-2, GML administration significantly increased survival rate and mitigated histopathological damage in the gills, liver, spleen, and kidney. At 3 days post-infection (dpi), expression levels of pro-inflammatory cytokines il-1β, il-6, and tnf-α were significantly lower in the GML-treated group compared to the virus-infected control group, whereas the anti-inflammatory cytokine il-10 was significantly upregulated. By 7 dpi, differences in inflammatory cytokine expression between groups had diminished, suggesting that GML not only exerts direct antiviral activity but may also modulate host immune responses during the early stage of infection. Collectively, these findings provide a theoretical basis for the practical application of LA and GML and propose a novel strategy for developing safe, effective, and environmentally friendly anti-CyHV-2 agents.

Vibrio vaccines play an important role in the prevention of Vibrio infections and in the sustainable development of the aquaculture industry. However, the genetic diversity and antigenic variation among Vibrio species make broad-spectrum protection a major challenge for conventional vaccine development, which often relies on inactivated whole cells or single-subunit antigens. To overcome this limitation, we developed a novel single/multi-target multi-derived epitope strategy, defined by two parameters: the number of target outer membrane proteins (OMPs) (single- or multi-target) and the incorporation of homologous epitope sequences from diverse Vibrio species (multi-derived). Four conserved epitopes, two immunodominant (K2, B3) and two cryptic (K7, U2), were identified from OMPs (OmpK, OmpU, LamB). Two multi-epitope vaccines were designed and constructed: EVK-10 (single-target multi-derived) and EVK-20 (multi-target multi-derived). Following prokaryotic expression and purification, the resulting vaccines were used to immunise zebrafish. Both vaccines subsequently induced substantial antibody responses, with the titers peaking at 21 days post-vaccination. Challenge trials demonstrated that both vaccines provided broad cross-immunoprotection against five heterologous Vibrio strains (V. parahaemolyticus, V. alginolyticus, V. harveyi, V. anguillarum, V. vulnificus). EVK-10 achieved relative percent survival (RPS) values of 59.3%, 67.9%, 53.3%, 36.0% and 45.8%, respectively, while EVK-20 showed superior RPS of 74.1%, 75.0%, 73.3%, 68.0% and 66.7%. In contrast, the single-derived rOmpK (V. parahaemolyticus derived) control induced strong protection only against closely related species (V. parahaemolyticus: 85.5%; V. alginolyticus: 82.1%) but lower protection against distantly related ones (V. anguillarum: 24.0%; V. vulnificus: 29.2%). These results demonstrate that the multi-derived strategy confers broader cross-protection than single-derived vaccines, and the multi-target approach (EVK-20) provides stronger and more redundant immunity than the single-target design (EVK-10). This study validates the single/multi-target multi-derived epitope strategy as an effective platform for developing universal vaccines against complex bacterial pathogens like Vibrio.

In this study, we investigated the immunomodulatory effects of St. John's Wort (Hypericum perforatum) oil on the growth performance, immune responses and resistance against Lactococcus garvieae in rainbow trout (Oncorhynchus mykiss). The plant oil was incorporated into feeds at concentrations of 1, 3, and 5 g kg^-1, designated as K1, K3, and K5, using a spray-coating method. A total of 12 tanks (100 L each) were used, with three replicates per group and 40 fish (10-11 g) stocked per tank. Trout fed with St. John's Wort oil exhibited significant improvements in immune parameters, particularly at the transcriptional level. Notably, after 40 and 60 days of feeding, the K3 group showed significant upregulation of immune-related genes, including IL-1β, IL-8, IFN-1, TNF-α, COX-2, TGF-β, and MHC-II in kidney and intestinal tissues. Following experimental challenge, survival rates reached 60% in K5, 50% in K3, and 35% in K1, compared with 30% in the control group. After 60 days, final body weights were 32.60 ± 0.34 g (K1), 34.83 ± 0.29 g (K3), 30.80 ± 1.46 g (K5), and 26.42 ± 0.36 g (control). The K3 group showed significantly higher final weight compared with all other groups (p < 0.05), whereas no significant differences were detected among the remaining treatments. These findings indicate that dietary inclusion of H. perforatum oil at 3 g kg^-1 enhances immune gene expression and disease resistance without compromising growth performance, suggesting its potential as an immunonutritional strategy in trout aquaculture.

Streptococcus agalactiae was confirmed as the etiologic agent of streptococcosis in Nile tilapia (Oreochromis niloticus) from Campeche, Mexico, during a cross-sectional investigation of eight semi-intensive farms (2021-2022) that yielded virulent strains in a challenge assay. Standard bacteriology, histopathology, API-20 Strep phenotyping, one-step PCR, and real-time PCR confirmed S. agalactiae in five farms (62.5%). Gross and histological lesions were characterised by mid-axial and peduncular muscle abscesses, extensive myonecrosis, granuloma formation, polyserositis, and occasional suppurative meningitis. Experimental infections fulfilled Koch's postulates. Juveniles of O. niloticus (20 g) infected with strain ASG-2290 at 10⁶ CFU/mL produced 53% mortality at 96 h. Whole-genome sequencing of the strains ASG-2290 and ASG-2292 identified them as S. agalactiae Ia, marking the first report of this serotype isolated from farmed tilapia in Mexico. Frequent interstate fry movements are likely to be a risk factor for pathogen dissemination. These findings document the geographic expansion of pathogenic S. agalactiae into the Yucatan Peninsula, underscoring the need for strengthened surveillance, biosecurity, and diagnostic capacity to mitigate economic losses in Mexican tilapia aquaculture. Considering that tilapia aquaculture is carried out all year-round in the Yucatan Peninsula, the implications of these findings are of major concern.

Piscirickettsiosis is the most prevalent bacterial disease affecting Chilean aquaculture and responsible for the majority of mortality in salmonids. Currently, large quantities of antibiotics, predominantly florfenicol, are used in the Chilean aquaculture industry, and sub-MIC concentrations of this antibiotic, similar to what occurs in the marine environment, have been shown to induce biofilm formation on both biotic and abiotic surfaces when sub-MIC doses of florfenicol, raising concerns about the emergence of antibiotic-resistant bacterial strains. Thus, the aim of this study was to evaluate whether in vitro sub-MIC concentrations of florfenicol induce the expression of genes associated with biofilm formation and antibiotic resistance in the biofilm-embedded P. salmonis. Interestingly, in vitro analyses showed that sub-MIC dilutions of antibiotic significantly modulated the expression of an efflux pump acrAB and the two-component systems cpxAR, and qseBC, as well as the antibiotic resistance-associated genes tclor/tflor and t.flor in the biofilm-embedded P. salmonis isolates tested. Thus, this study highlights the negative consequences of the extensive use of antibiotics in aquaculture, which can promote biofilm formation in marine bacterial pathogens, potentially facilitating the spread of resistance genes among different bacterial species in the aquatic environment and increasing the risk of reinfection within culture systems.

Harmful algal blooms (HABs) are a threat to fish welfare, occurring suddenly and unexpectedly causing significant consequences for fish and salmon farmers worldwide. Norwegian farmers have been facing this challenge at irregular intervals since the very beginning of the industry. This report describes the events on the first fish farm affected by the spring HAB of 2025 in northern Norway. Specifically at the production site Fornes, which at the time of the strike had more than 1 million Atlantic salmon (Salmo salar) with an average weight above 3 kg in its sea cages. First signs of the event were reduced fish appetite and cloudy water appearance, followed by changes in fish behaviour and acute mortality of 39.5%. Water samples showed dominance of the phytoplankton species Phaeocystis pouchetii, followed by Chrysochromulina leadbeateri. Necropsy of the fish and histopathological lesions on the gills and liver supported that the mortality was caused by the algae. The farm's contingency plan was enforced immediately when the acute high mortality was observed. Including, but not limited to, feed withdrawal followed by emergency harvest of the whole production site. The dramatic incidence at Fornes highlights the urgent need for monitoring and early-warning systems for HABs, as well as further development of suitable mitigation strategies and contingency plans to minimise the effect of HABs once they have been forecasted or detected.

Spring viremia of carp virus (SVCV) is a contagious pathogen associated with significant mortality and economic losses in freshwater aquaculture. Although reverse transcription quantitative PCR (RT-qPCR) enables rapid detection, it does not distinguish between infectious and non-infectious viral particles, which may lead to overestimation of infection risk. In this study, a viability RT-qPCR (vqPCR) assay targeting the glycoprotein (G) gene of SVCV was developed and analytically evaluated. Primers and probes were designed based on 45 full-length G-gene sequences representing genogroups Ia-Id to ensure broad genetic coverage. The assay achieved a 95% limit of detection (LoD_95%) of 6.82 copies per reaction and showed no cross-reactivity with other tested aquatic pathogens. Optimisation using 25 μM PMAxx effectively reduced amplification from inactivated virus and free viral RNA while preserving detection of infectious SVCV. Under controlled laboratory conditions, the vqPCR assay successfully differentiated varying proportions of infectious virus in mixed samples. In laboratory-simulated environmental freshwater, both qPCR and vqPCR signals declined with increasing temperature; notably, vqPCR reflected the loss of infectivity earlier than qPCR. The assay demonstrated good repeatability and reproducibility across three independent laboratories and showed overall agreement with cell culture and WOAH-recommended nested PCR results. Although further validation under diverse environmental conditions is warranted, these findings indicate that the proposed vqPCR approach may serve as a complementary method for monitoring infectious SVCV in freshwater aquaculture.