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<br />CHAPTER 6 <br />The Role of Infectious <br />Roger L. Herman <br />Introduction <br />Outbreaks of bacterial disease are seldom the re- <br />sult of a single factor. Three factors are involved in <br />every potential disease situation: susceptible hosts, <br />pathogenic organisms, and predisposing environ- <br />mental conditions. All must be present when an <br />epizootic occurs. Snieszko (1964) listed decreased <br />immunological response, poor genetic resistance, <br />temperature stresses, pollution, unfavorable water <br />chemistry, and other adverse conditions as some of <br />the possible predisposing factors. Adverse condi- <br />tions may include factors such as crowding, inade- <br />quate food supply, spawning activity, storms, and <br />seasonal changes. Although bacteria may be the <br />ultimate cause of death in a particular situation, <br />some other factor is often more important. Consider, <br />for example, the massive losses of tilapia that occur <br />when the water temperature falls below the opti- <br />Bacterial infections are <br />usually characterized by <br />lesions on or in the body <br />of fish. This channel cat- <br />fish shows the type of <br />lesion associated with <br />bacterial hemorrhagic <br />septicemia. <br />RESOURCE PUBLICATION 177, 1990 45 <br />ants in Fish Kills <br />mum for these species. A sudden cold wave may <br />result in massive losses. Survivors or moribund in- <br />dividuals often yield heavy cultures of bacterial <br />pathogens and, unless the observer is alert to the <br />circumstances involved, a diagnosis of a bacterial <br />epidemic might be given. Temporary intrusions of <br />salt water into freshwater environments (or vice <br />versa) can cause similar situations. <br />In the examples given above, outbreaks of disease <br />that result in mass mortalities in natural waters are <br />associated with stressful environmental changes, <br />high population densities, or shortages of food. <br />Whenever any of these factors compromises the <br />immunological capability of the fish, disease often <br />occurs. Pathogens rarely overwhelm a healthy <br />population of fish. Therefore, it is important to look <br />for underlying factors that may have contributed to <br />the occurrence of a pathogen-caused fish kill in <br />natural waters. <br />I