World aquaculture has grown tremendously during last few years becoming an economically important industry. However, disease is a primary constraint to the growth of many aquaculture species and is now responsible for severely impeding both economic and socio-economic development in many countries of the world. Prophylaxis refers to all the preventive steps such as vaccination, immustimulation and use of probiotics including group improve routine husbandry practices that are taken during a hatchery and farming operation to minimize the load of pathogen and to prevent the occurrence of disease.
In vertebrates, survivors of pathogens infection become resistance to subsequent infection by the same pathogen. This resistance card adaptive immunity is specified to challenging pathogen and persists for a relative longer period of tome (immunomemory).
The following components are undertaken in prophylactic measure for disease prevention in aquaculture:
Prebiotics are non-digestible food material that beneficially affects the host by stimulating the growth and/or activity of one or a limited number of bacteria in the colony and thus improves host health status and promoting organism’s intestinal balance. The health-promoting bacteria most commonly augmented by prebiotics include those of the genus Lactobacillus and Bifidobacter which tend to limit the presence of harmful bacteria. Examples of prebiotics include mannanoligo-saccharides, Lactose, galactogluco-mannans, oligofructose and inulin.
The probiotics were defined as live microbial feed supplements that improve health of man and terrestrial livestock. The gastrointestinal microbiota of fish and shellfish are peculiarly dependent on the external environment, due to the water flow passing through the digestive tract. Most bacterial cells are transient in the gut, with continuous form water and food. Some commercial products are referred to as probiotics, though they were designed to treat the rearing medium, not to supplement the diet. Probiotics are administered through water of diet and inhibit the growth of pathogenic microorganisms, contribute digestive enzymes to increase feed utilization, provide other growth-promoting factors, and stimulate the immune response of the organism. Biotics that may influence fish immunity, disease resistance, and other performance indices include those of the genus Bacillus and various lactic acid bacteria (Lactobacillus, Lactococcus, Carnobacterium, Pediococcus, Enterococcus and Streptococcus).
What are aquatic probiotics?
Aquatic probiotics are intended for an aquatic usage it is important to consider certain influencing to consider certain influencing factors that ate fundamentally different from terrestrial based probiotics. Aquatic animals have a much closer relationship with their external environment. There are the big differences between terrestrial and aquatic animals in the level of interaction between the intestinal microbiota and the surrounding environment. On the other hand, potential pathogens are able to maintain themselves in the external environment of the aquatic organisms and proliferate independently of the host.
Aquatic probiotics are two types:
- Gut probiotics which can be blended with feed and administrated orally to enhance the useful microbial flora of the gut. These probiotics ate mainly used on fish aquaculture.
- Water probiotics which can proliferate in water medium and exclude the pathogenic bacteria by consuming all available nutrients. Thus, the pathogenic bacteria are eliminated through starvation. These probiotics are mainly used in shrimp aquaculture.
A vaccine is any biologically based preparation intended to establish or to improve immunity to a particular disease or group of diseases. Vaccines are normally administers to healthy animals prior to a disease outbreak. Vaccines work by exposing the immune system of an animal to an antigen or a piece of pathogen then allowing time for the immune system to develop a response and “memory” to accelerate this response in later infections by the targeted disease causing organism.
Different types of vaccines
There are many different types of vaccines, and new kinds are continuously under development.
Bacterins are vaccines comprised of killed, formerly pathogenic bacteria. Bacterins stimulate the antibody-related portion of the immune response (i.e., the humoral immune response).
Live, attenuated vaccines
Live, attenuated vaccines are comprised of live micro-organisms (bacteria, viruses) that have been grown in culture and no longer have the properties that cause significant disease. Live attenuated vaccines will stimulate additional parts of the immune system (i.e., a cell-mediated, as well as a humoral[antibody] response).
Toxoids are vaccines comprised of toxic compounds that have been inactivated, so they no longer cause disease. An example, used in humans, is the tetanus toxoid vaccine.
Recombinant vector vaccines
Recombinant vector vaccines allow a weak pathogen to produce antigen.
DNA vaccines are composed of a circular portion of genetic material that can, after being incorporated into the animal, produce a particular immune-stimulating portion of a pathogen (i.e., antigen) continuously, thus providing an “internal” source of vaccine material. Other vaccine strategies are also undergoing research and development.
Immunostimulants is chemicals, drugs, stressors or action that elevates the specific or non-specific immunoresponse. Immunostimulants enhance the humoral and cellular response in both specific and non-specific ways. These agents are widely used for impaired immune function and to stabilize the improved immune status. The use of immunostimulants in fish/shrimp culture or in aquaculture of other species for prevention of diseases is a promising new development.
Kinds of Immuno-stimulants
- Bacterial derivatives- if toxicity and harmful inflammatory factors are deleted, bacterial derivates can be good immunostimulants for e.g. Muramyladipetide(MDP), the mycobacterium peptidoglycan derivative has been used experimentally in clinical use for immunostimulating response in cancer in patient.
- Yeast derivatives- Gllucans , long chain poly saccharide extracted from yeast are good stimulators of non-specific defence mechanism in animal including fish and shellfish.
Bioremediation is use of living organisms to reduce/eliminate toxic pollutants (NH3) and other nitrogenous wastes. Living organisms includes- bacteria, fungi, actinomycetes, cyanobacteria and plants.
- Organic derivatives-micro organism’s should have good enzymatic capability. E.g. Bacillus spp.
- Nitrogenous compounds- Bacterial nitrification through sand and gravel biofilter in circulating water e.g. Nitromonas, Nitrosovibrio etc
- Phosphorus derivatives- PO4 generated from organic compounds by bacteria releasing fates phosphates and phytase enzymes.
Prophylactic Husbandry Measures in aquaculture
In addition to vaccine, immunostimulants and probiotics, some of the important prophylactic steps that are to be routinely adopted for successful farming hatchery operations.
- Use of good quality water ( UV filter, sand filter)
- Use of healthy pathogen free brood stock
- Following strict sanitary and hygienic conditions
- Adopting strict sanitary and hygienic conditions
- Regular health monitoring
- Use of good quality live feeds
- Use of treated water
- Proper pond preparation (drying, waste removal, liming etc.)
- Use of good quality water
- Use of good quality seed( seeds screening, healthy seed selection)
- Rational feeding and fertilization schedule
- Adopting rational stocking policy in order to reduce stress
- Following strict hygienic condition and quarantine programme
- Regular health monitoring programmes
- Use of good quality live feeds