Novel immunomodulators in aquaculture

Due to the negative impact of antibiotics, there is a need to develop eco-friendly alternatives to control or prevent the bacterial infections in aquaculture. At this point, immunostimulants appear to be the most promising and useful tools for the prophylactic treatment of farmed fish and shrimp. It is safer than chemotherapeutics and their range of efficacy is wider than vaccination.

In the last few decades aquaculture has been growing rapidly for food production. World fisheries and aquaculture production has attained around 177.8 million tons (FAO, 2020). Several commercially important fish species have been intensively cultured in narrow or enclosed spaces viz., tanks or cages under high density leading to adverse effects on the cultured fish with potentially stressful environment and infectious diseases (Sakai, 1999).

Antibiotics and chemotherapeutics have been utilized to control or prevent the bacterial infections in aquaculture for about 20 years (Sakai, 1999). Due to increasing trend in antibiotic-resistant bacteria, regrettably the usage of antibiotics for treatment is not sustainable and successful, negative consequence on the indigenous microflora of juveniles or adult fish (Misra, 2006), and the aggregation of antibiotic residues in tissues of fish and environment inducing animal and human health issues.

Due to their negative impact on interactions, there is a need to develop eco-friendly alternatives to antibiotics that may keep fish healthy, such as probiotics, immunostimulants and applications of indigenous technological knowledge for treating diseases in fish health and disease management (Sahu et al., 2007).

An immunostimulant is specified as a chemical, drug, stressor that enhances the non-specific or innate immune response by interacting with the system that is activated by cells. Immunostimulants can be specified under bacterial preparations, chemical agents, polysaccharides, nutritional factors, cytokines and animal or plant extracts. Immunostimulants are chemical components which trigger leukocytes (Lunden, 2000).

TYPES OF IMMUNOSTIMULANTS USED IN FISH AND SHRIMP
Synthetic chemical
• Levamisole

Biological substances
• LPS (lipopolysaccharide)
• Yeast derivatives- Glucans
• Polysaccharides – Chitin and Chitosan
• Nutritional factors – Vitamin C and Vitamin E

Levamisole: Levamisole is an antihelminthic chemical compound that is used as curative medicine for the nematodes infection in animals and humans as well. It can induce an immune response in vitro. It enhances phagocytic activity, increases antibody producing cells and the NBT reaction. Oral administration of levamisole elevates the lysozyme activities in serum, number of leucocytes, phagocytic index of phagocytic cells and the stimulated NBT reduction (Ispir, 2005). Whereas, no differences were found in the levels of leucocrit or immunoglobulin, hematocrit using levamisole in rainbow trout (Ispir,2005). It has been found that rainbow trout exposed to levamisole at the concentration of 5, 10, 25 µg/ml through bath for 2 hr period showed resistance to Y.ruckeri (Ispir, 2009).

LPS (lipopolysaccharide): LPS is obtained from components of Gram-negative bacterial cell walls. It was reported in preventing A. hydrophilla disease and inducing innate immune response of rainbow trout (Nya,2010). Salati et al. (1987) described that LPS can induce the production of superoxide anions and phagocytosis in Atlantic salmon. Macrophages and LPS can enhance macrophage phagocytic activity and stimulate B-cell proliferation in red sea bream Pagrus major. Also, LPS stimulates the macrophage activating factor production in goldfish lymphocytes (Neumann, 1995). These components are high potent even at low doses. LPS enhance phagocytic activity as well as stimulate hemocytes proliferation and microbicidal activity of shrimp (Karunasagar, 1996).

Yeast derivatives: Glucans are polysaccharide extracted from yeast, they are good stimulators of non-specific immune response like phagocytic activity and protection against bacterial pathogens in animals including fish and shellfish. Many types of glucan has been reported in fish viz., yeast glucan, glucan (VST), peptide-glucan β -1,3, Yeast glucan and β-1,3 glucan (VST) is obtained from cell walls of baker’s yeast like Schizophyllum commune and Saccharomyces cerevisiae, respectively (Pais, 2008). β-glucans constitute a diverse group of polysaccharides of D-glucose monomers associated with β-glycosidic bonds. β-glucan increased the activity of cellular and non-cellular defense mechanisms like phagocyte activity, lysozyme activity, complement activity and bactericidal activity of macrophages.

Polysaccharides: Chitin is a polysaccharide which comprises the component of crustacean and insect exoskeletons, and few fungal cell walls (Sakai, 1999) It can trigger macrophage activity and give protection from certain bacteria.

Nutritional Factor – Vitamin C: Vitamin C is involved in various physiological functions viz, growth, wound healing, development, reproduction, response to stressors and possibly lipid metabolism by its action on carnitine synthesis while administering in feed. Vitamin C (Ascorbic acid) is a co-factor in many biological processes such as cellular functions related to neuromodulation, collagen synthesis, hormone and immune systems.

Nutritional Factor – Vitamin E: Vitamin E can increase specific cell-mediated immunity and macrophage phagocytosis in fishes

CONCLUSION
Immunostimulants appear to be the most promising and useful tools for prophylactic treatment of farmed fish and shrimp. It is safer than chemotherapeutics and their range of efficacy is wider than vaccination. Additional research is needed to define the specific dosage rates and efficacy of various compounds for a variety of aquatic species and their pathogens and to decrease costs of the immunostimulants.

References
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