IDENTIFYING THE REAL CAUSE OF A DISEASE; KOCH’S POSTULATES
Not all bacteria present in the body of a fish or crustacean are pathogenic or may cause disease. Some bacteria may be harmless or even beneficial. By care-fully noting the observations suggested here, one might be able to tell if the isolated bacterium suspected of causing the disease is the causative organism.
Koch, a German physician and bacteriologist, enunciated the following criteria in 1891 to establish unequivocally a causal relationship between an organism and a specific disease.
• The organism should be found in all cases of the disease in question, and its distribution in the body should be in accordance with the lesions observed.
• The organism should be cultivated outside the body of the host, in pure culture, for several generations.
• The organism so isolated should reproduce the disease when introduced into other susceptible animals.
• The organism must be reisolated from the experimentally-infected animal.
To produce disease, microorganisms must be able to:
• Enter the host;
• Multiply under the physical and chemical conditions of the host tissues;
• Interfere with the action of humoral (in body fluids) and cellular defense mechanisms of the host; and
• Damage tissues thereby producing the unpleasant and possibly lethal ef-fects.
Bacteria are ubiquitous. This means that they can be found or are present al-most everywhere in the aquatic environment. The actual role of these microor-ganisms may vary from being beneficial to that of being a secondary opportu-nistic invader, attacking only when the host is weakened or injured, or a pri-mary pathogen that may cause the death of the species.
In this chapter, the important bacterial diseases of fish and crustaceans are discussed as well as the different diagnostic methods, gross signs and preven-tive measures and treatments of these diseases.
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