Fish Health in Aquaculture: Diagnosis, Prevention and Control of Infectious Disease of Fish

India is the second largest fish producing nation in the world. It can support sustainable tourism, aquaculture and marine renewable energy, because if we can establish more protected areas and enforce sustainable fishing practices.Fish diseases is a substantial source of monetary loss to aquaculturists.  Production costs are increased by fish disease outbreaks because of the investment lost in dead fish, cost of treatment and decreased growth during convalescence. Parasites and bacteria may be of minimal significance under natural conditions, but can cause substantial problems when animals are crowed and stressed under culture conditions.

In this review the term fish refers to the abundant and diverse any fish, mainly those that are formed. Aquatic feeds have evolved over the last half a century and the feed industry is aiming to optimize the quality of their products and to develop alternate ingredients that meet the physiological requirements of the animal. The review of available literature revealed the promising effects of probiotics on disease resistance of fish. Therefore, it can be speculated that this environment friendly dietary supplement will receive increasing attention as an alternative for antibiotic in aquaculture.

Protecting Aquatic Species: Human activity is indelibly impacting the aquatic species. Sewage and industrial effluents, oil spills and chemicals used in agriculture are depleting oxygen and nutrients and killing marine and fresh water plants and animals life. Over fishing is driving the crisis. Almost 90% of the world’s fisheries are now over- exploited, fully exploited, depleted or nearing collapse, from sharks to sword fish to marlin face extinction. Climate change is harming aquatic life, air pollution is adding one-third of toxic contaminants, global warming is causing sea acidification. About 60% of marine ecosystems globally have been degraded.There are some special types of aquaculture:

1. Aquaculture such as sea weed farming

2. Shrimp farming

3. Integrated Multi Trophic Aquaculture ( IMTA)

4. Recirculating system- culture in reconditioned water and in closed system

5. Culture of rice fields

6. Aquaculture in race ways, cages, open net pens and enclosures

7. Cultivation of ornamental fish

Prevention and control of fish diseases:  There are three major causes of fish diseases-

1. Presence of environmental pathogens

2. Low resistance of the fish stock

3. Unsatisfactory water environment

Pathogens (e.g. bacteria, viruses fungi and parasites) exist in all natural water bodies, yet healthy fish have adequate resistances against them. They can also adapt to reasonable environmental changes and in turn avoid diseases due to pathogenic infection. When the pathogen level of a water body rises sharply due to external factors and the natural resistance of the fish stock cannot cope with the increased pathogens, the fish will become vulnerable to pathogenic infection and diseases. In addition, external factors may also cause drastic changes in water quality, resulting in poor health and low resistance of fish stock. The ricks of pathogenic infection and fish diseases or deaths are heightened. (2009).

We should maintain a good culture environment and prevent the deterioration of water environment; and use hygienic and nutritious fish feed to boost resistance of the fish stock and to minimize the chance of introducing pathogens to the water body.

Culture-dependent techniques: Biofloc Technology(BFT) is considered as new ‘ blue revolution’ since nutrients can be continuously recycled and reused in the culture medium, benefited by the minimum or zero- water exchange. This means that laboratories need to encompass a wide range of approaches to insure the best chance of recovering the pathogens in culture. Aqua farming grow fish and other aquatic animals plants differently. They use farm techniques and places that work well for what they grow.Most fish farms use ponds. Farmers make ponds with soil or cement walls. The ponds hold water for the fish. Other places help too. Some farms grow fish in big tanks. s Farmers also grow fish in cages. Cages are like big wire boxes in lakes or the oceans. Fish swim inside the cages.

The Role of Vaccines in Control and Prevention of Infectious Disease in Aquaculture:There are two categories of disease that affect fish, infectious and non- infectious diseases. Infectious diseases are caused by pathogenic organisms present in the environment or carried by other fishes.  But non- infectious diseases are caused by environment problems, nutritional deficiencies, or genetic anomalies. Infectious diseases pose a unique problem of diagnosis. It can be occurred both externally and internally.

1. Infectious diseases are contagious diseases caused by parasites, bacteria, viruses or fungi.

2. Non-infectious diseases are broadly categories as environmental, nutritional, or genetic. These problems are often correctly by changing management practices.

Fish vaccination was started by vaccinating against Aeromonas salmonicida infection in Cutthroat in 1942 . Vaccines available are oil adjuvant, injectable vaccines .

The salmon genome is now fully sequenced ( Lien,Koop, Sandev, 2016), the genome and several other fish species as well. These findings can lead to novel vaccine development strategies in near future (Earle and Hintz, 2014) . Vaccines against intracellular bacterial and viral pathogens will be one of the big challenges for the coming years. DNA vaccine will play a role in such cases (Gudding and Muiswinkel, 2013) .

1. Killed Vaccines

Killed vaccines are conventional types of vaccines prepared by killing the infectious agent and using it as an antigen to induce an induce response. Most of the commercial vaccines currently used in aquaculture are killed vaccine types. The advantages of these vaccines are as follows: they are easy to design, are stable in storage, and are less expensive and there are no virulence issues( Pridgeon and Klesius, 2012). Preparation strategies of these vaccines is that they most of the time target the outer surface of microorganism or inner parts without avoiding the ability to replicate when administered to the host . Infectious hematopoietic necrosis virus.Attenuated Vaccines

These are also conventional vaccines which are used in food producing animals and humans to prevent disease (Craig and Klesius, 2014).  They are prepared by repeated laboratory passage, physical and chemical attenuation of the organism to lose there virulence without killing them. Laboratory studies have shown the effectiveness of live vaccines in fish. They induce mucosal, cellular, and humoral immunity. Attenuated organism replicates in the target host without any clinical signs (Lillehaug, 2014).

2. Genetically Modified Vaccines

In vitro passaging of organisms results in a build up of genome mutations that make the organism weaken. Genetically, microorganisms may be attenuated by molecular approaches that involve removal of genes responsible for its pathogenicity. Aeromonas salmonicidain salmon can be prevented by these preparations ( Dadar, Dhama and Vakharia, 2016).

3. Injection Vaccination

Vaccination by injection is the delivery method generally resulting in best protection and is the only choice for adjuvant vaccines.  vaccine can be administered. The disadvantages of this administration are as follows: not suitable for small fishes, adhesion formation, stress in fish and feed intake reduction, damage during injection may cause multiple deaths in fish, and not being administered in very young stock due to immunity development may not sufficient .

The use of Antibiotics in Aquaculture:

Currently, there is a common understanding that antibiotic for protecting fish from disease should remain low (Aly and Albutti, 2014)  . Extensive use of antibiotics may result in resistance development. The use of antibiotic in aquaculture is no more primary treatment option. Even though using antibacterial agents in food animal species, including fish, is controlled by regulations, particularly in Europe and USA, a wide range of medically and veterinary inhibitory compounds are in use in aquaculture (Austin, 2017).  This difference particularly adding drugs to the water with or without feeds results in environmental disturbance of the microbiota. Aqua culturists are advised to use other prevention approaches rather than antibiotics administering but if the use antibiotics is a must in some circumstances, they have to administer only approved drugs for fish(Rogers and Basurco, 2009).

The use Probiotics in Aquaculture:

Biological disease control in aquaculture is among the best approaches in infectious disease control (Maqsood, Singh Samoon and Munir, 2011)  . Probiotics are a bacterial culture of bacterial strains which are nonpathogenic to fish (Sharifuzzaman and Austin, 2017) . The other definition of probiotics is that live microorganisms, administered to hosts to develop a protective immune status. After being administered to fish they multiply  themselves to occupy the gut of the fish, they help normal microflora, and they maintain microbial balance in the hosts(Mastan, 2015).

They stimulate the immune system of fish, avoid stress and act as antibacterial and antiparasitic agents due to their active chemical ingredients(Reverter, Bontemps, Lecchini, Banaigs and Sasal, 2014) . They can be administered by extracting their active components and the whole plant material can be added to the aquarium directly. Depending on the type of plant part used and the season of harvest of the plant material, their active ingredients may be varied so knowledge of the plant and season of the collection is necessary. Medicinal plants can be administered to fish by injection oral administration and through immersion or baths . Injecting the extracted material is an effective method for large fish(Awad and Awaad, 2017) .

Importance diagnostic tests in prevention and control of infectious disease in aquaculture:

Diagnostic test for identification of fish disease include conventional microbiological, immune serological, and molecular methods. Rapid and accurate molecular methods have become important diagnostic tools. Lateral flow immunoassays, DNA microarray, proteins, or glycans can also be immobilized on a solid surface of the microarray to probe different target molecules labeled with fluorescence(Kim, Nguyen and Kim, 2017) . In diagnosing disease of fish, the detection of the pathogen in a tissue sample is conducted by lethal sampling rather than detection of antibodies that are an indicator of a particular disease, but in case of high valued fishes like ornamental fish, nonlethal sampling is recommended( Harikrishnan, Balasundaram and Heo, 2011) .

Challenges in prevention and control of fish disease:

The nature of the fish themselves because fish cannot be observed close enough like we do in terrestrial animal the environment can facilitate disease transmission quickly, fish are not cached easily, without stress, they often gather in groups, and disease is often difficult to detect and characterize( Peeler and Taylor, 2011) . The other important challenge is in diagnosing disease of fish; in terrestrial animal disease diagnosis, the individual animal is the unit of interest. The scenario is not the same in aquaculture disease investigation because of the nature of the water where fish lives. A disease can transmit quickly and the whole tank may be the source of disease to healthy stock. In this case, the unit of interest is not a single fish rather the whole tank need to be investigated and diagnosed. Samples should be collected not only from fishes but also from water to measure important characteristics like pH, soil bottom condition, and turbidity which makes aquatic animal diagnosis complicated and challenging(Kumar, Roy, Meena and Sarkar, 2016) .

Diseases encountered by aquaculture species:

Several diseases are encountered by an aqua culturist particularly in semi-intensive and intensive systems of culture. Some of the major diseases, causative organisms, symptoms and treatment are :

a. Nutritional diseases

Nutritional diseases an be said to be of three types:

i. Those arising from malnutrition.

ii. Those arising from imbalance in the major components of foods.

iii. Those arising from toxic effect of the diet.

Standard artificial diets tend to eliminate these diseases. Sub-standard diets used in supplementary feeding increases the possibility of their occurrence.

1. ‘Pin heads’ with enlarged head and slender body occur in nurseries due to starved larvae. Starvation occurs due to overstocking.

2. Lipoid hepatic degeneration disease is common among farm fishes caused due to overfeeding, where trash fish or pelleted diets have developed rancidity. The symptoms are swollen liver with rounded edge, anemia followed by gills looking pallor.

3. Nutritional gill disease occurs from panthodic acid deficiency in the diet. The symptoms are inappetence, respiratory troubles and anemia.

4. Vitamin deficiency results in diseases which are enlisted.

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