Characterization of Aeromonas salmonicida and A. sobria isolated from cultured salmonid fish in Korea and development of a vaccine against furunculosis

Previously, Aeromonas sobria and A. salmonicida were identified to be the most prevalent species in salmonid farms in Korea. In this study, we evaluated the biochemical characteristics, antibiotic susceptibility and pathogenicity of A. salmonicida (3 isolates) and A. sobria (8 isolates) isolated from salmonids, and further investigated efficacy of A. salmonicida vaccine. In antibiotic susceptibility test, all of A. sobria isolates were resistant to amoxicillin and ampicillin. Six A. sobria and two A. salmonicida isolates were resistant to oxytetracycline. In challenge test, A. sobria isolates exhibited low pathogenicity in rainbow trout (Oncorhynchus mykiss) while one A. salmonicida isolate showed high pathogenicity with LD₅₀ of 6.4 × 10³  CFU/fish in rainbow trout and coho salmon (Oncorhynchus kisutch). Among virulence factors, secretion apparatus (ascV and ascC) and transcription regulatory protein (exsA) of type 3 secretion system and A-layer protein genes were differentially detected in DNA or cDNA of A. salmonicida isolates, indicating their contribution to the pathogenicity. A formalin-killed vaccine of highly pathogenic A. salmonicida isolate exhibited a protective effect with relative survival rate of 81.8% and 82.9% at 8 weeks and 16 weeks post-vaccination, respectively, in challenge test.

A pentavalent vaccine for rainbow trout in Danish aquaculture

Mariculture in Denmark is based on production of rainbow trout grown two years in fresh water followed by one growth season in sea cages. Although the majority of rainbow trout are vaccinated against the most serious bacterial pathogens - Aeromonas salmonicida subsp. salmonicida, Vibrio anguillarum and Yersinia ruckeri, by the use of commercially available vaccines, disease outbreaks requiring treatment with antibiotics still occur. The present study tested the potential of a new experimental multicomponent vaccine that is based on local bacterial strains, isolated from rainbow trout in Danish waters, and thus custom-designed for Danish rainbow trout mariculture. The vaccination with the multicomponent vaccine resulted in protection against three relevant bacterial diseases (yersiniosis, furunculosis, vibriosis) under experimental conditions. We showed that i.p. injection of the vaccine induced specific antibody responses in trout against the different bacterial antigens and regulated expression of genes encoding SAA, C3, IL-1β, IL-6, IL-8, IgD and MHCII.

Four selenoprotein P genes exist in salmonids: Analysis of their origin and expression following Se supplementation and bacterial infection

The following research was conducted to elucidate the evolution and expression of salmonid selenoprotein P (SelP), a selenoprotein that is unique in having multiple selenocysteine (Sec) residues, following supranutritional selenium supplementation and infection in rainbow trout. We show that in salmonids SelP is present as four paralogues and that the diversification of SelP genes during vertebrate evolution relates to whole genome duplication events. With 17 and 16 selenocysteine residues for rainbow trout (Oncorhynchus mykiss)/Atlantic salmon (Salmo salar) SelPa1 and SelPa2 proteins respectively and 1 or 2 (trout or salmon) and 4 or 3 (trout or salmon) selenocysteine residues for salmonid SelPb1 and SelPb2 proteins respectively, this is the highest number of (predicted) multiple selenocysteine containing SelP proteins reported for any vertebrate species to date. To investigate the effects of selenium form on SelP expression we added different concentrations (1 nM– 10 μM) of organic or inorganic selenium to a trout cell line (RTG-2 cells) and analysed changes in mRNA abundance. We next studied the impact of supplementation on the potential modulation of these transcripts by PAMPs and proinflammatory cytokines in RTG-2 and RTS-11 cells. These experiments revealed that selenium type influenced the responses, and that SelP gene subfunctionalisation was apparent. To get an insight into the expression patterns in vivo we conducted a feeding trial with 2 diets differing in selenium content and 5 weeks later challenged the trout with a bacterial pathogen (Aeromonas salmonicida). Four tissues were analysed for SelP paralogue expression. The results show a significant induction of SelPa1 in gills and intestine following infection in selenium supplemented fish and for SelPa2 in gills. SelPb1 was significantly reduced in head kidney of both diet groups following infection, whilst SelPb2 was significantly upregulated in skin of both diet groups post infection. Overall these findings reveal differential expression profiles for the SelPa/SelPb paralogues in trout, influenced by selenium supply, cell type/tissue and stimulant. The increase of multiple Sec containing SelP proteins in salmonids could indicate an enhanced requirement for selenium in this lineage.

Pharmacokinetic model of florfenicol in turbot (Scophthalmus maximus): establishment of optimal dosage and administration in medicated feed

The pharmacokinetics of florfenicol (FF) in turbot (Scophthalmus maximus) was studied after single intravenous (10 mg kg^-1 ) and oral (100 mg kg^-1 ) administration. The plasma concentration-time data of florfenicol were described by an open one-compartment model. The elimination half-life (t_1/2 ) was estimated to be 21.0 h, and the total body clearance, Cl, was determined as 0.028 L kg h^-1 . The apparent volume distribution (V_d ) was calculated to be 0.86 L kg^-1 and the mean residence time (MRT_iv ) was 30.2 h. Following oral administration, the maximum plasma concentration (C_max ) of 55.4 μg mL^-1 was reached at 12 h (T_max ). The absorption constant (k_a ) was 0.158 h^-1 . The bioavailability was estimated to be 57.1%. The low bioavailability observed at higher doses was explained by the saturation of the mechanisms of absorption. The drug absorption process was limited by its inherent low solubility, which limited the amount of available FF absorbed in the gastrointestinal tract. Based on the pharmacokinetic data, an optimal dosing schedule for FF administration is hereby provided. Based on the minimum inhibitory concentration found for susceptible strains of Aeromonas salmonicida, oral FF administration of first, an initial dose of 30 mg FF kg^-1 , followed by 6 maintenance doses at 18 mg kg^-1 /daily could be effective against furunculosis in turbot.

The effects of increasing dietary levels of soy protein concentrate (SPC) on the immune responses and disease resistance (furunculosis) of vaccinated and non-vaccinated Atlantic salmon (Salmo salar L.) parr

Juvenile salmon, with an initial weight of 9 g, were fed three experimental diets, formulated to replace 35 (SPC35), 58 (SPC58) and 80 (SPC80) of high quality fishmeal (FM) with soy protein concentrate (SPC) in quadruplicate tanks. Higher dietary SPC inclusion was combined with increased supplementation of methionine, lysine, threonine and phosphorus. The experiment was carried out for 177 days. On day 92 salmon in each tank were bulk weighed. Post weighing eighty salmon from each tank were redistributed in two sets of 12 tanks. Salmon from the first set of tanks were vaccinated, while the second group was injected with phosphate buffer saline (PBS). Salmon were sampled on day 92 (pre-vaccination), day 94 (2 days post vaccination [dpv]/PBS injection [dpPBSinj]) and day 154 (62 dpv/dpPBSinj) of the trial for the assessment of their immune responses, prior to the performance of salmon bulk weights for each tank. On day 154, fish from each tank were again bulk weighed and then seventeen salmon per tank were redistributed in two sets of twelve tanks and intra-peritoneally infected with Aeromonas salmonicida. At Day 154, SPC80 demonstrated lower performance (weight gain, specific growth rate and thermal growth coefficient and feed conversion ratio) compared to SPC35 salmon. Reduced classical and total complement activities for salmon fed diets with over 58% of protein from SPC, were demonstrated prior to vaccination. Reduced alternative complement activity was detected for both SPC58 and SPC80 salmon at 2 dpv and for the SPC80 group at 62 dpv. Total and classical complement activities demonstrated no differences among the dietary groups after vaccination. Numerical increases in classical complement activity were apparent upon increased dietary SPC levels. Increased phagocytic activity (% phagocytosis and phagocytic index) was exhibited for the SPC58 group compared to SPC35 salmon at 62 dpPBSinj. No differences in serum lysozyme activity, total IgM, specific antibodies, protein, glucose and HKM respiratory burst were detected among the dietary groups at any timepoint or state. Mortalities as a result of the experimental infection only occurred in PBS-injected fish. No differences in mortality levels were demonstrated among the dietary groups. SPC58 diet supported both good growth and health in juvenile Atlantic salmon while SPC80 diet did not compromise salmon' immunity or resistance to intraperitoneally inflicted furunculosis.

A comparison of the response of diploid and triploid Atlantic salmon (Salmo salar) siblings to a commercial furunculosis vaccine and subsequent experimental infection with Aeromonas salmonicida

Sterile triploid fish represent a solution to the problems associated with sexual maturation and escapees in aquaculture. However, as disease outbreaks continue to cause significant economic losses to the industry, it is essential that the response of triploids to disease and disease treatments be characterised. The aim of this study was to compare the response of triploid Atlantic salmon to a commercial furunculosis vaccine with that of diploid fish, and to assess the vaccine efficacy in the two ploidies through an experimental infection with Aeromonas salmonicida. Diploid and triploid Atlantic salmon were injected intraperitoneally with either phosphate buffered saline, liquid paraffin adjuvant or a commercial furunculosis vaccine. Following vaccination, growth, adhesion scores and a variety of assays to assess immune function, such as respiratory burst and antibody response, were measured. Vaccination did not have a significant effect on the weight of either ploidy prior to challenge at 750° days. Adhesion scores were significantly higher in vaccinated fish compared to unvaccinated fish, although no effect of ploidy was observed. Ploidy significantly affected respiratory burst activity following vaccination, however, with triploids exhibiting higher activity than diploids. Combined with lower white blood cell numbers observed in the triploids, it may be that this low cell number is compensated for by increased cellular activity. Ploidy however, did not have a significant effect on complement activity or antibody response, with significantly higher antibody levels detected in all vaccinated fish compared to unvaccinated controls. In addition, both ploidy groups were well protected following challenge with no difference in the relative percentage survival. Based on these results, it appears that ploidy does not affect the severity of adhesions that result post-vaccinate or in the fish's immune response following vaccination, and the furunculosis vaccine performs equally well in both diploid and triploid Atlantic salmon.

Aeromonas salmonicida infection levels in pre- and post-stocked cleaner fish assessed by culture and an amended qPCR assay

Due to increasing resistance to chemical therapeutants, the use of 'cleaner fish' (primarily wrasse, Labridae, species) has become popular in European salmon farming for biocontrol of the salmon louse, Lepeophtheirus salmonis (Krøyer). While being efficient de-licers, cleaner fish mortality levels in salmon cages are commonly high, and systemic bacterial infections constitute a major problem. Atypical furunculosis, caused by Aeromonas salmonicida A-layer types V and VI, is among the most common diagnoses reached in clinical investigations. A previously described real-time PCR (qPCR), targeting the A. salmonicida A-layer gene (vapA), was modified and validated for specific and sensitive detection of all presently recognized A-layer types of this bacterium. Before stocking and during episodes of increased mortality in salmon cages, cleaner fish (primarily wild-caught wrasse) were sampled and screened for A. salmonicida by qPCR and culture. Culture indicated that systemic bacterial infections are mainly contracted after salmon farm stocking, and qPCR revealed A. salmonicida prevalences of approximately 4% and 68% in pre- and post-stocked cleaner fish, respectively. This underpins A. salmonicida's relevance as a contributing factor to cleaner fish mortality and emphasizes the need for implementation of preventive measures (e.g. vaccination) if current levels of cleaner fish use are to be continued or expanded.

Tissue-specific responses of oxidative stress biomarkers and antioxidant defenses in rainbow trout Oncorhynchus mykiss during a vaccination against furunculosis

The present study was conducted to evaluate the effects of vaccination against furunculosis on responses of oxidative stress and antioxidant defenses in rainbow trout Oncorhynchus mykiss muscle, gills, liver, and brain tissues. The oxidative stress markers (malondialdehyde and carbonyl derivatives of protein oxidative destruction levels), antioxidant defenses (superoxide dismutase, catalase, glutathione reductase, and glutathione peroxidase), and total antioxidant capacity in different tissues of rainbow trout were measured. Our data showed that exposure of trout to vaccine against furunculosis produced changes (either increase or decrease) in oxidative stress and antioxidant enzymes responses, and these responses showed marked organ differences, associated with tissue patterns. Our study demonstrated that vaccinated trout showed alteration in antioxidant defenses and oxidative stress responses, with higher severity in the liver, compared with other tissues. Our data also suggest that vaccination against furunculosis induced lipid peroxidation in gill and liver tissues. However, muscle and brain tissue are capable of restoring its pro- and antioxidant balance after vaccination.

Long-lasting protection induced by bath vaccination against Aeromonas salmonicida subsp. salmonicida in rainbow trout

For decades Aeromonas salmonicida subsp. salmonicida (from here referred to as A. salmonicida) has been recognized as the causative agent of typical furunculosis. This disease has had a major impact on aquaculture worldwide, making it a target for international research, particularly within the field of immunoprohylaxis. Initial studies attempted vaccination via oral route and immersion. However, these vaccination methods proved insufficient when compared to intraperitoneally (i.p.) injected vaccines. The focus of vaccine research regarding A. salmonicida shifted towards the i.p.-injected vaccines during the 1980's and -90's, resulting in oil-adjuvanted vaccines providing high levels of protection over longer periods of time. The majority of this research has been conducted using salmon, while rainbow trout, which is also a commercially important species, has played a much less central role. In this study, we have examined the effect of a bath vaccination using an experimental A. salmonicida bacterin. Rainbow trout were vaccinated by a 5 min bath in a formalin-inactivated bacterin. Half of these fish was booster vaccinated using 50% of the initial vaccine dose 10 weeks post primary immunization. Along with an un-vaccinated control group, the fish were challenged by waterborne infection 24 weeks post primary immunization. Both vaccinated groups showed a significantly increased survival (>93% survival) compared to a 70% survival in the un-vaccinated control group (P = 0.005 and P = 0.019 for single and dual immunizations, respectively). When comparing the survival of the single and dual immunization groups, there was no significant difference (P = 0.531). ELISA showed no significant induction of specific circulating antibodies in either vaccinated group. These results are interesting with regard to the protective mechanisms, seen in the light of previous results obtained using bath as well as i.p. vaccination against furunculosis in salmonid fishes.

Quantitative genetics of disease resistance in vaccinated and unvaccinated Atlantic salmon (Salmo salar L.)

Furunculosis (Aeromonoas salmonicida) is an important disease in Atlantic salmon (Salmo salar) farming. Vaccination and selective breeding for increased resistance to the disease on the basis of challenge tests of unvaccinated fish are used as complementary prophylactic methods. An important issue is whether genetic predisposition to infection is consistent across vaccinated and unvaccinated fish. Hence, the main objective of this study was to determine the magnitude of the genetic associations (correlations) between resistance to furunculosis in vaccinated and unvaccinated fish, and to estimate the magnitude of the correlation of resistance to furunculosis with resistance to the viral diseases infectious pancreatic necrosis (IPN) and infectious salmon anaemia (ISA). Sub-samples of unvaccinated and vaccinated salmon from 150 full-sib families were subjected to separate cohabitation challenge tests. Substantial genetic variation was found in resistance to furunculosis in both the unvaccinated (heritabilities of 0.51 ± 0.05) and vaccinated (0.39 ± 0.06) fish. However, the genetic correlation between resistance to furunculosis in the two groups was low (0.32 ± 0.13), indicating a weak genetic association between resistance in the two groups. Hence, the current selection strategy on the basis of challenge tests of unvaccinated fish is likely to produce low genetic improvement in resistance to furunculosis under field conditions, where fish are vaccinated with an effective vaccine. Evidence was found of significantly favourable genetic associations of resistance to furunculosis in unvaccinated (but less so for vaccinated) fish with resistance to both IPN and ISA (unvaccinated fish), indicating that vaccination 'mask' genetic associations between resistance to different diseases.

Atypical furunculosis vaccines for Atlantic cod Gadhus morhua: impact of reattached Aeromonas salmonicida A-layer protein on vaccine efficacy

Atypical furunculosis caused by atypical Aeromonas salmonicida bacteria is reported as an increasing problem in farmed Atlantic cod Gadus morhua in Norway. At present, furunculosis vaccines adapted for cod or other marine fish species are not available. To identify bacterial components important for inducing protection in cod, we compared oil-adjuvanted vaccines based on A. salmonicida isolates phenotypically differing in their major cell surface constituents, such as the A-layer protein and lipopolysaccharide O-chains. Also included was an A-layer-deficient isolate with physically reattached A-layer protein. Vaccines containing A. salmonicida A-layer-producing cells elicited significantly better protection than vaccines with A-layer-deficient cells or with a supernatant with secreted A-layer protein. The A. salmonicida cells with reattached A-layer-protein resulted in significant and equal protection to the A-layer-producing cells and protected significantly better than the A-layer-deficient isolate. These results indicate that the A-layer protein when attached to the cell surface plays a role in inducing protective immunity in cod.

Enhancement of the immune response and protection induced by probiotic lactic acid bacteria against furunculosis in rainbow trout (Oncorhynchus mykiss)

We analysed the effect of probiotic strains on the cellular and humoral immune responses of rainbow trout (Oncorhynchus mykiss), and their capacity to prevent furunculosis during a challenge trial. Probiotic strains (Lactococcus lactis ssp. lactis CLFP 100, Leuconostoc mesenteroides CLFP 196, and Lactobacillus sakei CLFP 202) were administered orally to fish for 2 weeks at 10(6) CFU g(-1) of feed. In comparison to untreated control fish, the phagocytic activity of head kidney leukocytes and the alternative complement activity in serum were significantly greater in all probiotic groups at the end of the second week. With the exception of the group fed with Lactobacillus sakei, superoxide anion production was also significantly increased in the probiotic groups. Analysis of lysozyme activity did not exhibit any significant difference in the probiotic and control groups. Fifteen days after the start of the probiotic feeding, fish were challenged with Aeromonas salmonicida ssp. salmonicida. The fish supplemented with probiotics exhibited survival rates ranging from 97.8% to 100%, whereas survival was 65.6% in fish not treated with the probiotics. These results demonstrate that probiotic supplementation to fish can reduce the severity of furunculosis, and suggest that this reduction may be associated with enhanced humoral and cellular immune response.

Vaccination against atypical furunculosis and winter ulcer disease of fish

Atypical furunculosis is a problem in farming of salmonids and various other fish species caused by a heterogeneous group of atypical Aeromonas salmonicida strains. Winter ulcer is a disease of salmonids and cod caused by Moritella viscosa, but a number of fish species are susceptible to the infection. Vaccines are available against atypical furunculosis of salmonids, but their efficacy is dependent on the characteristics of the infective strain. Vaccines for non-salmonid fish are currently not commercially available. Furunculosis vaccines for salmon can induce cross protection against some atypical A. salmonicida infections and only in some fish species. Polyvalent injection vaccines based on inactivated bacterial cells are available against winter ulcer disease of salmonids. Outbreaks of winter ulcer disease in vaccinated salmon are, however, continuously reported.

Time of vaccination influences development of adhesions, growth and spinal deformities in Atlantic salmon Salmo salar

In August 1998, 3000 Atlantic salmon Salmo salar L. parr were divided into 7 groups with 2 replicates. Every 6 wk until March of the following year 1 group was vaccinated. One group was held as an unvaccinated control. The fish were transferred to seawater in May 1999, and slaughtered in February 2000. Temperature, fish size and photoperiod at vaccination, and the time between vaccination and sea transfer thus varied among the groups. In all vaccinated groups, growth was reduced for 1 to 2 mo following vaccination. Intra-abdominal lesions developed faster, and stabilised at a higher level in the groups vaccinated early at the highest temperature and the smallest fish size. Growth in seawater was influenced by the time of vaccination. At the end of the experiment, the group vaccinated last (MAR) was the heaviest of the vaccinated groups (4.0 kg), and the group vaccinated first, i.e. in August (AUG) was smallest (3.2 kg). Growth rate in seawater differed only in the summer when specific growth rate was above 1.45 in all groups. There was a correlation between adhesion, condition factor and number of weeks from vaccination to sea transfer. The AUG group had the highest condition factor, with a top level of 1.64 in autumn, and this group also displayed the highest incidence of deformed vertebra. The experiment shows that side effects of vaccination can be significantly reduced when planning the vaccination strategy, by taking environmental factors and fish biology into consideration.

Comparison of antibody responses in Atlantic cod (Gadus morhua L.) to Aeromonas salmonicida and Vibrio anguillarum

The immune system of Atlantic cod (Gadus morhua L.) differ from other bony fish species in that no or only very low increases in antibody levels are detected post-immunization with Vibrio salmonicida or V. anguillarum. Here, we report the results from the first study on comparison of antibody responses in cod to Aeromonas salmonicida and V. anguillarum. A. salmonicida appear to induce a stronger antibody response in cod compared to V. anguillarum, and more individuals immunized with A. salmonicida revealed a response compared to those immunized with V. anguillarum. The antibody responses to both bacterial species were mainly toward LPS, and the results indicate that cod antibodies are able to differentiate between LPS antigens from typical and atypical A. salmonicida strains.

Expressed sequence tags analysis of Atlantic halibut (Hippoglossus hippoglossus) liver, kidney and spleen tissues following vaccination against Vibrio anguillarum and Aeromonas salmonicida

To investigate the response of Atlantic halibut to vaccination and pathogen exposure, a cDNA library was constructed from liver, kidney and spleen mRNA collected following vaccination against Vibrio anguillarum and Aeromonas salmonicida. After sequencing 1114 clones 1072 (96.23%) readable sequences were obtained of which 106 sequences are the first reported from the fish. Of these, 182 clones (16.98%) contained cell/organism defence genes including immunoglobulin light chain, MHC class I and II, interferon consensus sequence binding protein, B-cell receptor-associated protein, early B-cell factor, 10 complement components, heat shock protein 70 and 90, antimicrobial peptides hepcidin type 1 and 2, and CC chemokine (macrophage inflammatory protein-1 beta-like chemokine, MIP-1beta). Expression of MIP-1beta-like was elevated in the kidney and spleen at 1, 2, 7 and 14 days post vaccination. Functional genes involved in cellular processes of hematopoietic tissues were also identified. These results indicate that this cDNA library contains many important genes involved in the immune response, making it an important resource for studying the response of Atlantic halibut to vaccination or pathogen exposure.

Protection against experimental Aeromonas salmonicida infection in carp by oral immunisation with bacterial antigen entrapped liposomes

Liposome-entrapped atypical Aeromonas salmonicida antigen was prepared to investigate the potential protective efficacy for A. salmonicida infection. Carp (Cyprinus carpio) were immunised orally with liposome-entrapped A. salmonicida antigen. After immunisation, significantly higher antigen-specific antibodies were detected in serum, intestinal mucus and bile than non-immunised control group. Furthermore, immunised carp were challenged by immersion with 1 x 10(6) cfu ml(-1) of A. salmonicida for 60 min. Of the eight non-immunised carp, three carp died (62.5% survival), whereas five out of six (83.5%) immunised survived. Furthermore, the development of skin ulcers was significantly inhibited in carp immunised with liposomes containing A. salmonicida antigen. These results suggest that liposomes containing A. salmonicida antigen have the potential for the induction of a protective immune response against atypical A. salmonicida infection and also suggest the possibility of developing a vaccine that may ultimately be used for the prevention of fish diseases.

Experimental infection of turbot, Scophthalmus maximus (L.), by Aeromonas salmonicida subsp. achromogenes and evaluation of cross protection induced by a furunculosis vaccine

Turbot was shown to be sensitive to injection challenges by Aeromonas salmonicida subsp. achromogenes (Asa). A systemic disease was induced and the bacterium was isolated from various internal organs. Histopathological changes involved haemorrhages, necrosis and degeneration in skin and muscle, haemorrhages and necrosis in kidney, degeneration in the heart muscle, and fusion of the secondary gill lamellae. A polyvalent commercial salmon vaccine, containing A. salmonicida subsp. salmonicida as one of five antigens, did not confer protection in turbot against an experimental Asa infection 13 weeks post-vaccination. Vaccination induced a significant antibody response against Asa cells but not against extracellular products of the bacterium. The results of the study indicate that Asa may be a potential threat to turbot farming and that the development of new turbot vaccines is needed.

Efficacy of furunculosis vaccines in turbot, Scophthalmus maximus (L.): evaluation of immersion, oral and injection delivery

The commercial furunculosis vaccine Aquavac Furovac 5 and an autogenous vaccine, based on the challenge strain, induced immune protection in turbot, Scophthalmus maximus (L.), as shown in challenge tests 120 days post-immunization by injection (relative percentage of survival, RPS = 72-99%). This protective effect lasted for at least 6 months post-immunization at appreciable levels (RPS = 50-52%). Neither the autogenous vaccine nor the commercial vaccine was able to induce significant levels of protection against Aeromonas salmonicida in turbot when administered by immersion. Antibody levels were high or moderate in fish vaccinated by injection with the different vaccines and very low in fish vaccinated by immersion. The field results show that delivering an oral boost after the primary vaccination by injection did not enhance protection of turbot against furunculosis and that water-based (autogenous vaccine) and oil adjuvanted (Alpha Ject 1200) vaccines administered by injection conferred similar levels of protection (RPS > 80%) in turbot.

Potency testing of a live, genetically attenuated vaccine for salmonids

Studies have been performed on the use of a live vaccine for immunization of salmonids against the bacterial disease furunculosis. The protection elicited by a kanamycin-resistant aroA mutant of A. salmonicida (Brivax I) and an unmarked aroA deletion mutant (Brivax II) has been examined, and data compared with protection seen using a freeze-dried Brivax II preparation and a commercial, oil-adjuvanted killed vaccine for furunculosis. Whilst high relative percent survival (RPS) values were seen in fish vaccinated with broth-grown Brivax I after a natural exposure to furunculosis (70-100%), much lower RPS values (30-40%) were seen with Brivax II vaccinated fish after an experimental challenge. Nevertheless, the freeze-dried Brivax II formulation performed as well as the broth-grown Brivax II formulation and a commercial vaccine in these studies. In addition, the environmental impact in terms of bacterial shedding into the tank water has been estimated, and shown to approximately 0.03% of the total inoculum used. Lastly, the freeze-dried formulation has been tested for its ability to infect fish and prime for lymphocyte proliferation and antibody production, relative to broth-grown preparations. In all three experiments no significant differences were seen between fish given the broth-grown and freeze-dried formulations. Such data, together with observations that the freeze-dried live preparation had an extended shelf life with the same potency as freshly grown bacteria, show that the potential exists for a commercially viable live vaccine to be produced for use in aquaculture.

Immunostimulatory effects of dimerized lysozyme (KLP-602) on the nonspecific defense mechanisms and protection against furunculosis in salmonids

Utilization of natural immunostimulants in fish culture offers a wide range of attractive methods for inducing and building protection against diseases. Lysozyme is an enzyme with bacteriolytic properties and is ubiquitous in its distribution among living organisms. This enzyme has antiviral, antibacterial and anti-inflammatory properties. In nature, lysozyme is found as a monomer. Lysozyme dimer is significantly less toxic than its monomer, and its high biological activity has been ascertained in cases of both viral and bacterial infections. In our study, we examined the influence of dimerized lysozyme (KLP-602) on the nonspecific cellular and humoral defense mechanisms and protection against furunculosis in rainbow trout (Oncorhynchus mykiss). We have analyzed the immunomodulatory effects of KLP-602 after experimental infection by Aeromonas salmonicida. Application of dimerized lysozyme (KLP-602) by injection stimulated the cellular and humoral defense mechanisms and provided protection against furunculosis. By contrast, mortality rate was reduced to 45% (one injection) and 25% (three injections) using 10 or 100 microg/kg KLP-602. Mortality in the untreated control group was 85%.

Vaccination strategies in freshwater salmonid aquaculture

The focus of this chapter is antibacterial vaccines. The main salmonid species in freshwater aquaculture is the rainbow trout. Other salmonid species are produced on a limited scale. The most important bacterial fish diseases in European freshwater aquaculture are the rainbow trout fry syndrome-RTFS-(Flavobacterium psychrophilum) and enteric redmouth disease-ERM-(Yersinia ruckeri) which are widespread and cause serious epizootics, while furunculosis (Aeromonas salmonicida) is endemic, only giving overt disease under extremely stressing conditions. In the hatchery, there is a need for vaccination against RTFS (not commercially available) and ERM; in the ponds it is urgent to vaccinate against ERM, while the importance of furunculosis vaccination is not clear. The fish for ongrowing in saltwater should be vaccinated against ERM, furunculosis and vibriosis. Commercial vaccines are available against these diseases, either as single component or combination vaccines for immersion and injection-and oral vaccines are under registration. Hitherto, there has not been much strategic research on vaccination in freshwater; however recent results suggest that with the regime of vaccines available (and soon available), fish should be vaccinated with an ERM immersion vaccine in the hatchery approximately four weeks before transfer to the ponds. To cover the growth period in fresh water an oral booster should be given two to three months later. There is a need for development and research in strategic use of an RTFS and a furunculosis vaccine in freshwater aquaculture.

Immunization with bacterial antigens: furunculosis

Although the nature of the antigens and the immune responses they elicit to achieve immunity to furunculosis are still not well defined, the currently available vaccines comprising A. salmonicida bacterins emulsified in oil adjuvants and delivered by intraperitoneal injection provide remarkably high levels of long-lasting protection. Despite some concern over side-effects, these vaccines have been adopted by most Atlantic salmon farmers over the last four years, transforming a situation where furunculosis outbreaks were becoming catastrophic to one where losses from the disease are negligible. Present evidence indicates that antibody responses to the polysaccharide capsule and iron regulated outer membrane proteins are associated with protection. Furthermore, cell-mediated immune responses involving antigen-induced release of cytokines from lymphocytes and the resultant activation of macrophages with the ability to kill the pathogen are also considered important protective mechanisms. Vaccines comprising whole A. salmonicida cultures grown under iron-restricted conditions and delivered by injection in an oil adjuvant are expected to induce prolonged stimulation of all the above responses. While these vaccines are suitable and effective for administration to salmon smolts there is still a need for mass vaccination by immersion or oral routes for salmonid fry. Effective means of achieving this are still required.

Reduction of antibacterial drug use in Norwegian fish farming due to vaccination

Sales statistics for antibacterial drugs and vaccines intended for use in farmed salmonids in Norway were collected and analysed. In 1993, there was a substantial reduction in annual sales figures for antibacterial drugs, which continued in 1994 and 1995. Oil-adjuvanted vaccines containing antigens against furunculosis were introduced in the fall of 1992. Introduction of oil-adjuvanted vaccines has been the single most important cause for the substantial reduction in use of antibacterial drugs in Norwegian fish farming industry.

Vaccination strategies in seawater cage culture of salmonids

Successful vaccination depends both on the development of protective vaccines and their correct use. In addition to deciding which diseases to vaccinate against, the choice of the method, timing, and use of revaccination must be considered. In seawater culture of salmonids, vibriosis and furunculosis are the most important diseases against which to vaccinate in many parts of the world, while cold-water vibriosis is of great significance in Atlantic salmon in some areas with low water temperatures. A vaccine against infectious pancreatic necrosis (IPN) has also been introduced recently. For optimal protection of salmonids in sea-water, vaccination should be carried out some time before sea transfer, in order to give immunity sufficient time to develop, and to avoid handling stress during smoltification. On the other hand however, vaccination should not be carried out too early, as the degree of immunity declines with time. Water temperature is an important factor when deciding when to vaccinate. Recent research has demonstrated that Atlantic salmon may be vaccinated successfully at low water temperatures. In general, vaccination by the injection method gives superior protection. Vaccines against the Vibrio-infections can also be administered successfully by immersion. However, due to lower levels of immunity, the need for a booster vaccination is greater when such a method is used. As regards vaccines against furunculosis, adjuvants are needed in order to achieve good protection, and, consequently, administration by injection is the only option.

A live (delta aroA) Aeromonas salmonicida vaccine for furunculosis preferentially stimulates T-cell responses relative to B-cell responses in rainbow trout (Oncorhynchus mykiss)

We have previously described (L. M. Vaughan, P. R. Smith, and T. J. Foster, Infect. Immun. 61:2172-2181, 1993) the construction of a kanamycin-resistant aromatic-dependent mutant of Aeromonas salmonicida, the causative agent of furunculosis, and its use as a live vaccine. Here we describe the construction of an unmarked aroA deletion mutant and examine the nature and magnitude of immune responses in rainbow trout (Oncorhynchus mykiss) to this vaccine strain. Good proliferative and antibody responses were elicited by using a range of vaccine doses from 2 x 10(6) to 2 x 10(9) live bacteria per fish, and a clear vaccine dose effect was apparent. A significant positive effect of using live bacteria to prime for lymphocyte proliferation and antibody production was apparent. However, the problem of directly comparing the vaccine doses of live and killed bacterial preparations is discussed, since some replication of live bacteria in vivo is expected. Most importantly, the live vaccine preferentially stimulated enhanced T-cell responsiveness, as evidenced by significantly greater increases in T-cell proliferation than in B-cell proliferation, compared with responses by the respective cell populations from fish given a killed vaccine. The manner in which live vaccines elicit strong cell-mediated immune responses and the relevance to fish vaccine design are discussed.

Recombinant infectious hematopoietic necrosis virus and viral hemorrhagic septicemia virus glycoprotein epitopes expressed in Aeromonas salmonicida induce protective immunity in rainbow trout (Oncorhynchus mykiss)

Fragments of the glycoprotein genes of viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV) were cloned into a bacterial broad-host-range expression vector under the control of the plac promoter. Western blot (immunoblot) analysis with monoclonal antibodies specific to the glycoproteins demonstrated the inducible expression of the fusion proteins in Escherichia coli. Aeromonas salmonicida is the causative agent of furunculosis in salmonid fish. It was confirmed that an avirulent strain of A. salmonicida, A440, which contains a deletion in the structural gene for the paracrystalline surface protein array, will provide protective immunity against furunculosis when used as a live attenuated vaccine. The plasmid-encoded viral epitopes were then mobilized into A440 for use as a shuttle system for the expression of fragments of the glycoprotein genes of IHNV and VHSV. Vaccination of rainbow trout with A440 containing the viral epitopes resulted in the development of protective immunity against both VHSV and IHNV. This indicates that the use of cloned fragments of the glycoproteins and the use of A. salmonicida as a shuttle system constitute a feasible approach to fish vaccine development.

Dietary intake of immunostimulants by rainbow trout affects non-specific immunity and protection against furunculosis

Immunostimulant preparations Macrogard, Candida utilis, Saccharomyces cerevisiae, Evetsel, Chitosan, or FinnStim were mixed into semipurified diets and fed to groups of rainbow trout for 1 week. Fish were bled by non-lethal caudal puncture and blood samples assayed for changes in non-specific cellular immunity and humoral protein levels. In the immunostimulated fish, hematocrit levels and lymphocyte counts remained relatively stable; however, elevations were observed in oxidative radical release, myeloperoxidase activity, phagocytic indexes, and potential killing activities of phagocytic cells including neutrophils. Total plasma protein and total immunoglobulin levels were elevated by the dietary immunostimulants. A challenge with the virulent pathogen that causes furunculosis, Aeromonas salmonicida, showed that the immunostimulated groups of fish were more resistant to the disease, confirming the potential use of these substances in fish culture for the prevention of disease.

[Health risks of vaccination of farmed fish]

In 1991, three cases of anaphylactoid reactions among vaccinators of farmed fish were registered in Norway. The reactions developed after repeated accidental self-injection of vaccine against salmon furunculosis. In 1991, some 60 million salmon were vaccinated against furunculosis at approximately 50 grams body weight. Following such self-injection of vaccine, inflammatory reactions are seen in the injected finger, also spreading occasionally to the hand. More seldom a "flu" like syndrome with fever, muscle aches, and general malaise is experienced probably caused by endotoxins in the vaccine. Devices to protect the fingers against accidental self-injection have been developed and, so far, the results are promising.

An aromatic-dependent mutant of the fish pathogen Aeromonas salmonicida is attenuated in fish and is effective as a live vaccine against the salmonid disease furunculosis

Aeromonas salmonicida is the etiological agent of furunculosis in salmonid fish. The disease is responsible for severe economic losses in intensively cultured salmon and trout. Bacterin vaccines provide inadequate protection against infection. We have constructed an aromatic-dependent mutant of A. salmonicida in order to investigate the possibility of an effective live-attenuated vaccine. The aroA gene of A. salmonicida was cloned in Escherichia coli, and the nucleotide sequence was determined. The codon usage pattern of aroA was found to be quite distinct from that of the vapA gene coding for the surface array protein layer (A layer). The aroA gene was inactivated by inserting a fragment expressing kanamycin resistance within the coding sequence. The aroA::Kar mutation was introduced into the chromosome of virulent A. salmonicida 644Rb and 640V2 by allele replacement by using a suicide plasmid delivery system. The aroA mutation did not revert at a detectable frequency (< 10(-11). The mutation resulted in attenuation when bacteria were injected intramuscularly into Atlantic salmon (Salmo salar L.). Introduction of the wild-type aroA gene into the A. salmonicida mutants on a broad-host-range plasmid restored virulence. A. salmonicida mutant 644Rb aroA::Kar persisted in the kidney of brown trout (Salmo trutta L.) for 12 days at 10 degrees C. Vaccination of brown trout with 10(7) CFU of A. salmonicida 644Rb aroA by intraperitoneal injection resulted in a 253-fold increase in the 50% lethal dose (LD50) compared with unvaccinated controls challenged with a virulent clinical isolate 9 weeks later. A second vaccination after 6 weeks increased the LD50 by a further 16-fold.

Intractable chronic furunculosis: prevention of recurrences with pentoxifylline

A 60-year-old HIV-negative man with known noninsulin-dependent diabetes mellitus and glucose 6-phosphate-dehydrogenase deficiency anemia suffered from chronic recurrent furunculosis since the age of 30. In recent years, his condition had become increasingly severe and the recurrences increasingly frequent. Different measures including continuous therapy with large doses of systemic antibiotics for a period of 6 months failed to prevent the recurrences. Oral treatment with pentoxifylline 400 mg t.i.d. was prescribed, and 2 months later the patient experienced a dramatic and complete remission of his furunculosis. Six months later he was still totally free of lesions while continuing to take the same medication. Pentoxifylline may provide a new and effective approach to the previously difficult and often disappointing problem of the management of patients with chronic recurrent furunculosis.

Protection of rainbow trout against vibriosis and furunculosis by the use of attenuated strains of Vibrio anguillarum

The fish pathogen Vibrio anguillarum causes a lethal infection in rainbow trout (Salmo gairdneri). Three different avirulent mutants, constructed by transposon insertion mutagenesis (VAN20 and VAN70) or as antibiotic-resistant mutants (VAN1000), were isolated by screening 200 individual isolated mutants for avirulence. When used as live vaccines, all three avirulent mutants were able to induce protective immunity against the homologous as well as a heterologous strain of V. anguillarum. When VAN1000 was used, protective immunity could be recorded 1 week after bath vaccination with 10(7) bacteria per ml of water for 30 min. A single-dose immunization was effective for at least 12 weeks. Western immunoblotting showed that strains of V. anguillarum have antigenic determinants in common with Aeromonas strains. Therefore, we tested and confirmed that VAN1000 also was able to induce protective immunity against challenge with Aeromonas salmonicida.

Strategies for the management of recurrent furunculosis

Furuncles are common infections of hair follicles. Staphylococcus aureus is frequently the causative agent, though other bacteria may also be pathogenic, especially for furuncles in the vulvovaginal area, the perirectal area, and the buttocks. A simple furuncle can be treated by incision and drainage. Systemic antibiotics are indicated in only special circumstances, and Gram stain is helpful in choosing an appropriate one. Some patients are plagued by recurrent furuncles because of follicular abnormalities, climatic conditions, colonization by pathogenic strains, reinfection, debility, or immunodeficiency. I present various strategies for treatment and prevention of recurrent furunculosis.

Immunity to Aeromonas salmonicida in coho salmon (Oncorhynchus kisutch) induced by modified Freund's complete adjuvant: its non-specific nature and the probable role of macrophages in the phenomenon

Juvenile coho salmon (Oncorhynchus kisutch), vaccinated with one intraperitoneal injection of formalin-killed virulent Aeromonas salmonicida cells suspended in saline, showed increased protection against approximately one LD60 of homologous challenge administered at 30 days post-vaccination. Under similar conditions, coho vaccinated with a modified complete Freund's adjuvant (MFCA) alone were also equally protected. When measured against a more severe A. salmonicida challenge of approximately one LD95, the strength of the MFCA-induced protection was found to exceed that produced by the homologous bacterin administered in saline or incomplete adjuvant, and the protection was still evident at 90 days post-treatment. Other more precise measurements indicated the LD50 for MFCA-treated coho to be up to 450 times that for saline-treated coho. Two other tested adjuvants, levamisole and MDP (N-acetyl-muramyl-L-alanyl-D-isoglutamine), administered in a modified Freund's incomplete adjuvant, also enhanced anti-A. salmonicida immunity but to a lesser degree. The active factor in MFCA was a killed Mycobacterium butyricum preparation, and the anti-A. salmonicida immunity it induced was non-specific because the immunity extended to two other serologically distinct fish pathogens tested: A. hydrophila (LD50 increase of 5.3-fold) and Vibrio ordalii (LD50 increase of 560-fold). Macrophages are believed to account for the M. butyricum-induced anti-A. salmonicida immunity because the immunity was a) non-specific, b) very rapid in onset (it was measurable by 4 days), and c) influenced by particulate preparations, known to affect macrophage function and immunity in mammals. The possible benefits of adjuvant-induced non-specific immunity in cultured fish are discussed.

Treatment and prevention of recurrent staphylococcal furunculosis: clinical and bacteriological follow-up

Various therapeutic and preventive methods were evaluated in 80 patients with recurrent staphylococcal furunculosis. The most appropriate treatment was peroral antibiotics for 10-14 days, mainly flucloxacillin twice daily. Fusidic acid ointment was used for prevention of relapses. Patients and healthy family members who carried the patient strain applied the ointment in nares twice daily every 4th week during 4-15 months. The method had a permanent effect in 80%. Implantation of strain 502 A was less effective when evaluated during a 2-3 yr period. 40-80 patients were checked up to 8 yr after their last furuncle. Three still had furuncles, in 2 of these cases the original strain was found in nares. 37 patients, now healthy after a mean observation time of 4.5 yr, showed either new nasal strains or negative cultures. A significantly lower frequency of phage group II strains in nares was noted in comparison to the previous findings during active furunculosis.

Vaccination of fish: a practical view

Commercial vaccines giving protection against three important bacterial fish diseases (enteric redmouth, furunculosis and vibriosis) became available in the United Kingdom in 1983. The degree of protection obtained and the cost effectiveness vary according to the method of presentation (injection, dipping or spraying and oral dosing). These methods and the optimum time for vaccination are described.