Society of Applied Bacteriology, Society of General Microbiology, European Asso-ciation of Fish Pathologists, and the U.K. Federation of Culture Collections; and has written previous books on bacterial taxonomy, marine microbiology, methods in aquatic bacteriology, methods for the microbiological examination of fish and shell-fish, and pathogens in the environment. Dawn Austinis a Research Associate at Heriot-Watt University, a position she has held since 1986. Prior to this she was Research Assistant at the University of Maryland (1977-1979), Lecturer in Microbiology, University of Surrey (1983-1984), and Research Fellow of the Freshwater Biological Association, The River Laboratory, Dorset (1984-85). Dr Austin gained a B.S. (1974) from City College, The City University, New York; an M.S. (1979) and a Ph.D. (1982) both from the University of Maryland.
1 Introduction Representatives of many bacterial taxa have, at one time or another, been associated with fish diseases. However, not all of these bacteria constitute primary pathogens. Many should be categorised as opportunistic pathogens, which colonise and cause disease in already damaged hosts. Here, the initial weakening process may involve pollution or a natural physiological state (e.g. during the reproductive phase) in the life cycle of the fish. There remains doubt about whether some bacteria should be considered as fish pathogens. In such cases, the supportive evidence is weak or non-existent. Possibly, such organisms constitute contaminants or even innocent sapro-phytes. However, it is readily apparent that there is great confusion about the precise meaning of disease. A definition, from the medical literature, states that: "... a disease is the sum of the abnormal phenomena displayed by a group of living organisms in association with a specified common characteristic or set of characteristics by which they differ from the norm of their species in such a way as to place them at a biological disadvantage ..." (Campbellet aL,1979) This definition is certainly complex, and the average reader may be excused for being only a little wiser about its actual meaning. Dictionary definitions of disease are more concise, and include "an unhealthy condition" and "infection with a pathogen [= something that causes a disease]". One conclusion is that disease is a complex phenomenon, leading to some form of measurable damage to the host. Yet, it is anticipated that there might be profound differences between scientists about just what constitutes a disease. Fortunately, infection by micro-organisms is one aspect of disease that finds ready acceptance within the general category of disease. For his detailed treatise on diseases of marine animals, Kinne (1980) considered that disease may be caused by:
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