Author: marina02

Antimicrobial Susceptibility of Streptococcus suis serotype VI and Streptococcus agalactiae serotype VII Causing Streptococcosis in Snakeskin Gourami (Trichopodus pectoralis) during year 2021-2025 in Thailand

Streptococcosis is a major bacterial disease impacting the health and productivity of snakeskin gourami (Trichopodus pectoralis), a freshwater species of economic importance in Southeast Asian aquaculture. The disease is primarily caused by Streptococcus suis and Streptococcus agalactiae, with affected fish commonly exhibiting exophthalmos, darkened skin, and brain hemorrhage. The increased use of antibiotics in aquaculture has raised serious concerns about antimicrobial resistance (AMR), making routine surveillance essential to ensure effective disease control and reduce resistance risks across animal and human health. From year 2021 to 2025, 40 bacterial isolates were recovered from diseased snakeskin gourami in Thailand: 24 identified as S. suis serotype VI and 16 as S. agalactiae serotype VII Species identification was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) and PCR. The serotype identification was performed by PCR and genomic analysis using bioinformatics programs. Antimicrobial susceptibility testing was conducted using the agar-based disk diffusion method. Interpretation of inhibition zones was based on breakpoints established by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST), where applicable. The antibiotics tested included amoxicillin, amoxicillin with clavulanic acid, oxytetracycline, enrofloxacin, and sulfamethoxazole-trimethoprim.All S. suis isolates were fully susceptible to the antibiotics tested, indicating no detectable resistance within this group. In contrast, S. agalactiae exhibited resistance to oxytetracycline, while remaining susceptible to amoxicillin, amoxicillin with clavulanic acid, enrofloxacin, and sulfamethoxazole-trimethoprim. This selective resistance pattern is likely a result of oxytetracycline overuse in aquaculture environments.This study highlights species-specific AMR trends, with S. suis remaining fully susceptible and S. agalactiae displaying resistance to oxytetracycline. These findings emphasize the importance of pathogen-specific identification and susceptibility testing to guide appropriate antibiotic therapy. Incorporating both CLSI and EUCAST guidelines strengthens the reliability of interpretation. Ongoing AMR surveillance and prudent antibiotic use are crucial to sustaining aquaculture health and productivity. A One Health approach remains essential for minimizing AMR risks across aquatic systems, ecosystems, and public health.

1. RODKHUM, CHANNARONG, FACULTY OF VETERINARY SCIENCE, CHULALONGKORN UNIVERSITY, THAILAND, Presenter
2. Chueahiran, Surawit, Faculty of Veterinary Science, Chulalongkorn University, Thailand, Author

Aeromonas veronii, in farmed European seabass (Dicentrarchus labrax) in the Ionian Sea, Greece.

Introduction

The intensification of aquaculture in the Mediterranean area together with climate change, has led to increased disease outbreaks, which threaten productivity and food security. Aeromonas veronii, is known to cause severe systemic infections in a range of fish species and has recently been associated with high mortality in European seabass (Dicentrarchus labrax). This study presents the first documented outbreak of A. veronii in seabass along the Greek Ionian coast, describing the clinical signs, diagnostics, and genomic data with the aim of supporting improved disease management strategies in aquaculture.

Methodology

The disease was first observed in late spring 2024 in Sagiada Bay, in the Ionian see in Greece, when the temperature of the water reached 18°C. Soon during Summer, the disease spread to nearby farms peaking when the temperature reached 26–29°C. Fish weighing over 250 g, exhibited clinical signs like anorexia, lethargy, skin paleness and ulcers, fin petechiae, hemorrhages, and ocular lesions. Mortality ranged from 0.01% to 0.05% daily, with temporary reductions following antibiotic treatment, reaching in the end of Autumn almost 15–20% cumulatively. Postmortem examinations were performed on samples from seven farms and tissue samples were collected for parasitological, microbiological, molecular, and histological analysis. Two Aeromonas veronii strains from geographically distant areas of the Ionian see, were subjected to whole-genome sequencing, followed by phylogenetic analysis.

Results

Postmortem examination revealed pale skin, petechiae, ulcers, and coelomic distention while in the abdominal cavity, enlargement of the spleen, kidney, and liver was detected, accompanied by necrotic foci, abscesses, and granulomas, as demonstrated by histopathological analysis. Phenotypic characterization reported the presence of gram-negative, motile bacteria exhibiting hemolytic activity and the ability to grow on various culture media. Molecular characterization using PCR detected Aeromonas veronii DNA in spleen tissues, while genomic and phylogenetic analyses demonstrated 100% identity among the Greek isolates, with minor divergence from Aegean strains and reference genomes, classifying them within sequence type ST23.

Conclusion

This study reports the first outbreak of Aeromonas veronii in farmed European seabass along Greece’s Ionian Sea coast, resulting in high morbidity, mortality, and economic losses. Clinical and pathological signs were similar to the ones described in previous infections from Aeromonas veronni, while isolates showed genetic similarity to the Aegean strains but differed phenotypically in motility, pigmentation, and metabolism. Whole-genome sequencing confirmed their classification within sequence type ST23. These findings highlight the need for ongoing surveillance and new vaccines, in order to support sustainable Mediterranean aquaculture.

1. Lampou, Eirini, Department of Aquaculture and Fish Health, Pharmaqua SA., 14451, Metamorfosi, Athens, Greece, Presenter
2. Psychari, Eleni, Department of Aquaculture and Fish Health, Pharmaqua SA., 14451, Metamorfosi, Athens, Greece, Author
3. Louka, Kostantina, Department of Aquaculture and Fish Health, Pharmaqua SA., 14451, Metamorfosi, Athens, Greece, Author
4. Kotzamanidis, Charalampos, Veterinary Research Institute, Hellenic Agricultural Organization DIMITRA, 57001, Thessaloniki, Greece, Author
5. Malousi, Andigoni, Laboratory of Biological Chemistry, Medical School. Aristotle University, 54124, Thessaloniki, Greece, Author
6. Bitchava, Konstantina, Laboratory of Applied Hydrobiology, Department of Animal Science, School of Animal Biosciences, Agricultural University of Athen, Author

Flavobacterium psychrophilum: what’s up bug?

Flavobacterium psychrophilum is the etiological agent of rainbow trout fry syndrome and bacterial cold-water disease in salmonid fish reared in freshwater. F. psychrophilum is currently one of the main bacterial pathogens hampering the productivity of salmonid farming worldwide. Our team has a long-lasting love story with bacteria of this family ranging from classification guidelines provided by Jean-François Bernardet in the late 90, the very first complete genome sequence of a bacterial fish pathogen in 2007, to transcriptomics and phenotypic characterization of relevant bacterial isolates. Over the years, we developed molecular typing tools and conducted comparative genomic analysis. We revealed that F. psychrophilum is a bacterial species harboring a limited genomic diversity in terms of both nucleotide diversity and gene repertoire but is also highly recombinogenic. We demonstrated that bacterial isolates cluster in clonal complexes (CC) according to their host-fish species suggesting a panmictic population structure with the expansion of highly virulent epidemic clones in rainbow trout farms.

Combining genome-wide association study, mutagenesis and phenotypic screens we identified relevant molecular traits explaining previously unresolved phenotypic characteristics such as serological diversity, proteolytic capabilities and nutrient acquisition systems.

1. DUCHAUD, ERIC, INRAE, Presenter
2. NICOLAS, PIERRE, INRAE, Author
3. ROCHAT, TATIANA, INRAE, Author

Characterization of Edwardsiella strains isolated from fish in the Netherlands (1995-2020) shows a relationship between Edwardsiella species and source.

Edwardsiella species are motile, facultative anaerobic, rod-shaped Gram-negative bacteria responsible for the disease edwardsiellosis, which leads to mass mortality in various populations and age groups of fish. Furthermore, E. tarda can infect mammals and cause severe disease in humans after consumption of contaminated food. Five different species have been identified so far, of which four are considered fish pathogens. However, it is not clear which Edwardsiella species are pathogenic to which fish species. The aim of this study was to characterise Edwardsiella species isolated from different fish species in the Netherlands. Furthermore, the Matrix Assisted Laser Desorption Ionization – Time of Flight Mass Spectrometry (MALDI-TOF MS) database was updated with profiles from Edwardsiella species to facilitate correct and rapid diagnosis.

Thirty-six Edwardsiella strains isolated from fish along a 25-year period were identified using Multilocus Sequence Analysis (MLSA) based on sequence analysis of three genes: gyrB, pgi and phoU. A selection of these strains was used to create a new Edwardsiella MALDI-TOF database by analyzing peptide spectra of strains belonging to the four fish pathogenic species.

MALDI-TOF MS analysis of the Edwardsiella isolates using the newly created database revealed a 100% agreement with MLSA. Furthermore, the presence of Edwardsiella species was associated with specific fish species: 1) E. piscicida was only present in sole, turbot, koi carp and eel, 2) E. tarda was solely isolated from ornamental fish generally kept at a higher water temperature and 3) E. ictaluri was recovered from both catfish and electric eel. In addition, E. anguillarum was not isolated from fish in the Netherlands and one strain isolated from electric eel was clearly different in both sequence analysis and MALDI-TOF profile from those of the other four species.

Thus, both MLSA and MALDI-TOF analysis can be used for determination of Edwardsiella species. Moreover, our results may indicate a host or environmental preference of the different species. Finally, it would be interesting to further investigate the pathogenic potential of the different Edwardsiella species in different fish hosts and under various environmental conditions.

1. Allaart, Janneke, Wageningen Bioveterinary Research, Lelystad, The Netherlands, Presenter
2. Voorbergen-Laarman, Michal, Wageningen Bioveterinary Research, Lelystad, The Netherlands, Author
3. van Gelderen, Betty, Wageningen Bioveterinary Research, Lelystad, The Netherlands, Author
4. Fouz, Belén, University Institute of Biotechnology and Biomedicine (BIOTECMED), University of Valencia, Valencia, Spain, Author
5. Engelsma, Marc, Wageningen Bioveterinary Research, Lelystad, The Netherlands, Author

Moritella viscosa and Tenacibaculum finnmarkense infection pressure in seawater

FUNDING: Norwegian Seafood Research Fund (FHF), project no. 901838

INTRODUCTION:

Skin ulcer conditions involving bacterial infections are currently among the most detrimental health and welfare challenges experienced in sea-based farming of Atlantic salmon in Norway, threatening the industry’s sustainability. In particular winter ulcer and tenacibaculosis, caused primarily by Moritella viscosa and Tenacibaculum finnmarkense, lead to fish suffering, mortalities, downgrading at slaughter, and a negative consumer perception. These bacteria often occur in mixed infections, alongside other Tenacibaculum, Aliivibrio, and Vibrio species. Recent studies have further revealed notable genetic diversity within M. viscosa and T. finnmarkense, with individual fish commonly carrying multiple variants of the same species. Still, a few genetic variants are particularly intimately linked to ulcer development specifically in Atlantic salmon, in the sense that they are almost always present amongst the variants recovered through culture-based ulcer diagnostics on this fish species. Whether this reflects host-specific pathogenicity and/or dominance of particular genotypes in the marine water column remains unclear.

METHODOLOGY:

Throughout one full year, weekly seawater samples were collected in triplicates at seven meters depth from within and outside (upstream control point) seven geographically distributed salmon farms along the Norwegian coastline. At the time of writing, these samples are being examined using a battery of seven PCRs, designed for specificity towards Tenacibaculum spp., T. finnmarkense and M. viscosa at various taxonomic levels, including also assays specific for the ‘salmon ulcer’-associated variants of these bacteria.

RESULTS:

Overall, the preliminary results indicate a generally higher incidence of the targeted ulcer bacteria within salmon farms compared to at the nearby control points, although findings vary between farms as well as over the seasons. Sample and data processing is still ongoing, and further results will be available for the presentation.

CONCLUSION:

Updated results from the PCR screening, across the investigated bacterial taxa, will be presented in relation to geography and time, as well diagnostic/clinical records from participating salmon farms. This will e.g. inform us with regard to the relative impact of environmental infection pressures in terms of ulcer development in sea-farmed Atlantic salmon.

1. GULLA, SNORRE, NORWEGIAN VETERINARY INSTITUTE, Presenter
2. Benedicenti, Ottavia, NORWEGIAN VETERINARY INSTITUTE, Author
3. Spilsberg, Bjørn, NORWEGIAN VETERINARY INSTITUTE, Author
4. Nilsen, Hanne, NORWEGIAN VETERINARY INSTITUTE, Author
5. Olsen, Anne Berit, NORWEGIAN VETERINARY INSTITUTE, Author
6. Colquhoun, Duncan, NORWEGIAN VETERINARY INSTITUTE, Author

Piscirickettsia salmonis in turbot

Introduction

Pisicirickettsia salmonis is a well-established intracellular pathogen of farmed Atlantic salmon around the world. The bacterium has also been detected from a number of other hosts including lumpfish, sea-bass and rainbow trout.

In February 2025, increased mortality was observed in a population of turbot, Scophthalmus maximus (80-160g), farmed in Norway. Macrospcopic signs of disease included bleedings in the stomach mucosa.

Method

Pisicirickettsiosis was diagnosed based on histopathological findings including rickettsia-like inclusions immunostaining positive for Pisicirickettsia salmonis in internal organs and gills, qPCR and in situ hybridisation using RNAscope. Culture on CHAB was attempted, and bacteriological and sequencing investigations continue at the time of writing.

Result and conclusion

While this represents to the best of our knowledge the first case of Pisicirickettsia salmonis reported from turbot confirmed by several independent methodologies, similar pathology and positive immunostaining against P. salmonis were previously reported in 2011 in turbot from the same site.

1. NILSEN, HANNE KATRINE, NORWEGIAN VETERINARY INSTITUTE, Presenter
2. Hansen, Magne Kjerulf, Agder Fish Health service, Author
3. Kuiper, Raoul, NORWEGIAN VETERINARY INSTITUTE, Author
4. Øvrebø, Jan Inge, NORWEGIAN VETERINARY INSTITUTE, Author
5. Sturød, Kjersti, NORWEGIAN VETERINARY INSTITUTE, Author
6. Saghafian, Maryam, NORWEGIAN VETERINARY INSTITUTE, Author
7. Gulla, Snorre, NORWEGIAN VETERINARY INSTITUTE, Author
8. Colquhoun, Duncan, NORWEGIAN VETERINARY INSTITUTE, Author
9. Olsen, Anne Berit, NORWEGIAN VETERINARY INSTITUTE, Author
10. Dale, Ole Bendik, NORWEGIAN VETERINARY INSTITUTE, Author

Characterization of Korean Edwardsiella isolates and development of species-specific identification tools using multiplex qPCR and MALDI-TOF MS

Edwardsiella species—including E. tarda, E. piscicida, and E. anguillarum—are globally distributed fish pathogens with broad host ranges and significant economic impact. In Korea, accurate species identification has been challenging due to overlapping phenotypic traits and inconsistent diagnostic results. This study aimed to characterize Edwardsiella isolates from Korea and to develop rapid and reliable tools for differentiating these closely related species.

A total of 306 isolates collected between 2002 and 2023, along with type strains, were analyzed. While some species-specific traits were observed—such as mannitol and arabinose utilization in E. anguillarum, and citrate utilization in E. tarda—H₂S production results varied across media (API 20E, SIM, SS agar, TSIA), limiting the reliability of traditional biochemical assays.

Multilocus phylogenetic analysis (MLPA) using eight housekeeping genes showed that gyrB provided the highest discriminatory power. Based on this, a PNA probe-based multiplex qPCR assay was developed to accurately identify and quantify the three species with high amplification efficiency. Re-identification of isolates revealed that E. piscicida was dominant in olive flounder, while E. tarda was found only in isolates from Anguilla japonica, suggesting possible host associations. Pathogenicity assays showed E. piscicida as the most virulent (LD₅₀: 10¹–10² CFU/fish), followed by E. anguillarum; E. tarda showed no mortality in olive flounder.

To support rapid proteomic identification, a MALDI-TOF MS spectral library was constructed using 1,816 spectra from 182 genetically confirmed isolates. Species-specific Main Spectra Profiles (MSPs) were generated, and correlation indices confirmed interspecies separation (EP–EA: 0.84; EA–ET: 0.73; ET–EP: 0.71). Validation using 70 blind-tested isolates showed that the in-house library achieved high log score values (2.48–2.70), whereas the commercial database failed to distinguish species, identifying all strains as E. tarda (log score 1.87–2.62).

This study presents an integrated molecular and proteomic platform for reliable identification of Edwardsiella species, with potential application in diagnostics and surveillance across diverse aquaculture settings.

1. KIM, AHRAN, NATIONAL INSTITUTE OF FISHERIES SCIENCE, Presenter
2. KIM, A hyun, NATIONAL INSTITUTE OF FISHERIES SCIENCE, Author
3. KIM, Chi Yun, NATIONAL INSTITUTE OF FISHERIES SCIENCE, Author
4. Do, Jeong Wan, NATIONAL INSTITUTE OF FISHERIES SCIENCE, Author

Biofilm formation and viability of Renibacterium salmoninarum under varying salinities and temperatures

introduction: bacterial kidney disease (bkd), caused by renibacterium salmoninarum (r. salmoninarum), is a serious disease affecting both wild and farmed salmonid fish worldwide. reports indicate an 80% mortality rate in pacific salmon and 40% in atlantic salmon. in norway, where it was first detected in 1980, it is classified as a notifiable disease. bkd outbreaks were reported in 2023, and well boats have been identified as a critical risk factor for horizontal disease transmission between farms during these outbreaks. as industries have prioritized biosecurity measures in recent years, it is crucial to investigate the efficacy of washing detergents and disinfectants against r. salmoninarum that can be applied to well boats and other transportation equipment. however, the efficacy of washing detergents and disinfectants is influenced by the bacteria’s ability to form autoaggregation or biofilm. the capacity of r. salmoninarum to produce autoaggregation or biofilm has not been well-documented so far.
method and aim: the bkdsafeguard project aims to determine the bacterium’s survivability, auto-aggregation, and biofilm formation abilities in different sterilized and unsterilized media (fish tank outlet water, freshwater, and seawater) and under various temperature conditions (5, 10, and 15°c) that mimic the fish farm environment.
results: the 6-week survival experiment showed that the Ct-values (14.92-17.99) of bacteria were stable and viable throughout the experiment in all the sterilized media. in unsterilized media, a marked reduction of bacteria (Ct-value: 15.55- 27.34) was observed until the 3rd week, followed by a slow increase until the 6th week (Ct-value: 15.55- 17.11). Results also indicate better viability at 5°C compared to 10 and 15°C. based on macroscopic and microscopic observations, and tube sedimentation assays, the bacterium exhibited a high tendency to form autoaggregation in fresh and seawater and a lower tendency in fish tank outlet water. the highest autoaggregation occurred at 15°c, followed by 5°c, and the lowest was at 10°c.
moreover, the biofilm formation assay showed that the bacterium can form a strong biofilm in seawater and fish tank outlet water at 15 and 10°c, but a moderate biofilm formation at 5°c. we hypothesize that autoaggregation and biofilm-forming bacteria may be more resistant to disinfection than planktonic bacteria. further work will be performed within the project to investigate the efficacy of disinfection and washing detergents on r. salmoninarum and determine the effective dose, exposure time, and optimal temperature conditions.

1. WELEKIDAN, LETEMICHAEL NEGASH, NORWEGIAN UNIVERSITY OF LIFE SCIENCES, Presenter
2. Weman Persson, David, Norwegian University of Life Sciences, Author
3. Østevik, Liv, Norwegian University of Life Sciences, Author
4. Foyn Nørstebø, Simen, Norwegian Veterinary Institute, Author

First report on Flavobacterium columnare causing disease outbreaks in cultured black carp, Mylopharyngodon piceus

Columnaris disease has recently emerged as a concern for black carp (Mylopharyngodon piceus), an economically important aquaculture species in northern Vietnam. This study investigated the disease outbreaks from five affected farms. A total of 47 naturally diseased fish were sampled for bacterial isolation and identification. Five representative isolates resembling Flavobacterium were selected for phenotypic and genetic characterization. Through a combination of biochemical tests, PCR assays, and 16S rRNA sequencing, these bacterial isolates were identified as Flavobacterium columnare. Experimental challenge tests on black carp revealed a median lethal dose (LD₅₀) of F. columnare at 1.28 × 10⁶ CFU/ml via immersion. Clinical signs and histopathological features observed in experimentally infected fish closely resembled those in naturally diseased fish. F. columnare isolates exhibited reduced susceptibility to several common antibiotic uses in Vietnam. This study represents the first documentation of F. columnare infection in black carp culture, enhancing our understanding of columnaris disease in fish and underscoring the need for preventive measures tailored to this cultured fish species.

1. HOAI, TRUONG, VIETNAM NATIONAL UNIVERSITY OF AGRICULTURE, Presenter

Can stressed fish provoke ‘aggressive’ bacteria?

FUNDING: Research Council of Norway (RCN), project no. 336037

INTRODUCTION:

A resurgence of bacterial diseases, involving recognised fish pathogens as well as a few largely uncharacterised species, has occurred across Norwegian sea-based salmon farming in recent years. This trend closely parallels the widespread adoption and frequent use of non-chemical delousing techniques, particularly thermal treatments, which intrinsically involve physically invasive handling of fish. Such procedures will induce significant stress and may also inflict mechanical injuries.

It is well established that stress can suppress immune function in vertebrates. Additionally, research in mammals has demonstrated that catecholamines, a group of stress hormones, can enhance the expression of virulence factors in certain pathogenic bacteria. This avenue for host-pathogen interaction falls within the term microbial endocrinology, and operates through signalling pathways that are evolutionarily conserved across various bacterial phyla. It is therefore plausible that similar mechanisms may also play a part in an aquaculture setting, whereby stress in fish could promote bacterial virulence and thus trigger or amplify disease outbreaks.

METHODOLOGY:

Selected isolates of Yersinia ruckeri, Moritella viscosa and Aeromonas salmonicida subsp. salmonicida, at various inoculation concentrations, were cultured in minimal broth media supplemented with bovine serum and various concentrations of the catecholamine noradrenaline. Bacterial growth was monitored spectrophotometrically and colony forming units determined post incubation. Future work will also assess transcriptional responses to elucidate underlying molecular mechanisms.

RESULTS:

Exposure to ≥50 µM and ≥150 µM noradrenaline greatly increased growth M. viscosa and Y. ruckeri, respectively, while the reaction was much less pronounced for A. salmonicida subsp. salmonicida. Transcriptomic results are not available at the time of writing.

CONCLUSION:

Although preliminary, our results suggest that growth of Y. ruckeri and M. viscosa, and to some extent A. salmonicida subsp. salmonicida, is promoted by exposure to the catecholamine stress hormone noradrenaline. Whether this phenomenon also occurs within the fish remains to be established, but there can be little doubt that fish subjected to physical delousing will demonstrate an acute systemic release of this hormone (amongst others). During the course of this project, we will explore further microbial endocrinology mechanisms in an array of fish-pathogenic bacteria, including also its effects on bacterial gene expression.

1. GULLA, SNORRE, NORWEGIAN VETERINARY INSTITUTE, Author
2. Asal, Basma, NORWEGIAN VETERINARY INSTITUTE, Author
3. Trohjell, Markus, University of Bergen, Author
4. Lyte, Mark, Iowa State University, Author
5. Colquhoun, Duncan, NORWEGIAN VETERINARY INSTITUTE, Presenter