A pentavalent vaccine for rainbow trout in Danish aquaculture

Bacterial and viral co-infections in aquaculture under climate warming: co-evolutionary implications, diagnosis, and treatment

Climate change and the associated environmental temperature fluctuations are contributing to increases in the frequency and severity of disease outbreaks in both wild and farmed aquatic species. This has a significant impact on biodiversity and also puts global food production systems, such as aquaculture, at risk. Most infections are the result of complex interactions between multiple pathogens, and understanding these interactions and their co-evolutionary mechanisms is crucial for developing effective diagnosis and control strategies. In this review, we discuss current knowledge on bacteria-bacteria, virus-virus, and bacterial and viral co-infections in aquaculture as well as their co-evolution in the context of global warming. We also propose a framework and different novel methods (e.g. advanced molecular tools such as digital PCR and next-generation sequencing) to (1) precisely identify overlooked co-infections, (2) gain an understanding of the co-infection dynamics and mechanisms by knowing species interactions, and (3) facilitate the development multi-pathogen preventive measures such as polyvalent vaccines. As aquaculture disease outbreaks are forecasted to increase both due to the intensification of practices to meet the protein demand of the increasing global population and as a result of global warming, understanding and treating co-infections in aquatic species has important implications for global food security and the economy.

Validation of a QTL associated with resistance to Vibrio anguillarum in rainbow trout (Oncorhynchus mykiss)

Vibriosis is a bacterial disease in fish caused by the Gram negative bacterium Vibrio anguillarum with severe impact on rainbow trout (Oncorhynchus mykiss) farming. Sustainable control methods should be developed and we here show that marker assisted selective breeding of fish naturally resistant to the disease is feasible. We have validated the use of a single nucleotide polymorphism (SNP) marker SNP AX-89,945,921 (QTL on chromosome 21). The QTL was previously found associated with resistance to vibriosis and described following a genome wide association analysis (GWAS) of trout exposed to the bacterium. For this validation spawners were genotyped by use of the 57 K Axiom®Trout Microarray (Affymetrix) and homozygous male fish carrying the allele with the SNP AX-89,945,921 were then selected and used to fertilize eggs from outbred female trout resulting in fish all carrying the SNP (QTL-fish). Control fish (non-QTL fish) were produced by fertilizing the same batch of eggs by use of male parents negative for the SNP. The fish were exposed in freshwater to V. anguillarum (water bath infection) at 19 C°. A total of 900 fish were challenged in a common garden set-up in triplicate. A bacterial solution of V. anguillarum (serotype O1) was added to each of three freshwater fish tanks, each with 150 QTL and 150 non-QTL fish. Fish were tagged by tail fin cut (upper/lower) to discern the two groups, whereafter fish were monitored around the clock to detect disease signs and remove moribund fish. Clinical vibriosis developed within two days in non-QTL-fish (overall morbidity of 70%). QTL fish developed clinical signs later and the morbidity was significantly lower and did not reach 50%. Rainbow trout farming may benefit from using the QTL associated with higher resistance towards vibriosis. The effect may be optimized in the future by use of both male and female parents homozygous for the marker allele.

Bacterial Pathogenesis in Various Fish Diseases: Recent Advances and Specific Challenges in Vaccine Development

Aquaculture is a fast-growing food sector but is plagued by a plethora of bacterial pathogens that infect fish. The rearing of fish at high population densities in aquaculture facilities makes them highly susceptible to disease outbreaks, which can cause significant economic loss. Thus, immunity development in fish through vaccination against various pathogens of economically important aquaculture species has been extensively studied and has been largely accepted as a reliable method for preventing infections. Vaccination studies in aquaculture systems are strategically associated with the economically and environmentally sustainable management of aquaculture production worldwide. Historically, most licensed fish vaccines have been developed as inactivated pathogens combined with adjuvants and provided via immersion or injection. In comparison, live vaccines can simulate a whole pathogenic illness and elicit a strong immune response, making them better suited for oral or immersion-based therapy methods to control diseases. Advanced approaches in vaccine development involve targeting specific pathogenic components, including the use of recombinant genes and proteins. Vaccines produced using these techniques, some of which are currently commercially available, appear to elicit and promote higher levels of immunity than conventional fish vaccines. These technological advancements are promising for developing sustainable production processes for commercially important aquatic species. In this review, we explore the multitude of studies on fish bacterial pathogens undertaken in the last decade as well as the recent advances in vaccine development for aquaculture.

Antigenic variability of Vibrio anguillarum serotype O2a: A hurdle for vaccine efficacy against vibriosis in rainbow trout (Oncorhynchus mykiss)

Despite vaccination, outbreaks of vibriosis still occur in sea-reared rainbow trout in Denmark. Vibriosis outbreaks are caused mainly by V. anguillarum serotypes O1 and O2a, and bacterins of both serotypes are included in the commonly used vaccine against this disease in Danish aquaculture. However, while the strains belonging to serotype O1 are genetically similar, the strains belonging to serotype O2a are highly diverse. This work aimed first at examining how the antibody response and protection induced by bacterin-based vaccines were affected by the antigenic variability within V. anguillarum serotype O2a strains. Following vaccination of rainbow trout with either a commercial or an experimental vaccine, specific antibody reactivity in serum from vaccinated fish was examined by ELISA against 23 strains of V. anguillarum serotype O2a (VaO2a). The strains were divided into 4 distinct subgroups according to the observed detection pattern. Seven strains were strongly recognized only by sera from fish vaccinated with the experimental vaccine (EV-I antisera), while 13 other strains were primarily recognized by sera from fish vaccinated with the commercial vaccine (CV antisera). Two strains were recognized by both EV-I and CV antisera, but with intermediate reactivity, while one strain was not recognized at all. A partly similar recognition pattern was observed when purified lipopolysaccharide (LPS) was used as antigen in the examination of antibody reactivity in Western blotting. The level of protection was highly dependent on both the vaccine and the strain used for challenge and showed no consistent correlation with antibody reactivity. Secondly, we attempted to use a bacterin vaccine based on one of the V. anguillarum O2a strains intermediately recognized by both EV-I and CV antisera to investigate whether that could potentially provide protection across strain variability. The immunized fish did mount a cross-reactive antibody response, but protection still varied depending on the strain used for challenge. Interestingly, the grouping of strains according to antibody reactivity correlated not only with genotyping based on single nucleotides polymorphisms analysis (SNP) but also with variability in the accessory genome, indicating that presence or absence of protein antigens or proteins associated with the biosynthesis of antigenic epitopes may explain the observed distinct serological subgrouping within VaO2a strains by trout immune sera. In terms of vaccination against VaO2a, our results demonstrate that it is important to take (local) antigen variations into account when using bacterin-based vaccines but also that alternatives to traditional bacterin-based vaccines might be needed to induce protection against the highly virulent Vibrio anguillarum serotype O2a strains.

Flavobacterium psychrophilum: Response of Vaccinated Large Rainbow Trout to Different Strains

BACKGROUND

Although Flavobacterium psychrophilum is recognized as the causative pathogen of rainbow trout fry syndrome (RTFS), often resulting in high fry mortality, it is also responsible for bacterial cold water disease (BCWD) in large and older rainbow trout (Oncorhynchus mykiss). These older fish do not experience high mortality, but sustain, through the shedding of bacteria, a constant infection pressure at farm level, which exposes fry to an unnecessary infection risk. We have produced and assessed the immunogenicity of an experimental injection BCWD vaccine, which may be used to decrease the shedding of bacteria from older fish.

METHODS

A total of 800 fish were i.p.-injected: 200 fish received the bacterin with adjuvant, 200 fish received the bacterin alone, 200 fish received adjuvant alone and 200 fish were injected with physiological saline. Blood samples were taken at day 0 and at three different time points (4, 8 and 14 weeks) post-vaccination. Plasma antibody levels were measured by ELISA for reactivity against both the homologous F. psychrophilum vaccine strain (serotype Fd) and heterologous strains (serotype Th).

RESULTS

Significantly elevated antibody titers were found against all serotypes in vaccinated fish. Welfare parameters associated with the vaccination process were evaluated by analyzing trout plasma samples for six different biochemical parameters, but no adverse effects associated with injection were indicated.

CONCLUSIONS

The study suggests that an injection vaccine containing formalin-inactivated whole cells of F. psychrophilum (serotype Fd), adjuvanted with FIA, may also induce protection against heterologous strains. We advocate for, as the next step, the performance of field trials evaluating if the vaccination of older rainbow trout will (1) reduce the infection pressure in farms, (2) elevate the general health level in all groups and (3) minimize F. psychrophilum infection in fry at farm level. This may reduce the need for the administration of antibiotics in all age classes.

Immersion vaccines against Yersinia ruckeri infection in rainbow trout: Comparative effects of strain differences

The protective effects of autogenous and commercial ERM immersion vaccines (bacterins based on Yersinia ruckeri, serotype O1, biotypes 1 and 2) for rainbow trout (Oncorhynchus mykiss) were compared in order to evaluate whether the use of local pathogen strains for immunization can improve protection. In addition, the effect of the bacterin concentration was established for the commercial product. Following sublethal challenge of vaccinated and non-vaccinated control fish with live bacteria, we followed the bacterial count in the fish (gills, liver and spleen). The expression of genes encoding immune factors (IL-1β, IL-6, IL-8, IL-10, IFN-γ, MHCI, MHCII, CD4, CD8, TCRβ, IgM, IgT, IgD, cathelicidins 1 and 2, SAA and C3) and densities of immune cells in organs were recorded. Both vaccines conferred protection as judged from the reduced bacterial load in exposed fish. Innate immune genes were upregulated in all groups following bacterial challenge but significantly more in non-vaccinated naive fish in which densities of SAA-positive immune cells increased. Immunoglobulin genes were upregulated on day 5 post-challenge, and fish vaccinated with the high commercial bacterin dosage showed increased IgM levels by ELISA on day 14 post-challenge, reflecting that the vaccine dosage was correlated to protection. In conclusion, both vaccine types offered protection to rainbow trout when exposed to live Y. ruckeri and no significant difference between commercial and autogenous vaccines was established.

Transcriptome Analysis in the Head Kidney of Rainbow Trout (Oncorhynchus mykiss) Immunized with a Combined Vaccine of Formalin-Inactivated Aeromonas salmonicida and Vibrio anguillarum

This study aimed to identify the molecular mechanisms regulated by a combined vaccine against Aeromonas salmonicida and Vibrio anguillarum (O1 serotype). These bacteria cause furunculosis and vibriosis, respectively, and are associated with a high mortality in rainbow trout in Korea. The vaccine upregulated gene expression of TCRα, T-bet, sIgM, and mIgM, markers of an activated adaptive immune response. On days 1, 3, and 5, transcriptome analysis revealed 862 (430 up- and 432 downregulated), 492 (204 up- and 288 downregulated), and 741 (270 up- and 471 downregulated) differentially expressed genes (DEGs), respectively. Gene ontology (GO) enrichment analysis identified 377 (108 MF, 132 CC, 137 BP), 302 (60 MF, 180 CC, 62 BP), and 314 (115 MF, 129 CC, 70 BP) GOs at days 1, 3, and 5, respectively. Kyoto Encyclopedia of Genetic and Genomic enrichment analysis identified eight immune system-related pathways like cytokine-cytokine receptor interaction, NF-kappaB signaling pathway, TNF signaling pathway, NOD-like receptor signaling pathway, cytosolic DNA sensing pathway, cell adhesion molecule, complement and coagulation cascade, and antigen processing and presentation. In the analysis of the protein–protein interaction of immune-related DEGs, a total of 59, 21, and 21 interactional relationships were identified at days 1, 3, and 5, respectively, with TNF having the highest centrality at all three time points.

Vibrio harveyi isolated from marine aquaculture species in eastern China and virulence to the large yellow croaker (Larimichthys crocea)

AIMS

Diseases of maricultured species caused by Vibrio harveyi are increasing in China and other regions. This study examined the genetic diversity, antimicrobial susceptibility, plasmid profiles and virulence potential of the V. harveyi isolated from marine organisms farmed in two provinces in eastern China between 2014 and 2019.

METHODS AND RESULTS

A total of 54 V. harveyi were obtained from seven marine species. Enterobacterial repetitive intergenic consensus (ERIC)-PCR fingerprinting revealed substantial genetic heterogeneity among the V. harveyi isolates. There was no significant correlation between ERIC-PCR genotypes and host origins or fish farms. All the isolates were resistant to amoxicillin and ampicillin, and 79·6% to kanamycin. We found that 61·1% of the V. harveyi isolates had plasmid(s) and there were 14 different plasmid profiles. Most isolates from fish hosts (76·5%) contained plasmids; however, 75% of isolates from nonfish hosts lacked plasmids. Experimental infection results showed that isolates with plasmid(s) were more virulent to large yellow croaker than isolates lacking plasmids (P < 0·05).

CONCLUSIONS

This study confirmed that V. harveyi isolates obtained from animals farmed in the coastal region of east China were genetically diverse. Our results suggest that the virulence of various V. harveyi strains to fish is associated with the plasmids they carry.

SIGNIFICANCE AND IMPACT OF THE STUDY

More than 50% of the V. harveyi isolates carried one to 11 plasmids. The plasmid-borne traits of V. harveyi strains might be important for host adaptation and virulence, but they were not associated with susceptibility to the tested antibiotics.

A Major QTL for Resistance to Vibrio anguillarum in Rainbow Trout

Genetic selection of disease resistant fish is a major strategy to improve health, welfare and sustainability in aquaculture. Mapping of single nucleotide polymorphisms (SNP) in the fish genome may be a fruitful tool to define relevant quantitative trait loci (QTL) and we here show its use for characterization of Vibrio anguillarum resistant rainbow trout (Oncorhynchus mykiss). Fingerlings were exposed to the pathogen V. anguillarum serotype O1 in a solution of 1.5 × 10⁷ cfu/ml and observed for 14 days. Disease signs appeared 3 days post exposure (dpe) whereafter mortality progressed exponentially until 6 dpe reaching a total mortality of 55% within 11 days. DNA was sampled from all fish – including survivors – and analyzed on a 57 k Affymetrix SNP platform whereby it was shown that disease resistance was associated with a major QTL on chromosome 21 (Omy 21). Gene expression analyses showed that diseased fish activated genes associated with innate and adaptive immune responses. The possible genes associated with resistance are discussed.

Analysis of 44 Vibrio anguillarum genomes reveals high genetic diversity

Vibriosis, a hemorrhagic septicemic disease caused by the bacterium Vibrio anguillarum, is an important bacterial infection in Danish sea-reared rainbow trout. Despite of vaccination, outbreaks still occur, likely because the vaccine is based on V. anguillarum strains from abroad/other hosts than rainbow trout. Information about the genetic diversity of V. anguillarum specifically in Danish rainbow trout, is required to investigate this claim. Consequently, the aim of the present investigation was to sequence and to characterize a collection of 44 V. anguillarum strains obtained primarily from vibriosis outbreaks in Danish rainbow trout. The strains were sequenced, de novo assembled, and the genomes examined for the presence of plasmids, virulence, and acquired antibiotic resistance genes. To investigate the phylogeny, single nucleotide polymorphisms were identified, and the pan-genome was calculated. All strains carried tet(34) encoding tetracycline resistance, and 36 strains also contained qnrVC6 for increased fluoroquinolone/quinolone resistance. But interestingly, all strains were phenotypic sensitive to both oxytetracycline and oxolinic acid. Almost all serotype O1 strains contained a pJM1-like plasmid and nine serotype O2A strains carried the plasmid p15. The distribution of virulence genes was rather similar across the strains, although evident variance among serotypes was observed. Most significant, almost all serotype O2 and O3 strains, as well as the serotype O1 strain without a pJM1-like plasmid, carried genes encoding piscibactin biosynthesis. Hence supporting the hypothesis, that piscibactin plays a crucial role in virulence for pathogenic strains lacking the anguibactin system. The phylogenetic analysis and pan-genome calculations revealed great diversity within V. anguillarum. Serotype O1 strains were in general very similar, whereas considerable variation was found among serotype O2A strains. The great diversity within the V. anguillarum serotype O2A genomes is most likely the reason why vaccines provide good protection from some strains, but not from others. Hopefully, the new genomic data and knowledge provided in this study might help develop an optimized vaccine against V. anguillarum in the future to reduce the use of antibiotics, minimize economic losses and improve the welfare of the fish.

An understated danger: Antimicrobial resistance in aquaculture and pet fish in Switzerland, a retrospective study from 2000 to 2017

Aquaculture is a rapidly growing field of food production. However, morbidity and mortality are higher in aquaculture species than in domestic animals. Bacterial diseases are a leading cause of farmed fish morbidity and are often treated with antimicrobials. Since most Swiss fish farms release effluents directly into surface water without treatment and since aquaculture fish are consumed by humans, antimicrobial resistance (AMR) and multi-resistance in aquaculture fish are important for environmental and public health. In this study, AMR tests for 14 antimicrobials were performed on 1,448 isolates from 1,134 diagnostic laboratory submissions from farmed and ornamental fish submissions for the period from 2000 to 2017. Amoxicillin, gentamycin and norfloxacin had the lowest proportion of resistant samples. However, AMR was highly variable over time. Resistance proportions were higher in: (a) ornamental fish compared with farmed fish, (b) fish from recirculation systems compared with those from other farming systems and (c) isolates originating from skin compared with those originating from inner organs. Multiple resistances were common. The results of this study provide useful data for Swiss fish veterinarians and some interesting hypotheses about risk factors for AMR in aquaculture and pet fish in Switzerland. However, further research is needed to define risk factors.

Digital gene expression analysis in the liver of ScpB-vaccinated and Streptococcus agalactiae-challenged Nile tilapia

In recent years, streptococcal diseases have severely threatened the development of tilapia aquaculture, but effective prevention and control methods have not yet been established. To understand the immune responses of vaccinated Nile tilapia (Oreochromis niloticus), digital gene expression (DGE) technology was applied in this study to detect the gene expression profile of the Nile tilapia (O. niloticus) liver in response to ScpB (Streptococcal C5a peptidase from group B Streptococcus, ScpB) vaccination and a Streptococcus agalactiae-challenge. The control and the ScpB-vaccinated Nile tilapia yielded a total of 25,788,734 and 27,088,598 clean reads, respectively. A total of 1234 significant differentially expressed unigenes were detected (P < 0.05), of which 236 were significantly up-regulated, and 269 were significantly down-regulated (P < 0.05, |fold|>2, FDR<0.05). Of the differentially expressed gene, the identified genes which were enriched using databases of GO and KEGG could be categorized into a total of 67 functional groups and were mapped to 153 signaling pathways including 15 immune-related pathways. The differentially expressed genes (TLR1, TLR2, TLR3, TLR5, TLR9, MyD88, C3, IL-1β, IL-10) were detected in the expression profiles, and this was subsequently verified via quantitative real-time PCR (qPCR). The results of this study can serve as a basis for future research not only on the molecular mechanism of S. agalactiae invasion, but also on the anti-S. agalactiae mechanism in targeted tissues of Nile tilapia.

Distinct response of immune gene expression in peripheral blood leucocytes modulated by bacterin vaccine candidates in rainbow trout Oncorhynchus mykiss: A potential in vitro screening and batch testing system for vaccine development in aquaculture

Fish aquaculture is the world's fastest growing food production industry and infectious diseases are a major limiting factor. Vaccination is the most appropriate method for controlling infectious diseases and a key reason for the success of salmonid cultivation and has reduced the use of antibiotics. The development of fish vaccines requires the use of a great number of experimental animals that are challenged with virulent pathogens. In vitro cell culture systems have the potential to replace in vivo pathogen exposure for initial screening and testing of novel vaccine candidates/preparations, and for batch potency and safety tests. PBL contain major immune cells that enable the detection of both innate and adaptive immune responses in vitro. Fish PBL can be easily prepared using a hypotonic method and is the only way to obtain large numbers of immune cells non-lethally. Distinct gene expression profiles of innate and adaptive immunity have been observed between bacterins prepared from different bacterial species, as well as from different strains or culturing conditions of the same bacterial species. Distinct immune pathways are activated by pathogens or vaccines in vivo that can be detected in PBL in vitro. Immune gene expression in PBL after stimulation with vaccine candidates may shed light on the immune pathways involved that lead to vaccine-mediated protection. This study suggests that PBL are a suitable platform for initial screening of vaccine candidates, for evaluation of vaccine-induced immune responses, and a cheap alternative for potency testing to reduce animal use in aquaculture vaccine development.