Vaccination is crucial for Atlantic salmon farming, protecting against bacterial and viral infections, such as infectious salmon anemia (ISA) caused by infectious salmon anemia virus (ISAV). Salmon immune responses are highly temperature-dependent and optimal water temperatures are expected to impact vaccine efficacy, while those out of the optimal range may weaken immunity and compromise protection. Additionally, vaccination regime impacts the length of protection against most common pathogens. To assess the impact of temperature and dosing regime on commercial vaccination, we evaluated two commercial multivalent vaccines in salmon reared at 8°C, 12°C, and 15°C. Fish were intraperitoneally injected with 100 μL of either vaccine 1 (V1) or vaccine 2 (V2). Half of each group received a booster after, between 700 and 750-degree days (dd), including groups that were boosted with the opposite vaccination (V1/V2; V2/V1). After an additional ∼650 dd post boost (∼1400 dd post first vaccination), ten fish from each group were sampled for serum IgM detection. One week later, donor fish previously infected with ISAv (ISAV-HPR4 at TCID50 of 1 × 105/ml) were introduced to cohabitation tanks at a 6.5:1 ratio of cohabitants to donors. Post-infection, survival rate was recorded, and serum samples were collected for specific-IgM detection, as well as head kidney to determine ISAv load. The results show that elevated rearing temperatures (12-15°C) consistently enhanced specific antibody responses against ISAv, A. salmonicida, and V. anguillarum, whereas cold conditions (8°C) limited or delayed antibody-mediated immune responses in pre-exposed fish. Booster vaccinations and higher temperatures effectively increased and maintained IgM levels in pre-exposed Atlantic salmon, compensating for low-temperature suppression. Survival and viral load data further highlight the interaction between temperature and immune protection, with fish hold at warmer temperatures during vaccination exhibiting higher survival and more efficient ISAv clearance. These findings demonstrate that water temperature and vaccination strategy, including regimen and formulation, critically influence adaptive immunity in Atlantic salmon. Aligning vaccination protocols with seasonal and environmental conditions can maximize protection and limit pathogen persistence.

This study explored transcriptomic alterations in the skin and head kidney of Atlantic salmon (Salmo salar) from multiple families in response to infestation with the salmon louse (Lepeophtheirus salmonis) at the chalimus and adult stages, under elevated (20 degrees C) versus standard (10 degrees C) temperature conditions. Family-specific transcriptomic comparisons identified several candidate genes potentially associated with variation in host responses among families to sea lice, as reflected by differential expression patterns between families with varying lice densities. These biomarkers were primarily involved in immune function (e.g., cd209, cd22, btn2a1, h2-q9, c1qb, muc2, muc5ac, sntx-b, kbas), tissue repair (e.g., krt13, tgm2, mmp9, mmp13), inflammation (e.g., nlrp1, il1b, nlrc3, nlrp12), iron homeostasis (e.g., hbb1, ftm) and stress response (e.g., hspa8, herc3). Under elevated temperature conditions (20 degrees C), 124 and 324 shared differentially expressed genes (DEGs) were respectively identified in the skin and head kidney of lice-infested salmon. RNA-seq analysis revealed consistent modulation of several thermal stress biomarkers (e.g., serpinh1, hsp90aa1, hspa8, cirbp, fkbp10) in both chalimus- and adult-infested salmon under elevated temperature conditions, along with pronounced differences in the diversity of transcriptomic alterations and biological pathways compared to fish parasitized under standard temperature. Our results provide insights into molecular signatures that can help to differentiate more and less susceptible salmon families, supporting the development of selective breeding strategies for improved sea lice management. The temperaturerelated transcriptomic changes highlighted the climate adaptation strategies adopted by salmon, as well as their potential vulnerability to long-term shifts in water temperature.

Non-medicinal approaches for controlling sea lice (Lepeophtheirus salmonis) infestations, such as freshwater (FW) treatments, have been increasingly implemented to support more sustainable parasite control in aquaculture. This study evaluated the efficacy of FW treatment against sea lice under controlled experimental conditions and assessed the potential development of resistance across multiple generations. Sea lice populations were maintained over five consecutive generations, and FW efficacy was evaluated by quantifying lice abundance on Atlantic salmon before and after treatment. Resistance assessment involved comparative bioassays using copepodid and adult life stages from two distinct populations, one exposed to FW and the other a saltwater (SW) control group. Our results demonstrated that a 4 h FW treatment significantly reduced total lice abundance, while an extended 18-day exposure resulted in complete detachment of all lice from salmon. Importantly, bioassay comparisons revealed no evidence of resistance development, as both FW-exposed and SW-control populations maintained similar susceptibility across generations. The absence of measurable changes in salinity tolerance over multiple generations suggests that FW protocols may remain a viable and sustainable management strategy. However, the potential for genetic or epigenetic variation in FW tolerance among lice populations warrants further investigation to better understand long-term implications for treatment efficacy.

Hydrogen Peroxide is an oxidative chemical used as a topical treatment via bath exposures in salmonid aquaculture to mitigate external infections such as sea lice and amoebic gill disease. Protocols can depend on each individual site treatment, which varies in environmental temperature, dose concentration used and exposure time. Treatments such as this, in addition to environmental insults, increasing ocean temperatures and other farm management practices occurring simultaneously and/or chronically can lead to Complex Gill Disease (CGD). The purpose of this study was to assess the effects of hydrogen peroxide on the gills of Atlantic salmon (Salmo salar) following acute and chronic repeated exposure and dose and temperature dependent exposure. Our focus was to examine biological responses influencing gill function such as immune, healing, and redox reactions while understanding the cellular damage occurring in the gill. Histology and targeted gene expression through RT-qPCR analysis was used to examine these effects. Results showed minor to severe morphological changes in the gill, increasing damage in a linear relationship with increasing temperature and dose. The main morphological changes observed in the gill were lamellar epithelial lifting, lamellar edema, and lamellar fusion. Gene expression analysis revealed upregulation of genes associated with healing, redox, oxygen transport, cell death and DNA repair whereas suppression of immune responses genes was evident for both experimental groups. Differential regulation of genes associated with apparent adaptation to repeated hydrogen peroxide treatment could be useful in identifying markers to differentiate acute and chronic impact of treatment(s) that may not always de discerned through standard histological assessment.

Sea lice (Lepeophtheirus salmonis) infestation continues to pose a persistent and escalating challenge to the global salmon aquaculture industry. Given the complexity of host-parasite interactions, family-based transcriptomic studies provide crucial insights into genetic variation in host responses to sea lice, potentially guiding the development of selective breeding programs to manage parasite resistance in Atlantic salmon. This study investigated global gene expression (transcriptomic) responses of the skin and head kidney of Atlantic salmon (Salmo salar) from different families following infestation at two distinct stages of sea lice, chalimus II and adult, under varying temperature conditions (10°C and 20°C). RNA sequencing results revealed consistent expression of lice-responsive genes across different families under varying thermal conditions, which allowed the identification of potential biomarkers associated with adult-stage compared to chalimus-stage infestations. Our findings highlight critical physiological disruptions in salmon infested with advanced (adult) stages of lice, including uncontrolled and persistent inflammation, dampened/dysregulated immune responses, and impaired tissue repair at attachment sites. This study provides a comprehensive analysis of the transcriptomic responses of Atlantic salmon to different developmental stages of sea lice under specific temperature conditions (10°C and 20°C), and identifies several novel molecular markers from RNA-seq analysis that may be instrumental in developing targeted control strategies for this economically important parasite.

Sea lice infestations and winter ulcer disease caused by Moritella viscosa are two major challenges for the Atlantic salmon (Salmo salar) aquaculture industry. Despite their common tropisms for the skin/muscle we know very little about the interaction between these two significant pathogens. The objective of this study was to elucidate the transcriptomic response of Atlantic salmon to sea louse (Lepeophtheirus salmonis) and M. viscosa under single infection and different scenarios of co-infection (i.e. first, L. salmonis and then, M. viscosa and vice versa). After exposure, sampling was performed from the infestation site as well as the adjacent area on fish skin, followed by transcriptome analysis. RNA-seq analysis revealed that infection with either L. salmonis or M. viscosa induced a significant immune response from the skin and resulted in extensive transcriptomic changes. More differentially expressed genes (DEGs) were detected at M. viscosa lesion sites compared to L. salmonis attachment sites. In addition, the effect of the infection order was evaluated. Interestingly, we found that primary infection with lice was associated with a significantly higher number of DEGs during the co-infection process on fish skin compared to the fish first infected with M. viscosa (721 and 5336 DEGs from lice attachment and M. viscosa lesion sites, respectively, vs 291 and 3601 DEGs from lice attachment and M. viscosa lesion sites, respectively). We also found that lice infection caused localized effects on the skin of Atlantic salmon, while single infection with M. viscosa caused a moderate systemic impact and inhibited the tissue repair function of the skin, leading to severe ulceration. In addition, the C-type lectin receptor signaling pathway was drastically activated at the lice attachment sites during both lice alone- and co-infections, regardless of the order of infections, and similar results were observed in M. viscosa infection at both lesion and adjacent sites, indicating that this pathway played an essential role for immunity in Atlantic salmon. A more intense inflammatory and immune response was also observed at M. viscosa lesion sites. These results will promote our understanding of the immune interactions between L. salmonis and M. viscosa during the co-infection process and provide insights for the development of preventive and treatment strategies for these pathogens in salmon aquaculture.

Due to the nature of open-pen farming, salmon are exposed to numerous pathogens shared by other farms and their wild counterparts. Industry must manage these outbreaks through vaccination, oral or bath treatments, and more recently through functional feed administration. Globally, the most important pathogens of salmon are sea lice ( Lepeophtheirus salmonis , and Caligus rogercresseyi ) not only due to their direct impacts on the host, but indirectly by enhancing host susceptibility to co-infection. This study aims to characterize molecular responses during a co-infection of L. salmonis and a salmon orthomyxovirus (infectious salmon anemia virus; ISAv) under administration of four functional feed diets: a control feed with a low 0.3 % EPA/DHA + high- omega 6 (Ctrl), an EPA/ DHA enriched pro-inflammatory diet 1 % EPA/DHA + high- omega 6; FA + I), an EPA/DHA enriched antiinflammatory diet (1 % EPA/DHA+high- omega 3; FA-I), and a low EPA/DHA feed (0.3 %) with an immunostimulant added (0.3 % EPA/DHA + high- omega 6 + immunostimulant (IS); Ctrl + IS). Atlantic salmon (40 fish per tank; 8 tanks per feed) were acclimated to one of the four experimental diets. Sea lice copepodids were introduced to all experimental tanks and 10 fish sampled from each tank at each time point (prior to infection [-3], and 11, 33, 47 days post infection). A high virulence ISAv isolate (ISAV-HPR4) was intraperitoneally injected into donor fish 6 days prior to their transfer into the experimental tanks for cohabitation (ca. 10 - 15 % of tank density; 4 tanks per feed group) to achieve peak shedding rates at time of stocking. Fatty acid enriched (FA + I and FA-I) diets had a significant impact on sea lice abundance on fish infected with lice only. Gene expression profiles measured by reverse transcriptase-qPCR showed significant upregulation in several antiviral genes ( irf7b and mxb ) associated with the interferon system in all but the fish fed the immunostimulating diet. An increase in transcript levels ( irf7b, isg15a, mmp-9, mxb ) accounted for high lice and high viral loads in diets FA + I and Ctrl + IS. As feeds successful at reducing lice were the least successful in survival of co-infected fish, there appears to be a trade-off for better anti-parasitic responses, which is enhanced through dietary supplementation. This will require further research to ensure careful selection of feed combinations and expected co-infection challenges.

Moritella viscosa (M. viscosa) and sea lice (Lepeophtheirus salmonis) are severe pathogens that primarily infect the skin of Atlantic salmon (Salmo salar), which cause significant economic losses in the farming industry. However, the pathogenesis and molecular mechanisms underlying the host's immune defence at the post-transcriptional level remain unclear. Alternative splicing (AS) is an evolutionarily conserved post-transcriptional mechanism that can greatly increase the richness of the transcriptome and proteome. In this study, transcriptomic data derived from skin tissues of Atlantic salmon after M. viscosa and sea lice infections were used to examine the AS profiles and their differential expression patterns. In total, we identified 33,044 AS events (involving 13,718 genes) in the control (CON) group, 35,147 AS events (involving 14,340 genes) in the M. viscosa infection (MV) group, and 30,364 AS events (involving 13,142 genes) in the sea lice infection (LC) group, respectively. Among the five types of AS identified in our study (i.e., SE, A5SS, A3SS, MXE, and RI), SE was the most prevalent type in all three groups (i.e., CON, MV, and LC groups). Decreased percent-spliced-in (PSI) levels were observed in SE events under both MV- and LC-infected conditions, suggesting that MV or LC infection elevated exon-skipping isoforms and promoted the selection of shorter transcripts in numerous DAS genes. In addition, most of the differential AS genes were found to be associated with pathways related to mRNA regulation, epithelial or muscle development, and immune response. These findings provide novel insights into the role of AS in host-pathogen interactions and represent the first comparative analysis of AS in response to bacterial and parasitic infections in fish.

Antimicrobial peptides have been isolated from various organisms and play an essential role in defense against infections. They are small peptides (less than 60 amino acids) with broad-spectrum antibacterial activity. In teleosts, pituitary adenylate cyclase-activating polypeptide (PACAP) has been demonstrated to have direct antimicrobial activity against several aquatic pathogens, including those from the genus Flavobacterium. Our goal was to examine the impacts of PACAP and route of administration on the immune response of Nile tilapia (Oreochromis niloticus). Over the course of four studies, tilapia (416.1 ± 116.7 g) randomly assigned to replicate tanks were administered with either PACAP-38 or a modified form of PACAP-38 via intraperitoneal (i.p.) injection, bath, nares flush, or gill flush, and compared to PBS controls. In the first two studies, following individual treatments, tilapia underwent a bath exposure (40 L tank for 45 min) to F. columnare (isolate ALG-00-530; at 2.1 × 108 CFU/ml) or sham exposure without the addition of the bacterial culture. Fish were sampled before exposure, 48 h after stimulation, at 1 day after the onset of mortality in exposed tanks, and resolution of mortality. Tilapia that received i.p. injection of PACAP-38 showed significantly lower mortality from F. columnare (10 %) than those receiving PBS i.p. (25 %). However, bath immersion of fish, both with and without F. columnare, resulted in significant mortality due to secondary infections with Edwardsiella tarda. Administration of modified PACAP-38 via nares/gill flush, or i.p. injection did not, however, result in significantly fewer mortalities compared to sham/PBS. Furthermore, the same modified form of PACAP-38 also induced inflammatory gene expression in the spleen, and eosinophilic granule cell aggregation in the nares, following flushing. The data from the first study suggest that PACAP-38 induces protection against infection and stress mainly through a reduction of inflammatory-il1β expression. Moreover, data analysis of spleen samples from these two studies also indicates that PACAP-38 might have more anti-inflammatory impacts than its modified form. In the fourth trial, after exposing tilapia to low temperature (15–17 °C) for 30 min (cold stress), the PACAP-38 treated group had significantly lower cortisol levels compared to both PBS injected group and the negative control. Due to the observed impacts of PACAP-38 treatment on the immunophysiology of Nile tilapia, its potential for therapeutic use should be further investigated.

Ocean temperatures continue to rise annually due to the ever-growing consequences of global climate change. These temperature changes can have an impact on the immunological robustness of cultured fish, especially coldwater species such as Atlantic salmon. The salmon farming industry already loses hundreds of millions of dollars each year to infectious and non-infectious diseases. One particularly important and WOAH reportable disease is infectious salmon anemia caused by the orthomyxovirus ISAv. Considering the changing environment, it is necessary to find ways to mitigate the effect of diseases on the industry. For this study, 20 Atlantic salmon families were housed in each of 38 different tanks at the AVC, with half of the fish being kept at 10 & DEG;C and half being kept at 20 & DEG;C. Donor Atlantic salmon IP- injected with a highly virulent ISAv isolate (HPR4; TCID50 of 1 x 105/mL) were added to each tank as the source of co-habitation infection. Both temperatures were sampled at onset of mortality in co-habited fish and at resolution of mortality. Family background and temperature significantly impacted ISAv load, as assessed by qPCR, time to mortality and overall mortality. Mortality was more acute at 20 & DEG;C, but overall mortality was higher at 10 & DEG;C. Based on percent mortality calculated over the course of the study, different families demonstrated different levels of survival. The three families that demonstrated the highest percent mortality, and the three families with the lowest percent mortality were then assessed for their antiviral responses using relative gene expression. Genes significantly upregulated between the unexposed fish and ISAv exposed fish included mx1, il4/13a, il12rb2, and trim25, and these were further impacted by temperature. Understanding how ISAv resistance is impacted by temperature can help identify seasonal risks of ISAv outbreaks as well as ideal responses to be targeted through immunopotentiation.

Sea lice (Lepeophtheirus salmonis) and infectious salmon anemia virus (ISAv) are two of the most important pathogens in Atlantic salmon (Salmo salar) farming and typically cause substantial economic losses to the industry. However, the immune interactions between hosts and these pathogens are still unclear, especially in the scenario of co-infection. In this study, we artificially infected Atlantic salmon with sea lice and ISAv, and investigated the gene expression patterns of Atlantic salmon head kidneys in response to both lice only and coinfection with lice and ISAv by transcriptomic analysis. The challenge experiment indicated that co-infection resulted in a cumulative mortality rate of 47.8 %, while no mortality was observed in the lice alone infection. We identified 240 differentially expressed genes (DEGs) under the lice alone infection, of which 185 were downregulated and 55 were up-regulated, while a total of 994 DEGs were identified in the co-infection, of which 206 were down-regulated and 788 were significantly up-regulated. The pathway enrichment analysis revealed that single-infection significantly suppressed the innate immune system (e.g., the complement system), whereas coinfection induced a strong immune response, leading to the activation of immune-related signaling pathways such as Toll-like receptors and NOD-like receptors pathways, as well as significant upregulation of genes related to the activation of interferon and MH class I protein complex. Our results provide the first global transcriptomic study of gene expression in the Atlantic salmon head kidney in response to co-infection with sea lice and ISAv, and provided the baseline knowledge for understanding the immune responses during co-infection.

Hemiurid digeneans conspecific with Stomachicola muraenesocis Yamaguti, 1934 (the type species of the genus Stomachicola Yamaguti, 1934) were collected from the stomach of the daggertooth pike conger Muraenesox cinereus (Forsskål) off the Persian Gulf of Iran. This study aimed to provide a detailed characterization of Stom. muraenesocis , including measurements, illustrations and scanning electron microscopy ( s.e.m. ) representations. Comparisons with the original and previous descriptions revealed morphological and metrical variations in several features (i.e. body size and shape, arrangement of reproductive organs, soma to ecsoma length ratio, position of genital opening, number of vitelline tubules and extension of uterine coils) between Stom. muraenesocis from different hosts and localities. This study presents the first molecular sequence data associated with the small (18S) and large (28S) subunit nuclear ribosomal RNA genes (rDNA) for Stom. muraenesocis . Phylogenetic analyses of the 18S dataset placed Stom. muraenesocis as sister lineage to a clade formed of a group of species of Lecithaster Lühe, 1901 (Lecithasteridae Odhner, 1905). In contrast, phylogenetic analyses based on the 28S consistently recovered a sister relationship between Stom. muraenesocis and representatives of the Hemiuridae Looss, 1899. Further comprehensive phylogenetically based classification in light of morphology and taxonomic history of the Hemiuridae and Lecithasteridae is required to infer phylogenetic affinities and historical biogeography of Stomachicola . A comprehensive list of previously reported species of Stomachicola together with their associated hosts, localities and morphometric data is provided.

Sea lice ( Lepeophtheirus salmonis ) are ectoparasitic copepods that cause significant economic loss in marine salmoniculture. In commercial salmon farms, infestation with sea lice can enhance susceptibility to other significant pathogens, such as the highly contagious infectious salmon anemia virus (ISAv). In this study, transcriptomic analysis was used to evaluate the impact of four experimental functional feeds (i.e. 0.3% EPA/DHA+high-ω6, 0.3% EPA/DHA+high-ω6+immunostimulant (IS), 1% EPA/DHA+high-ω6, and 1% EPA/DHA+high-ω3) on Atlantic salmon ( Salmo salar ) during a single infection with sea lice ( L. salmonis ) and a co-infection with sea lice and ISAv. The overall objectives were to compare the transcriptomic profiles of skin between lice infection alone with co-infection groups and assess differences in gene expression response among animals with different experimental diets. Atlantic salmon smolts were challenged with L. salmonis following a 28-day feeding trial. Fish were then challenged with ISAv at 18 days post-sea lice infection (dpi), and maintained on individual diets, to establish a co-infection model. Skin tissues sampled at 33 dpi were subjected to RNA-seq analysis. The co-infection’s overall survival rates were between 37%-50%, while no mortality was observed in the single infection with lice. With regard to the infection status, 756 and 1303 consensus differentially expressed genes (DEGs) among the four diets were identified in “lice infection vs. pre-infection” and “co-infection vs. pre-infection” groups, respectively, that were shared between the four experimental diets. The co-infection groups (co-infection vs. pre-infection) included up-regulated genes associated with glycolysis, the interferon pathway, complement cascade activity, and heat shock protein family, while the down-regulated genes were related to antigen presentation and processing, T-cell activation, collagen formation, and extracellular matrix. Pathway enrichment analysis conducted between infected groups (lice infection vs. co-infection) resulted in several immune-related significant GO terms and pathways unique to this group, such as “autophagosome”, “cytosolic DNA-sensing pathway” and “response to type I interferons”. Understanding how experimental functional feeds can impact the host response and the trajectory of co-infections will be an essential step in identifying efficacious intervention strategies that account for the complexities of disease in open cage culture.

Hydrogen peroxide (H2O2) is used to treat sea lice infections of farmed salmonids in the Atlantic and Pacific Oceans and issues with resistance to this treatment, and others are a major threat to the sustainability of the industry. The objectives of this study were to determine how H2O2 exposure affects survival and antioxidant-related gene expression in salmon lice (Lepeophtheirus salmonis) collected from the Bay of Fundy, New Brunswick. The maximum recommended dose of H2O2 is 1,800 mg/L, while the EC50 values (with 95% CI) for the population tested were 1,486 (457, 2,515) mg/L for males and 2,126 (984, 3,268) mg/L for females. Neither temperature nor pretreatment with emamectin benzoate (EMB) impacted survival after H2O2 exposure. RT-qPCR was performed on pre-adult sea lice exposed to H2O2 and showed that four genes classically involved in the response to oxidative stress were unchanged between treated and control groups. Seven genes were found to be significantly upregulated in males and one in females. This is the first report on the efficacy and molecular responses of Atlantic Canada sea lice to H2O2 treatment.

This study was conducted to determine the effects of a co-infection with Moritella viscosa at different exposure levels of sea lice Lepeophtheirus salmonis in Atlantic salmon (Salmo salar). M. viscosa (1.14 x 10(6) cfu/ml) was introduced to all experimental tanks at 10 days post-lice infection (dpLs). Mean lice counts decreased over time in both the medium lice co-infection (31.5 +/- 19.0 at 7 dpLs; 16.9 +/- 9.3 at 46 dpLs) and high lice co-infection (62.0 +/- 10.8 at 7 dpLs; 37.6 +/- 11.3 at 46 dpLs). There were significantly higher mortalities and more severe skin lesions in the high lice co-infected group compared to medium lice co-infected group or M. viscosa-only infection. Quantitative gene expression analysis detected a significant upregulation of genes in skin from the high lice co-infection group consistent with severe inflammation (il-8, mmp-9, hep, saa). Skin lesions retrieved throughout the study were positive for M. viscosa growth, but these were rarely located in regions associated with lice. These results suggest that while M. viscosa infection itself may induce skin lesion development in salmon, co-infection with high numbers of lice can enhance this impact and significantly reduce the ability of these lesions to resolve, resulting in increased mortality.

Pyroptosis, a type of programmed cell death that along with inflammation, is mainly regulated by two main pathways, cysteinyl aspartate specific proteinase (caspase)-1-induced canonical inflammatory pathway and caspase-11-induced non-canonical inflammatory pathway. The non-canonical inflammatory pathway-induced pyroptosis is a unique immune response in response to gram-negative (G−) bacteria. It is induced by lipopolysaccharide (LPS) on the surface of G− bacteria. This activates caspase-11 which, in turn, activates a series of downstream proteins eventually forming protein pores on the cell membrane and inducing cell sacrificial processes. Caspase-11 belongs to the caspase family and is an homologous protein of caspase-1. It has the ability to specifically hydrolyze proteins, but it is still unclear how it regulates cell death caused by non-canonical inflammatory pathways. The present study describes a pathway that enables LPS to directly enter the cell and activate caspase-11, and the key role caspase-11 plays in the activation of pyroptosis and inflammation.

With respect to salmonid aquaculture, one of the most important bacterial pathogens due to high mortality and antibiotic usage is the causative agent of typical furunculosis, Aeromonas salmonicida spp. salmonicida (Asal). In Atlantic salmon, Salmo salar, the host response during infections with Asal is well-documented, with furunculosis outbreaks resulting in significant mortality in commercial settings. However, less is known about the host-pathogen interactions in the emerging aquaculture species, Arctic charr Salvelinus alpinus. Furthermore, there is no data on the efficacy or response of this species after vaccination with commonly administered vaccines against furunculosis. To this end, we examined the immunological response of S. alpinus during infection with Asal, with or without administration of vaccines (Forte Micro®, Forte Micro® + Renogen®, Elanco Animal Health). Artic charr (vaccinated or unvaccinated) were i.p.-injected with a virulent strain of Asal (106 CFUs/mL) and tissues were collected pre-infection/post-vaccination, 8, and 29 days post-infection. Unvaccinated Arctic charr were susceptible to Asal with 72% mortalities observed after 31 days. However, there was 72–82% protection in fish vaccinated with either the single or dual-vaccine, respectively. Protection in vaccinated fish was concordant with significantly higher serum IgM concentrations, and following RNA sequencing and transcriptome assembly, differential expression analysis revealed several patterns and pathways associated with the improved survival of vaccinated fish. Most striking was the dramatically higher basal expression of complement/coagulation factors, acute phase-proteins, and iron hemostasis proteins in pre-challenged, vaccinated fish. Remarkably, following Asal infection, this response was abrogated and instead the transcriptome was characterized by a lack of immune-stimulation compared to that of unvaccinated fish. Furthermore, where pathways of actin assembly and FcγR-mediated phagocytosis were significantly differentially regulated in unvaccinated fish, vaccinated fish showed either the opposite regulation (ForteMicro®), or no impact at all (ForteMicro®Renogen®). The present data indicates that vaccine-induced protection against Asal relies on the pre-activation and immediate control of humoral immune parameters that is coincident with reduced activation of apoptotic (e.g., NF-κB) and actin-associated pathways.

Arctic charr (Salvelinus alpinus) farming is an emerging component in today's aquaculture sector; however, intensive aquaculture sites provide optimal conditions for disease. Although vaccines have been developed and tested for other major cultured salmonids, such as Atlantic salmon (Salmo salar), it is unknown if such vaccines provide protection in Arctic charr. Therefore, the present study aimed to evaluate the efficacy of two vaccines, Forte Micro® and Renogen®, administered solely or in combination, in the prevention of atypical furunculosis in Arctic charr after challenge with atypical Aeromonas salmonicida (RPC strain FFA-43). Furthermore, vaccination efficacy was assessed after short or long post-vaccination periods prior to bacterial challenge. Improved survival was observed in charr vaccinated with both single and combined vaccines in the short-term efficacy study, and this positively correlated with increased circulating IgM specific for atypical A. salmonicida. However, circulating IgM was reduced in the long-term study. Forte Micro® provided improved protection when Arctic charr were exposed to infection in the short-term vaccine efficacy compared to the long-term vaccine efficacy, where no significant protection was observed. Gene expression analysis revealed upregulation of complement, acute phase, and cellular signalling post-vaccination and post-challenge for fish in the short-term vaccine efficacy study, with similar to muted responses observed in fish in the long-term vaccine efficacy study. This study provides insight on the efficacy and molecular mechanisms of protection of two commercial salmonid vaccines in Arctic charr during infections with atypical A. salmonicida.

The avermectins, emamectin benzoate (EMB) and ivermectin (IVM) have been commonly used in North America over the last two decades to control the salmon louse, Lepeophtheirus salmonis, infections in farmed Atlantic salmon. Emamectin benzoate, trade name SLICE (TM), was used heavily in the Eastern Canadian industry between the years 2000-2008, due to its long lasting protection and efficacy against all parasitic life stages. However, over reliance on this drug soon resulted in reduced sensitivity in many L. salmonis populations, resulting more recently in uses of higher treatment dosages and switching to the use of IVM. For these reasons, we investigated the effects of different dosages of EMB and multiple IVM treatments on baseline immunophysiological indicators, anti-viral responses and protection against subsequent salmon lice exposure in salmon smolts. Different doses of EMB or repeated treatment with IVM did not affect feeding behaviour in salmon, however by the end of the second IVM treatment, some neurotoxicity was observed. A single (1 x) EMB dose (50 mu g/Kg) administered for 7 consecutive days had no significant effect on L. salmonis abundance and development, whereas triple the dosage (150 mu g/Kg) significantly reduced lice development, thereby eliminating subsequent stress responses in salmon associated with lice development to pre-adult stages. Emamectin benzoate and IVM treatment did not significantly impact expression of resting antigen presentation molecules in salmon (MH class I or II), however they did inhibit short-term (6 h) induced responses to the ISA virus. The impact of gender, as previously shown, had the greatest effect on louse transcriptomic regulation, but avermectin treatment also caused perturbations in gene expression. Transcriptome differences between lice on control and 1 x EMB treated fish were larger than those observed for IVM or 3 x EMB. Nearly half of the transcripts differentially expressed by IVM were also affected by one of the EMB treatments. Transcriptomic results from the louse suggest a high degree of similarity and concordance within and across studies in avermectin treatment, with gender of louse and dosage of drug significantly impacting the outcomes.