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Chapter: Health Management in Aquaculture: Immunity and biological methods of disease prevention and control

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Avoidance of Pathogens - biological methods of disease prevention and control in Aquaculture

At the turn of this century, the concept and practice of biological control was developed by agriculturists to control insect pests.

BIOLOGICAL CONTROL

 

At the turn of this century, the concept and practice of biological control was developed by agriculturists to control insect pests. In agriculture, biological control is generally defined as the use of a specially chosen living organism to control a particular insect pest. It is basically the use of one chosen organism to control another organism. This chosen organism might be a predator, parasite or disease, which will attack the harmful insect. It is a form of manipulating nature to increase a desired effect. In agriculture, a complete biological control program may range from choosing a pesticide, which will be least harmful to beneficial insects, to raising and releasing one insect to have it attack another, almost like a “living insecticide”.

The scope of the concept of biological control was later expanded to include all kinds of pests. Such as the use of different species of carps to control the prolif-eration of water lilies and other plant pests that choke ponds and rivers. More recently, the concept of biological control was further expanded to include all forms of biological manipulation of the host, the environment and the pest (or pathogen) to minimize or control infestation or infection. It is this expanded scope of biological control that we will discuss here and we will focus on the use of biological control in preventing, minimizing or eradicating disease in aquaculture stocks.

Avoidance of Pathogens

Aquaculture facilities that use intensive techniques for production often pro-vide an environment for the cultured organism that is unnatural and stressful. A primary factor is crowding or over-crowding. Under this condition the fish or the cultured organism has to compete with each other for space and dissolved oxygen and are confronted with the stress of exposure to increased metabolites such as ammonia, carbon dioxide and dissolve or suspended organic matter. It is not surprising that disease outbreaks often occur in these facilities. An aquaculturist’s main concern then is to eliminate all possible sources of these disease agents.

Pathogen-free water supply

A pathogen-free water supply is obtained through the sterilization of fresh wa-ter or seawater. Sterilization of in-coming seawater may control the number of potentially harmful microorganisms entering the aquaculture systems but not necessarily totally eradicate these microorganisms in the water. The most com-monly employed methods for controlling the presence of these microorganisms are by filtration, ultraviolet light treatment, ozonation and chlorination.

1. Filtration

 

Ultrafiltration of water supplies through sterile 0.2µm membrane filters has the advantage of having none of the detrimental effects associated with the other methods of water sterilization. However, it could be a slightly more expensive method than the other three and that absolute retention of bacte-ria (e.g. 100%) will not necessarily be achieved.

2. Ultraviolet light

 

Ultraviolet light at a wavelength of 254nm or within the range 240 to 280nm disinfects seawater but not sterilizes it. It reduces the reproductive capacity of bacteria and fungi and is therefore bacteriostatic (or fungistatic) rather than bactericidal. Organic matter in seawater may also be oxidized depending on the ultraviolet energy emitted. The efficiency of ultraviolet light treatment is dependent upon the amount of particulate material sus-pended in the water and of the presence of natural pigments seawater. To improve efficiency, it is advisable to reduce particulate matter content by mechanical filtration prior to UV light exposure.

3. Ozone treatment

 

Ozone is a highly oxidizing form of oxygen, if used with caution; it can be a powerful means of improving seawater quality. Ozone dissociates rapidly in seawater to provide a highly active oxygen atom. Ozone will disinfect and sterilize seawater, oxidize organic material and oxidize toxic nitrite to less toxic nitrate. However, ozone is highly toxic to both man and the organism being cultured. It is also highly corrosive to aquaculture equipment, be it metal or plastic. The effective dosage of ozone can be affected by factors that also consume ozone such as: chemical oxygen demand, salinity, dis-solved substances, and microbial and plankton densities.

The equipment manufacturers provide guidance on the use of ozonators and UV light cartridges.

4. Chlorination

 

Chlorination, although less effective than ozonation, is the more popular form of seawater sterilization in aquaculture facilities in Southeast Asia. This could be due to the fact that chlorination does not involve special equipment. However, it shares many of the disadvantages of ozone in that it is toxic to both man and the cultured organisms, corrosive, and may form toxic stable complexes with organic compounds (e.g. chloramines). Chlo-rine as calcium or sodium hypochlorite is commonly used in hatcheries in south Asia.

Pathogen-free diets

Most manufacturers of artificial diets claim that their products are pathogen-free. In most cases the preparation of commercial feeds involves high tempera-tures of about 120ºC for short periods that should be enough to pasteurize if not sterilize the feed.

 

On the other hand, the feeding of live food may present a different set of prob-lems in that some germicides that are used to disinfect live feed could also be toxic to the live food organism. In Japan, live foods are managed under sani-tary conditions to prevent intestinal infections of larval fish and shellfish. Bath treatments with a nitrofuran derivative, sodium nifurstyrenate, in live diets, such as rotifers and brine shrimp, are effective approaches to decreasing the number of bacteria in these diets.

 

Specific-pathogen-free (SPF) stocks

SPF animals are defined as “animals that are free of specified microorganisms and parasites but not necessarily free of others.” Although specific pathogen free technology has been practiced in agriculture for many decades it has only been recently applied in aquaculture.

In prawn farming, some pathogens causing epizootics during the grow-out phase have been traced to be vertically transmitted from wild broodstock that are carriers of diseases. The vertical transmission of pathogens from mother to larvae is a continuous threat to production. In Taiwan, the screening of wild broodstock and the production of captive reared broodstock, which are certi-fied SPF with regards to white spot syndrome virus (WSSV), have resulted in grow-out cycles free of WSSV epizootics. Although the use of SPF broodstock does not result in disease resistant or even disease tolerant stock, it is proving to be one effective managerial control measure, which minimizes the likeli-hood of epizootics due to an identifiable pathogen.

Disease inspection, quarantine, and international regulations

A fish health inspection is a procedure by which a sample of fish collected from a defined fish population and examined for the presence of certain specific pathogens. Knowledge of the presence of pathogens can be used to prevent the introduction of serious fish diseases into areas where they do not presently occur and to better manage those diseases.

A number of serious diseases are caused by organisms that can survive only for very short periods of time outside of the fish they infect. These organisms are calledobligate pathogens. Fish health inspections are conducted to detect this obligate pathogen group because one of the most likely methods by which these are spread to new areas is through infected fish. Another broad group of pathogens are called facultative pathogens. They are commonly found in all aquatic environments and may cause disease only when the host fish is stressed. Fish health inspections are not typically conducted to detect organ-isms in the facultative group.

Quarantine is defined as the holding or rearing of animals under conditions that prevent their escape or the escape of a disease agent. Quarantine can pro-vide a useful environment for “filtering out” disease in new stocks, especially if cures are available. Unfortunately post-treatment checks of individuals in large aquatic animal populations are generally not practical, and few cures are known for important fish and shrimp diseases. Quarantines are sometimes used as a means to avoid a particular disease agent. Such a concept often emerges as a regulatory effort to establish “disease free” or “pathogen free” status of imports. While it is true that the quarantine technique is a means for disease avoidance, there is a great deal of difference between the presence of disease and the presence of pathogens.

A number of international organizations have been developing guidelines or codes of practice for the introduction and transfer of species. These guidelines establish specific diagnostic techniques for pathogens, define sanitary regula-tions of the individual countries, and develop health certificates for facilities to accompany shipments and prohibit the international transfer of aquatic ani-mals that are not accompanied by these certificates. The organizations that have developed protocols include the Office International des Epizooties (OIE), the European Inland Fisheries Advisory Commission (EIFAC), and the Interna-tional Council for the Exploration of the Sea (ICES). The ICES code of practice details how the species approved for importation are to be handled, the follow-ing are the steps of the protocol: 1) imported stocks are examined for potential pathogen or parasites, 2) stocks that are pathogen-free are grown into broodstock in an approved quarantine facility where they are regularly exam-ined for pathogens, 3) if no pathogens are detected, the first generation off-spring are released to the farmer, the original imported stock never leaves the quarantine site and disease studies are continued on the transplanted individu-als, and 4) to eliminate the need for further importations of this species, it is recommended that F1 individuals be used to establish a local broodstock.

Biology of the pathogen

The control of disease in an aquatic environment is particularly unique be-cause water, as a universal solvent, makes prevention and control of physical, chemical and biological contamination of water and water sources much more difficult compared to land-based agriculture. Aside from the water itself, aquatic animals in all stages of their life cycles are carriers and reservoirs of disease.

Knowledge on the biology of the pathogen is one tool in breaking the cycle of infection and transmission. In shrimp hatcheries, for example, there is evi-dence that the virulence of facultative pathogens (e.g. Vibrio harveyi) increases from cycle to cycle. Successive passages from host to host provide a mecha-nism where the characteristic of virulence is selected. In order to limit the spread, adaptation and selection of these virulent strains, it is advisable to separate each group of larvae in time, (such as by batch culture, or by giving rest periods to the whole facility) and to isolate each group in space (by giving extra distance between tanks or using enclosures in some tanks). Any vector of contamination (e.g. water, workers, equipment and the fish themselves) must be controlled continuously to prevent vertical and horizontal transmission of pathogens. Another example is in the use of antibiotics in hatcheries. The use or overuse of antibiotics in hatcheries and farms may encourage outbreaks of oomycetes (Lagenidium, Sirolpidium and Haliphthoros) by removing the com-petitive bacterial microflora allowing these organisms to proliferate. An addi-tional example where knowledge of the physiology of the pathogen can be put to good use is in cases of vibriosis. The Vibrio species that infect marine fish and shellfish are not tolerant to freshwater or very low salinities. If the infected cultured stocks happens to be euryhaline like the sea bass then the salinity of  the rearing water can be lowered gradually even down to zero parts per thou-sand salinity to eliminate the pathogen.

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