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Diseases caused by bacteria may cause heavy mortality in both wild and cul-tured fish and crustaceans. Bacteria are found everywhere in the aquatic envi-ronment. Most bacterial disease agents are part of the normal microflora of the marine environment and are generally considered as secondary or opportunis-tic pathogens. Almost all fish bacterial pathogens are capable of independent existence outside the fish. There are only a few obligatory pathogens. Even these, however, are capable of living for a long time in the tissues of their host without causing injury. Clinical infections and disease usually occur only after the onset of some major changes in the physiology or body of the host. Thus, to understand bacterial diseases of fish, one must understand the relationship of bacteria with their host and with their environment.
As in all animal production systems, bacterial disease is one of the major prob-lems facing production, development and expansion of the aquaculture industry. The control of disease is particularly difficult because fish are often farmed in systems where production is dependent on natural environmental conditions. Changes or deterioration in the aquatic environment cause most of the bacterial diseases encountered, and environmental effects give rise to many other adverse conditions. A second major constraint on disease control is the relatively limited number of therapeutic agents available for the control of bacterial disease agents. Even recommended therapies and preventive measures pose limitations. Their application to aquatic animals is often difficult in actual practice, and sometimes impossible to implement.
Outbreaks of major bacterial diseases in aquaculture can be significantly re-duced if proper attention is paid to good husbandry practices and the mainte-nance of optimum environmental conditions, especially water quality. Another important consideration involves identifying the predisposing factors that may lead to a disease state. Once predisposing factors are identified, appropriate corrective measures should be initiated in the culture system.
Most bacterial disease show similar signs, especially in fishes. Bacterial infec-tion may appear on the skin or fins of fish, exoskeleton or appendages of crus-taceans, in the muscles and in the internal organs. In nearly all cases, red spots, brown or black spots, or necrotic tissues can be observed. Inflammation may also occur. Proper identification of the causative agent is important to ensure successful treatment.
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