Development of a novel Glycovaccine against Aeromonas hydrophila for tilapia aquaculture in India

Summary

Aquaculture is the fastest growing food-production sector globally, with over 1 billion people relying on fish as their major protein source. Reaching harvest size in just 6 months, tilapia (Oreochromis sp.) provides essential revenue for many low-income families, and is a major trade commodity for many low to middle-income countries (LMIC). As the tilapia aquaculture sector is expanding, problems related to its farming is also increasing mostly due to infectious bacterial diseases.

Bacterial disease is one of the major threats in fish production. Common bacterial infections in tilapia include Aeromonas, Streptococcus and Flavobacterium infections which cause severe economic losses in tilapia culture. Within the aeromonads group, Aeromonas hydrophila (52.94%) was found to be involved in the greatest number of disease cases. Aeromonas has emerged as one of most important bacterial diseases for farmed tilapia, for which there is no commercial vaccine, and farmers resort to using antibiotics to control this disease. With increasing concerns about using antibiotics in aquaculture, vaccination has raised attention for disease control. The shortage of new drugs to combat antibiotic resistant bacteria has motivated researchers to discover new therapeutic modality to curtail severe infections.

Herein, we propose a novel method to target the bacterial cell surface glycan using nanoconjugates developed with naturally existing lipid signalling molecules and plant seed lectin. Considering the negative surface charge of the bacterium and the understanding that the bacterial membranes are composed of lipopolysaccharide, the project plans to synthesize novel cationic nanoparticles and functionalize them with glycan binding protein to develop a dual targeting antimicrobial agent. Vaccination has proven a successful too for controlling diseases in aquaculture, with formulations tested by vaccinating fish which are subsequently given a pathogen challenge. Many tilapia farmers will not vaccinate by injection once the fish have been moved onto the farm and would prefer fish to be vaccinated in the hatchery. We will therefore investigate a novel and innovative lectin NP antigen formulation for mucosal (oral, immersion and injection) delivery of the Aeromonas vaccine and develop a cost-effective, easily administered vaccine for use in a large scale tilapia-production both for use in India and globally.

Project Outcomes

Tilapia aquaculture provides essential revenue for many low-income families and is a major trade commodity for many low to middle-income countries (LMIC). However, the intensification of tilapia aquaculture has led to an increase in infectious disease outbreaks resulting in significant losses for tilapia farmers. Vaccination has proved to be effective in protecting farmed tilapia from disease. Although efficacious, intraperitoneal (i.p) administration of vaccines is stressful for the fish and logistically challenging. Alternative vaccine delivery methods, such as oral or immersion delivery, are ideal methods for vaccine delivery in tilapia aquaculture. Advanced antigen delivery technologies such as nano vaccines and glycovaccines are exciting alternatives for administering vaccines to tilapia. The present study aimed to assess the effectiveness of a novel lectin-based glycovaccine in protecting Nile tilapia against Aeromonas hydrophila by comparing different routes of administration. The biophysical characteristics of the synthesised lectin-nanoconjugate vaccine containing formalin-killed A. hydrophila were examined by transmission electron microscopy. The glycovaccine was administered to fish by injection, immersion and oral (through their feed for seven days), followed by a booster dose 22 days post-vaccination (dpv). The vaccinated fish were infected with a virulent strain of A. hydrophila, and the infection monitored for three weeks. Samples were collected at set time points during the trial to assess specific IgM titres, histopathology, immunohistochemistry, and immune gene expression using RT-qPCR. A significant increase in immune parameters was observed with increased IgM, IgT, IgD, CD4, CD8α, IL1β, and TNFα expression (p<0.05) in the head kidney of vaccinated fish. Histopathological examination of tissues sampled from vaccinated fish showed infiltration of lymphocytes suggestive of gill and gut mucosal immune responses to the vaccine. Relative percentage survival ranged from 70% to 100% in vaccinated fish relative to the unvaccinated control after challenging fish with A. hydrophila. Greatest protection was observed in fish immunised orally with the A. hydrophila glycovaccine. The study's results suggest that this novel glycovaccine can induce a protective immune response against this problematic bacterial disease, offering an easy and safe approach for tilapia farmers to vaccinate their fish against A. hydrophila. Follow-up work will establish whether the vaccine can protect against more virulent strains of the bacterium and possibilities for commercialising the vaccine for use in tilapia aquaculture in India.