Feed-based vaccination regime against streptococcosis in red tilapia, Oreochromis niloticus x Oreochromis mossambicus

Evaluation of the efficacy of MONTANIDE™ GR01, a new adjuvant for feed-based vaccines, on the immune response and protection against Streptococcus agalactiae in oral vaccinated Nile tilapia (Oreochromis niloticus) under laboratory and on-farm conditions

Streptococcosis, an emerging infectious disease caused by Streptococcus agalactiae, has had adverse effects on farmed tilapia. Several vaccines have been developed to prevent this disease and induce a specific immune response against S. agalactiae infection. In this study the use of MONTANIDE™ GR01, a new adjuvant for oral vaccination, was optimized for use in tilapia under laboratory and field studies. In the laboratory trial the immune response and protective efficacy of two doses of MONTANIDE™ GR01, 20 % (w/w) and 2 % (w/w), included into the feed-based adjuvanted vaccines were assessed comparatively. Following immunization, the innate immune parameters studied in serum, including lysozyme, myeloperoxidase, catalase and glutathione peroxidase activity, were all increased significantly. Furthermore, specific IgM antibodies against S. agalactiae were induced significantly in serum post-vaccination, with higher levels observed in both groups that received the feed-based adjuvanted vaccine. Under both injection and immersion challenge conditions, the relative percent survival for the feed-based adjuvanted vaccine groups ranged from 78 % to 84 %. Following use of the low dose concentration of MONTANIDE™ GR01 for oral vaccination of tilapia in cage culture systems, several innate immune parameters were effectively enhanced in the immunized fish. Similarly, the levels of specific IgM antibodies in the serum of feed-based vaccinated fish were significantly enhanced, reaching their highest levels 2-5 months post-vaccination. Cytokines associated with innate and adaptive immunity were also examined, and the expression levels of several genes showed significant up-regulation. This indicates that both cellular and humoral immune responses were induced by the feed-based adjuvanted vaccine. The economic impact of a feed-based adjuvanted vaccine was examined following vaccination, considering the growth performance and feed utilization of the fish. It was found that the Economic Performance Index and Economic Conversion Ratio were unaffected by vaccination, further demonstrating that there are no negative impacts associated with administering a feed-based vaccine to fish. In conclusion, the data from this study indicate that MONTANIDE™ GR01 is a highly valuable adjuvant for oral vaccination, as demonstrated by its ability to induce a strong immune response and effectively prevent streptococcal disease in Nile tilapia.

Antimicrobial resistance in aquaculture: Occurrence and strategies in Southeast Asia

Aquaculture is a highly important and expanding industry in Southeast Asia (SEA). An upcoming problem is the emergence of antibiotic resistant pathogens due to the unchecked use of antibiotics and human clinical practices. This review focused insight into the occurrence of antimicrobial resistance (AMR) and strategies from SEA aquaculture based on the original research publication over the period 2002 to 2023. Amongst the 11 SEA countries, the most AMR report has come from Vietnam, Malaysia, and Thailand, respectively. The AMR found in SEA aquaculture were classified into 17 drug classes. The most reported AMR are aminoglycosides, beta-lactams, (fluoro)quinolones, tetracycline, sulpha group and multi-drug. Beta-lactams, tetracycline, sulpha group are reported in each country with the reported frequencies higher than 40 %. Escherichia coli, Aeromonas and Vibrio are the most widely and frequently reported ARB in SEA aquaculture. Multiple antibiotic resistance (MAR) indexes for the sample containing multiple bacterial isolates were generally low, while the medium numbers of MAR indexes for the typical bacteria species were higher than 0.2 and showed higher MAR levels than the global mean. Most of the detected ARGs are related to beta-lactams, tetracycline, sulpha group, and aminoglycosides. Amongst the beta-lactam resistance genes, blaTEM, and blaSHV are the most frequently detected. Almost all the available information of antibiotics, ARB and ARGs in SEA aquaculture was consistent with the global scale analysis. In addition, factors that contribute to the development and spread of AMR in SEA aquaculture were discussed. Moreover, the national action plan to combat AMR in SEA countries and the available technologies that already applied in the SEA aquaculture are also included in this review. Such findings underline the need for synergistic efforts from scientists, engineers, policy makers, government managers, entrepreneurs, and communities to manage and reduce the burden of AMR in aquaculture of SEA countries.

Immersion prime and oral boost vaccination with an inactivated Vibrio harveyi vaccine confers a specific immune response and protection in Asian seabass (Lates calcarifer)

Asian seabass (Lates calcarifer) holds significant economic value in fish farming in the Asia-Pacific region. Vibriosis caused by Vibrio harveyi (Vh) is a severe infectious disease affecting intensive farming of this species, for which prevention strategies by vaccination have been developed. This study investigated an alternative approach to injectable vaccination to prevent vibriosis in Asian seabass juveniles. The strategy begins with an immersion prime vaccination with a heat-inactivated Vh vaccine, followed by two oral booster doses administered at 14- and 28-days post-vaccination (dpv). Expression of five immune genes TNFα, IL1β, CD4, CD8, and IgM in the head kidney and spleen, along with investigation of anti-Vh antibody response (IgM) in both systemic and mucosal systems, was conducted on a weekly basis. The efficacy of the vaccines was assessed by a laboratory challenge test at 43 dpv. The results showed that the immunized fish displayed higher levels of mRNA transcripts of the immune genes after the immersion prime and the first oral booster dose compared to the control group. The expression levels peaked at 14 and 28 dpv and then declined to baseline at 35 and 42 dpv. Serum specific IgM antibodies were detected as early as 7 dpv (the first time point investigated) and exhibited a steady increase, reaching the first peak at 21 dpv, and a second peak at 35 dpv. Although the antibody levels gradually declined over subsequent weeks, they remained significantly higher than the control group throughout the experiment. A similar antibody response pattern was also observed in the mucosal compartment. The laboratory challenge test demonstrated high protection by injection with 1.65 × 104 CFU/fish, with a relative percent of survival (RPS) of 72.22 ± 7.86 %. In conclusion, our findings highlight the potential of an immersion prime-oral booster vaccination strategy as a promising approach for preventing vibriosis in Asian seabass.

Biomimetic nanovaccine based on erythrocyte membrane enhances immune response and protection against tilapia lake virus

An infectious disease emerged in recent years, Tilapia Lake Virus Disease (TiLVD), has severely restricted the development of global tilapia industry. Vaccination has proved potential strategy to prevent its causative agent Tilapia Lake Virus (TiLV) infectious. However, the response intensity of subunit vaccine is limited by its low immunogenicity, thus inclusion of adjuvants is required. Thus, we prepared a biomimetic nano-system (Cs-S2@M-M) with a particle size of ∼100 nm and an encapsulation efficiency of about 79.15% based on erythrocyte membrane. The immune response was detected after intramuscular injection to assess the effectiveness of the vaccine. The biomimetic system significantly up-regulates the expression of immune genes, enhances the activity of non-specific immune-related enzymes (P < 0.05) and improved relative percentage survival by 17.4%-26.1% in TiLV challenge. The biomimetic nano-system based on erythrocyte membrane induced significant immune response in tilapia and enhanced protection against TiLV, promising as a model for fish vaccines.

Field Efficacy of a Feed-Based Inactivated Vaccine against Vibriosis in Cage-Cultured Asian Seabass, Lates calcarifer, in Malaysia

Vibrio spp. are important aquaculture pathogens that cause vibriosis, affecting large numbers of marine fish species. This study determines the field efficacy of a feed-based inactivated vaccine against vibriosis in cage-cultured Asian seabass. A total of 4800 Asian seabass, kept in a field environment, were separated equally into two groups (vaccinated and non-vaccinated) in duplicate. Fish of Group 1 were orally administered the feed-based vaccine on weeks 0 (prime vaccination), 2 (booster), and 6 (second booster) at 4% body weight, while the non-vaccinated fish of Group 2 were fed with a commercial formulated pellet without the vaccine. Fish gut, mucus, and serum were collected, the length and weight of the fish were noted, while the mortality was recorded at 2-week intervals for a period of 16 weeks. The non-specific lysozyme activities were significantly (p < 0.05) higher in the fish of Group 1 than the non-vaccinated fish of Group 2. Similarly, the specific IgM antibody levels in serum and mucus were significantly (p < 0.05) higher in Group 1 than in Group 2, as seen in the second week, with the highest level 8 weeks after primary immunization. At week 16, the growth performance was significantly (p < 0.05) better in Group 1 and showed lower bacterial isolation in the gut than Group 2. Despite the statistical insignificance (p > 0.05), the survival rate was slightly higher in Group 1 (71.3%) than Group 2 (67.7%). This study revealed that feed-based vaccination improves growth performance, stimulates innate and adaptive immune responses, and increases protection of cultured Asian seabass, L. calcarifer, against vibriosis.

Current status and advances of fish vaccines in Malaysia

Fish diseases have a significant negative influence on the Malaysian aquaculture industry. Since the 1980s, the sector has grown in size, which has resulted in a rise in the prevalence of infectious outbreaks affecting both freshwater and marine cultured fish species. Demand for commercially available fish vaccinations is predicted to increase as infectious disease outbreaks continue to occur. In Malaysia, aquaculture vaccine research and development (R&D) are still in its infancy, with most efforts concentrating on producing vaccines against bacterial infections, most notably streptococcosis, vibriosis, and motile Aeromonas septicemia. Despite several attempts, no homegrown vaccine has been effectively introduced into the manufacturing pipeline to date. At the moment, only three imported aquatic vaccines have received full permission, a far cry from the 314 and 60 vaccines licensed in the poultry and porcine industries, respectively. This review will describe recent findings regarding the development of aquaculture vaccines for certain fish species and diseases in Malaysia. In our opinion, R&D on fish vaccines is critical to the aquaculture industry's viability.

Laboratory and Field Assessments of Oral Vibrio Vaccine Indicate the Potential for Protection against Vibriosis in Cultured Marine Fishes

Vibriosis is one of the most common threats to farmed grouper; thus, substantial efforts are underway to control the disease. This study presents an oral vaccination against multiple Vibrio spp. in a marine fish with double booster immunisation. The Vibrio harveyi strain VH1 vaccine candidate was selected from infected groupers Epinephelus sp. in a local farm and was formalin inactivated and combined with commercial feed at a 10% ratio (v/w). A laboratory vaccination trial was conducted for seventy days. The induction of IgM antibody responses in the serum of Asian seabass Lates calcarifer immunised with the oral Vibrio harveyi strain VH1 was significantly (p < 0.05) increased as early as week one post-primary vaccination. Subsequent administration of the first and second booster for 5 consecutive days, starting on days 14 and 42, respectively, improved the specific antibody level and reached a highly significant (p < 0.05) value at days 35 and 49 before slightly decreasing from day 56 onwards. Antibody titres of the control unvaccinated group remained relatively stable and low throughout the experimental period. At the end of the 70-day vaccination trial, 23 days post final boost, an intraperitoneal challenge with a field strain of Vibrio harveyi, V. alginolyticus, and V. parahaemolyticus was carried out. Our challenge study showed that oral Vibrio harveyi strain VH1 vaccine candidate could induce significant protection, with an RPS of 70–80% against different Vibrio species. Thereafter, a field trial was conducted in a mariculture farm to study the effect of field vaccination using the oral Vibrio harveyi strain VH1 vaccine candidate. A total of 3000 hybrid grouper juveniles were divided into two groups in triplicate. Fish of Group 1 were not vaccinated, while Group 2 were vaccinated with the feed-based vaccine. Vaccinations were carried out on days 0, 14, and 42 via feeding the fish with the vaccine at 4% body weight for 5 consecutive days. At the end of the study period, the fish survival rate was 80% for the vaccinated group, significantly (p < 0.05) higher than the 65% seen in the control unvaccinated group. Furthermore, the vaccinated fish showed significantly (p < 0.05) better growth performances. Therefore, the oral Vibrio vaccine from the inactivated Vibrio harveyi strain VH1 is a potential versatile vaccine candidate that could stimulate good immune responses and confer high protection in both Asian seabass, Lates calcarifer, and farm hybrid grouper Epinephelus fuscoguttatus × Epinephelus lanceolatus.

Immunogenicity in Oreochromis niloticus vaccinated with sonicated antigens against streptococcosis

Streptococcosis causes great economic losses in intensive culture of tilapia. Vaccination is the most effective and safest way to tackle infectious diseases. Thus, this study sought the more effective and safer antigenic fraction after sonication of Streptococcus agalactiae to elaborate a vaccine against streptococcosis in Nile tilapia. For this, twenty-one days after vaccination with different fractions (soluble and insoluble) of S. agalactiae, the fish were challenged with the homologous strain (LD50). Then, samples were taken at zero, 14, 28, 60 and 90 days post-vaccination (DPV, n = 7). Blood and organs (cranial kidney, spleen and liver) were collected from vaccinated and unvaccinated fish. Finally, insoluble fraction vaccine presented the best effect, resulting in a 100% relative percent of survival (RPS) and without clinical manifestations. In view of the results, it was to evaluate the role of the insoluble fraction of the antigen in the protective immunity against streptococcosis. The results indicate that the spleen might be the main organ in the vaccine response in Nile tilapia due to the great morphological and immunological differences in vaccinated fish, evidenced by the greater of melanomacrophage centers (MMC) and IgM + lymphocytes in relation to the non-vaccinated fish. At 60 DPV, it was observed the peak of the protective immunity related to the maximum concentration of proteins, circulating leukocytes, antibody titers in the serum and tissue changes with greater expression of IgM + and MMC number in the spleen and kidney of Oreochromis niloticus. Vaccination with insoluble fraction of S. agalactiae was safe and provided effective protection against streptococcosis with maximum protective response at 60 DPV.

Immuno-protective efficiency of feed-based whole-cell inactivated bivalent vaccine against Streptococcus and Aeromonas infections in red hybrid tilapia (Oreochromis niloticus × Oreochromis mossambicus)

Streptococcosis and motile aeromonad septicemia (MAS) are well-known diseases in tilapia culture, which cause mass mortality with significant economic losses. The development of feed-based bivalent vaccines in controlling these diseases has been initiated, however, the mechanisms of immunities and cross-protection in fish remain unclear. This study was conducted to assess the immuno-protective as well as the cross-protective efficacy of a newly developed feed-based bivalent vaccine against Streptococcus and Aeromonas infections in red hybrid tilapia. A total of five groups of fish were vaccinated orally through two different techniques; bivalent vaccine (inactivated Streptococcus iniae and Aeromonas hydrophila) sprayed on feed pellets (BS group); bivalent vaccine (inactivated S. iniae and A. hydrophila) incorporated in feed (BI group); monovalent inactivated S. iniae and A. hydrophila vaccine separately incorporated into feed as monovalent S. iniae (MS group) and monovalent A. hydrophila (MA group); and control group (without vaccine). The feed-based vaccine was delivered orally at 5% of body weight for five consecutive days. The booster doses were given in the same manner on weeks 2 and 6. Serum and skin mucus samples were collected to assess the IgM responses using indirect ELISA. The first administration of the feed-based vaccine stimulated the IgM levels that lasted until week 3, while the second booster ensured that the IgM levels remained high for a period of 16 weeks in the BI, MS and MA groups. The BI group developed a strong and significantly (P < 0.05) higher systemic and mucosal IgM responses against both S. iniae and A. hydrophila and also cross-protective antigen S. agalactiae and A. veronii compared to the BS and control groups. Quantitative real-time PCR results also showed that the relative expressions of IL-8, INF-γ and IgM in the BI immunized fish spleen, head kidney and hindgut exhibited various significant (P < 0.05) rising trends following both the vaccination and the challenge phase. On weeks 10, all fish were challenged through the intraperitoneal route, where relative percent survivals (RPS) of 82.22 ± 3.85% when challenged with S. iniae, 77.78 ± 3.85% when challenged with A. hydrophila and 77.78 ± 3.85% when co-challenged with both S. iniae and A. hydrophila were observed in the BI group, which were significantly higher (P < 0.05) compared to the other groups. The BI group also showed significantly (P < 0.05) higher partial cross-protections following challenges with S. agalactiae (RPS at 60.00 ± 6.67%) and A. veronii (RPS at 57.78 ± 7.70%). This study demonstrated that immunization with feed-based BI vaccine elicited immune responses that were capable of protecting red hybrid tilapia against streptococcosis and MAS.

Vaccine Efficacy of a Newly Developed Feed-Based Whole-Cell Polyvalent Vaccine against Vibriosis, Streptococcosis and Motile Aeromonad Septicemia in Asian Seabass, Lates calcarifer

Multiple infections of several bacterial species are often observed under natural farm conditions. The infections would cause a much more significant loss compared to a single infectious agent. Vaccination is an essential strategy to prevent diseases in aquaculture, and oral vaccination has been proposed as a promising technique since it requires no handling of the fish and is easy to perform. This research attempts to develop and evaluate a potential feed-based polyvalent vaccine that can be used to treat multiple infections by Vibrios spp., Streptococcus agalactiae, and Aeromonas hydrophila, simultaneously. The oral polyvalent vaccine was prepared by mixing formalin-killed vaccine of V. harveyi, S. agalactiae, and A. hydrophila strains with commercial feed pellet, and palm oil as an adjuvant was added to improve their antigenicity. Thereafter, a vaccinated feed pellet was tested for feed quality analysis in terms of feed stability in water, proximate nutrient analysis, and palatability, safety, and growth performance using Asian seabass, Lates calcarifer as a fish host model. For immune response analysis, a total of 300 Asian seabass juveniles (15.8 ± 2.6 g) were divided into two groups in triplicate. Fish of group 1 were not vaccinated, while group 2 was vaccinated with the feed-based polyvalent vaccine. Vaccinations were carried out on days 0 and 14 with oral administration of the feed containing the bacterin at 5% body weight. Samples of serum for antibody and lysozyme study and the spleen and gut for gene expression analysis were collected at 7-day intervals for 6 weeks. Its efficacy in protecting fish was evaluated in aquarium challenge. Following vaccination by the polyvalent feed-based vaccine, IgM antibody levels showed a significant (p < 0.05) increase in serum against Vibrio harveyi, Aeromonas hydrophila, and Streptococcus agalactiae and reached the peak at week 3, 5, and 6, respectively. The high-stimulated antibody in the serum remained significantly higher than the control (p < 0.05) at the end of the 6 weeks vaccination trial. Not only that, but the serum lysozyme level was also increased significantly at week 4 (p < 0.05) as compared to the control treatment. The immune-related gene, dendritic cells, C3, Chemokine ligand 4 (CCL4), and major histocompatibility complex class I (MHC I) showed significantly higher expression (p < 0.05) after the fish were vaccinated with the oral vaccine. In the aquarium challenge, the vaccine provided a relative percentage survival of 75 ± 7.1%, 80 ± 0.0%, and 80 ± 0.0% after challenge with V. harveyi, A. hydrophila, and S. agalactiae, respectively. Combining our results demonstrate that the feed-based polyvalent vaccine could elicit significant innate and adaptive immunological responses, and this offers an opportunity for a comprehensive immunization against vibriosis, streptococcosis, and motile aeromonad septicemia in Asian seabass, Lates calcarifer. Nevertheless, this newly developed feed-based polyvalent vaccination can be a promising technique for effective and large-scale fish immunization in the aquaculture industry shortly.

Systemic and Mucosal B and T Cell Responses Upon Mucosal Vaccination of Teleost Fish

The development of mucosal vaccines against pathogens is currently a highly explored area of research in both humans and animals. This is due to the fact that mucosal vaccines have the potential to best elicit protective responses at these mucosal surfaces, which represent the frontline of host defense, thus blocking the pathogen at its initial replication sites. However, in order to provide an efficient long-lasting protection, these mucosal vaccines have to be capable of eliciting an adequate systemic immune response in addition to local responses. In aquaculture, the need for mucosal vaccines has further practical implications, as these vaccines would avoid the individual manipulation of fish out of the water, being beneficial from both an economic and animal welfare point of view. However, how B and T cells are organized in teleost fish within these mucosal sites and how they respond to mucosally delivered antigens varies greatly when compared to mammals. For this reason, it is important to establish which mucosally delivered antigens have the capacity to induce strong and long-lasting B and T cell responses. Hence, in this review, we have summarized what is currently known regarding the adaptive immune mechanisms that are induced both locally and systemically in fish after mucosal immunization through different routes of administration including oral and nasal vaccination, anal intubation and immersion vaccination. Finally, based on the data presented, we discuss how mucosal vaccination strategies could be improved to reach significant protection levels in these species.

Efficacy of Feed-Based Formalin-Killed Vaccine of Streptococcus iniae Stimulates the Gut-Associated Lymphoid Tissues and Immune Response of Red Hybrid Tilapia

Red hybrid tilapia were fed a formalin-killed oral Streptococcus iniae vaccine (FKV) in the present study was assessed. Three hundred Red hybrid tilapia 80 ± 10 g were divided into five groups (1A, 1B, 2A, 2B, and Cx), each consisting of 60 fish. Fish from Groups 1A, 1B, 2A, and 2B were fed with FKV over different periods of administration, while Group 2B was the only group of fish to receive an oral booster vaccination on day 14- and 21-days post-vaccination (dpv). Group Cx was fed with normal pellets containing no vaccine as a control group. At four weeks post-vaccination (wpv), all fish were experimentally infected with S. iniae. Groups 2A and 2B had the lowest level of mortalities following vaccination (45% and 30%, respectively) compared to Groups 1A and 1B (80% and 55%, respectively), while the level of mortalities in Group Cx was 100%. All vaccinated groups showed a significant increase in anti-S. iniae IgM levels (p < 0.05) in serum, mucus, and gut-lavage, while Group Cx did not (p > 0.05) and all fish in this group died by five weeks post-infection. In conclusion, fish fed with the S. iniae FKV had a greater level of protection against S. iniae, with increased specific antibody response to the vaccine and there was also evidence of GALT stimulation by the vaccine.

Efficacy of Whole Cell Inactivated Vibrio harveyi Vaccine against Vibriosis in a Marine Red Hybrid Tilapia (Oreochromis niloticus × O. mossambicus) Model

Vibrio harveyi causes vibriosis in various commercial marine fish species. The infection leads to significant economic losses for aquaculture farms, and vaccination is an alternative approach for the prevention and control of fish diseases for aquaculture sustainability. This study describes the use of formalin-killed Vibrio harveyi (FKVh) strain Vh1 as a vaccine candidate to stimulate innate and adaptive immunities against vibriosis in a marine red hybrid tilapia model. Tilapia are fast growing; cheap; resistant to diseases; and tolerant to adverse environmental conditions of fresh water, brackish water, and marine water and because of these advantages, marine red hybrid tilapia is a suitable candidate as a model to study fish diseases and vaccinations against vibriosis. A total of 180 healthy red hybrid tilapias were gradually adapted to the marine environment before being divided into two groups, with 90 fish in each group and were kept in triplicate with 30 fish per tank. Group 1 was vaccinated intraperitoneally with 100 µL of FKVh on week 0, and a booster dose was similarly administered on week 2. Group 2 was similarly injected with PBS. Skin mucus, serum, and gut lavage were collected weekly for enzyme-linked immunosorbent assay (ELISA) and a lysozyme activity assay from a total of 30 fish of each group. On week 4, the remaining 60 fish of Groups 1 and 2 were challenged with 10⁸ cfu/fish of live Vibrio harveyi. The clinical signs were monitored while the survival rate was recorded for 48 h post-challenge. Vaccination with FKVh resulted in a significantly (p < 0.05) higher rate of survival (87%) compared to the control (20%). The IgM antibody titer and lysozyme activities of Group 1 were significantly (p < 0.05) higher than the unvaccinated Groups 2 in most weeks throughout the experiment. Therefore, the intraperitoneal exposure of marine red hybrid tilapia to killed V. harveyi enhanced the resistance and antibody response of the fish against vibriosis.

Yasin IS, Saad MZ, Abd. Rahaman NY, Barkham T, Amal MNA. Co-Infections of Tilapia Lake Virus, Aeromonas hydrophila and Streptococcus agalactiae in Farmed Red Hybrid Tilapia

A high death rate among red hybrid tilapias was observed in a farm in Selangor, Malaysia, in January 2020. The affected fish appeared lethargic, isolated from schooling group, showed loss of appetite, red and haemorrhagic skin, exophthalmia and enlarged gall bladders. Histopathological assessment revealed deformation of kidney tubules, and severe congestion with infiltrations of inflammatory cells in the brains and kidneys. Syncytial cells and intracytoplasmic inclusion bodies were occasionally observed in the liver and brain sections. Tilapia Lake Virus (TiLV), Aeromonas hydrophila and Streptococcus agalactiae were identified in the affected fish, either through isolation or through PCR and sequencing analysis. The phylogenetic tree analysis revealed that the TiLV strain in this study was closely related to the previously reported Malaysian strain that was isolated in 2019. On the other hand, A. hydrophila and S. agalactiae were closer to Algerian and Brazilian strains, respectively. The multiple antibiotic resistance index for A. hydrophila and S. agalactiae was 0.50 and 0.25, respectively. Co-infections of virus and bacteria in cultured tilapia is a new threat for the tilapia industry.

Development of fish vaccine in Southeast Asia: A challenge for the sustainability of SE Asia aquaculture

Southeast (SE) Asia plays an important role in global food security as this region has been regarded as one of the major producers of aquaculture product and, to date, freshwater fish accounted for one-third of the total aquaculture in SE Asia. The intensification of freshwater farming corresponding to increase of consumer demands has inevitably led to the emergence and re-emergence of diseases causing tremendous economic loss in the region. Nile tilapia (Oreochromis niloticus) and striped catfish (Pangasianodon hypophthalmus), the major freshwater fish species of SE Asia, have been reported susceptible to several bacterial pathogens, e.g. Streptococcus agalactiae, Edwardsiella ictalurid and Flavobacterium columnare. Since only a limited number of vaccines being registered and marketed, these pathogenic organisms still represent a severe threat to aquaculture industry in SE Asia. However, there is profound advancement in the understanding of disease epidemiology, pathogenic mechanisms, teleost mucosal immunity and vaccine delivery system over the last few years. This review aimed to summarize those recent findings which hopefully can provide novel insight into the future development of suitable vaccine and vaccination regime against bacterial infection in SE Asia region.

Haemato-immunological responses and effectiveness of feed-based bivalent vaccine against Streptococcus iniae and Aeromonas hydrophila infections in hybrid red tilapia (Oreochromis mossambicus × O. niloticus)

Background

Streptococcosis and Motile Aeromonad Septicemia (MAS) are important diseases of tilapia, Oreochromis spp. and causes huge economic losses in aquaculture globally. The feed-based vaccination may be an alternative to minimize major infectious diseases in tilapia. Thus, this study aims to evaluate the haemato-immunological responses and effectiveness of a newly developed feed-based killed bivalent vaccine against Streptococcus iniae and Aeromonas hydrophila in hybrid red tilapia. A total of 495 hybrid red tilapia of 61.23 ± 4.95 g were distributed into 5 groups (each with triplicate). The fish were immunized orally through bivalent (combined S. iniae and A. hydrophila) spray vaccine (BS group), bivalent formulate vaccine (BF group), monovalent S. iniae vaccine (MS group), monovalent A. hydrophila vaccine (MA group) and unvaccinated as a control group. The vaccine was orally administered on days 0, 14 and 42 applied feed-based bacterin at 5% body weight. The blood and spleen samples were collected from all groups on 7, 21 and 49 days post-vaccination, and also 96 h post-infection to assess their haemato-immune responses.

Results

Compared with the unvaccinated group, leukocyte, lymphocytes, monocytes, granulocytes counts in vaccinated groups were significantly (P < 0.05) increased on 21, 49 days post-vaccination and also 96 h post-infection, while erythrocytes, haemoglobin and haematocrit in vaccinated groups were significantly (P < 0.05) enhanced only 96 h post-infection. Additionally, the lysozyme and phagocytic activity and, serum antibody (IgM) were significantly higher (P < 0.05) against S. iniae and A. hydrophila in vaccinated groups compared to the unvaccinated group in the pre- and post-infection. Results from the challenge through co-infection with S. iniae and A. hydrophila showed the relative percent survival (RPS) in BF group was 76.67 ± 4.71%, which had the capacity to induce significant protection (P < 0.05) compared to others groups.

Conclusions

This study demonstrates the bivalent formulate (BF) group could elicit significant non-specific and specific immunological responses with higher protection in hybrid red tilapia. In addition, this newly developed feed-based bivalent vaccination can be a promising technique for effective and large scale fish immunization in the aquaculture industry.

Efficacy of live attenuated vaccine derived from the Streptococcus agalactiae on the immune responses of Oreochromis niloticus

Streptococcus agalactiae species have been recognized as the main pathogen causing high mortality in fish leading to significant worldwide economical losses to the aquaculture industries. Vaccine development has become a priority in combating multidrug resistance in bacteria; however, there is a lack of commercial live attenuated vaccine (LAV) against S. agalactiae in Malaysia. The aim of this study is to compare two methods using attenuated bacteria as live vaccine and to evaluate the efficacy of selected LAV on the immune responses and resistance of Oreochromis niloticus (tilapia) against S. agalactiae. The LAV derived from S. agalactiae had been weakened using the chemical agent Acriflavine dye (LAV1), whereas the second vaccine was weakened using serial passages of bacteria on broth media (LAV2). Initial immunization was carried out only on day one, given twice-in the morning and evening, for the 42 day period. Serum samples were collected to determine the systemic antibody (IgM) responses and lysozymal (LSZ) activity using ELISA. On day 43 after immunization, the fish were injected intraperitoneally (i.p) with 0.1 mL of S. agalactiae at LD₅₀ = 1.5 × 10⁵ (CFU)/fish. Fish were monitored daily for 10 days. Clinical signs, mortality and the relative percent of survival (RPS) were recorded. Trial 1 results showed a significant increased (P < 0.05) in serum IgM titers and LSZ activity as compared to LAV2 and the control group (unvaccinated fish). The efficacy of LAV1 was proven effective as determined by the RPS values, LAV1 at 81.58% as compared to LAV2 at 65.79%. Trial 2 of LAV1 and control group were further determined by administering primary and booster doses revealed a RPS value for LAV1 of 82.05%, with the significant enhancement on the immune responses of tilapia as compared to control group. In conclusion, LAV revealed to elevate antibody IgM levels, LSZ activity and provide long-term protection when added to feed. LAV is a low-cost vaccine shown to rapidly increase the immune response of fish and increase survival rates of fish against S. agalactiae infection.

Secondary immune response of rainbow trout following repeated immersion vaccination

Teleosts are able to raise a protective immune response, comprising both innate and adaptive elements, against various pathogens. This is the basis for a widespread use of vaccines, administered as injection or immersion, in the aquaculture industry. It has been described that repeated injection vaccination of fish raises a secondary immune response, consisting of rapid, accelerated and increased antibody reaction. This study reports how rainbow trout responds to repeated immersion vaccination against yersiniosis (ERM) caused by the bacterial pathogen Yersinia ruckeri. It was found that rainbow trout does not raise a classical secondary response following repeated immersion vaccination. Serum antibody titres were merely slightly increased even after three immunizations, using 30-s immersion into a bacterin consisting of formalin-inactivated Y. ruckeri (serotype O1, biotypes 1 and 2), performed over a 3-month period. The densities of IgM-positive lymphocytes in spleen of fish immunized three times were increased compared to control fish, but no general trend for an increase with the number of immunizations was noted. The lack of a classical secondary response following repeated immersion vaccination may partly be explained by limited uptake of antigen by immersion compared to injection.

The effect of feed-based vaccination on tilapia farm endemic for streptococcosis

A tilapia farm experiencing endemic streptococcosis was selected to study the effect of vaccination with a feed-based vaccine on naturally ocurring streptococcosis. A total of 9000 red tilapia, Oreochromis niloticus × Oreochromis mossambicus of 100 ± 20 g were divided into 9 cages. Fish of Group 1 in cages 1, 2 and 3 were not vaccinated. Group 2 in cages 4, 5 and 6 were vaccinated on days 0 and 14 (single booster) while Group 3 in cages 7, 8 and 9 were vaccinated on days 0, 14 and 42 (double booster). Vaccination was done by oral administration of the feed-based bacterin vaccine at 4% bodyweight. Samples of serum for antibody study and the brain, eyes and kidney for bacterial isolation were collected at 14-day intervals. The study was carried out during the critical months between April and June. Following vaccination and booster, there was significant (p < 0.05) increase in the antibody levels in all vaccinated groups from week 1 that reached the peak at week 3 before declining gradually until week 6. However, second booster on week 6 significantly (p < 0.05) increased the antibody level that remained high until the end of the 16-week study period (double booster). Streptococcus agalactiae was isolated at the start of the experiment (day 0) at an average of 10 ± 5.0% of the sampled fish. In week 4, the isolation rate was 13 ± 5.7% but increased to 18 ± 7.6% in week 8, to 25 ± 10.0% in week 10, to 28 ± 5.8% in week 12 and 25 ± 7.3% in week 14. The average isolation rate was 28 ± 7.2%, 18 ± 7.1% and 13 ± 8.2% of the fish sampled from unvaccinated, single booster and double booster groups, respectively. At the end of the study period, the survival rate was 45.2 ± 2.45% for unvaccinated, 65.3 ± 4.8% for single booster and 75.1 ± 2.1% for double booster groups. Vaccinating fish in endemic farm might not eliminate the disease but was able to significantly improve the survival rate.

Immunization Strategies against Piscirickettsia salmonis Infections: Review of Vaccination Approaches and Modalities and Their Associated Immune Response Profiles

Salmonid rickettsial septicemia (SRS) is a serious, infectious disease in Chilean salmon farming caused by Piscirickettsia salmonis, causing heavy losses to the salmonid industry. P. salmonis belongs to the Gammaproteobacteria, order Thiotrichales. SRS was first described in Chile in 1989, and infection with P. salmonis has since been described from a high number of fish species and in several geographic regions globally. P. salmonis infection of salmonids causes multifocal, necrotic areas of internal organs such as liver, kidney, and spleen. Histologically and immunologically, the tissue response is the formation of granulomas, often with central suppuration. The exact sequence of infection is not known, but bacteria likely gain access to internal organs through mucosal surfaces and when infected, fish carry bacteria in macrophages. It has not been fully determined if the bacterium resides in the cytosol or “hide” within vesicular structures intracellularly, although there are indications that in vitro infection results in actin reorganization and formation of actin-coated vesicle within which the bacterium resides. Protection against lethal challenge is well documented in lab scale experiments, but protection from vaccination has proven more difficult to attain long term under field conditions. Current vaccination protocols include whole cell, inactivated and adjuvanted vaccines for injection for primary immunization followed by oral boost where timing of boost delivery is followed by measuring circulating antibody levels against the pathogen. Documentation also exist that there is correlation between antibody titers and protection against mortality. Future vaccination regimes will likely also include live-attenuated vaccines or other technologies such as DNA vaccination. So far, there is no documentation available for live vaccines and, for DNA vaccines, studies have been unsuccessful under laboratory conditions.