3
the reproductive phase) in the life cycle of the fi sh. A weakened host is then
prone to infection.
Notwithstanding these concerns, representatives of many bacterial taxa have, at
one time or another, been associated with fi sh diseases. There remains doubt about
whether some of these bacteria should really be considered as true fi sh pathogens.
In some cases, the supportive evidence is either weak or non-existent – scientists
may have failed to prove that the isolate from diseased fi sh is actually pathogenic -,
or there have been only single reports of disease without any repeat cases over many
years. Possibly, such organisms constitute contaminants or even innocuous saprophytes. However, it is readily apparent that there is great confusion about the precise
meaning of disease. A defi nition, 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 specifi ed 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…”
(Campbell et al. 1979 )
This defi nition is certainly complex, and the average reader may be excused for
being only a little wiser about its actual meaning. Dictionary defi nitions 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 among scientists about just
what constitutes a disease. Fortunately, infection by micro-organisms is one aspect
of disease that fi nds ready acceptance within the general category of disease.
For his detailed treatise on diseases of marine animals, Kinne ( 1980 ) considered
that disease might be caused by
– genetic disorders
– physical injury
– nutritional imbalance
– pathogens
– pollution.
This list of possible causes illustrates the complexity of disease. An initial conclusion is that disease may result from biological (= biotic) factors, such as pathogens, and abiotic causes, e.g. the emotive issue of pollution. Disease may also be
categorised in terms of epizootiology (Kinne 1980 ), as:
Sporadic diseases, which occur sporadically in comparatively small members of a fi sh
population;
Epizootics , which are large-scale outbreaks of communicable disease occurring temporarily in limited geographical areas;
Panzootics, which are large-scale outbreaks of communicable disease occurring over
large geographical areas;
Enzootics, which are diseases persisting or re-occurring as low level outbreaks in certain
defi ned areas.
1 Introduction
the reproductive phase) in the life cycle of the fi sh. A weakened host is then
prone to infection.
Notwithstanding these concerns, representatives of many bacterial taxa have, at
one time or another, been associated with fi sh diseases. There remains doubt about
whether some of these bacteria should really be considered as true fi sh pathogens.
In some cases, the supportive evidence is either weak or non-existent – scientists
may have failed to prove that the isolate from diseased fi sh is actually pathogenic -,
or there have been only single reports of disease without any repeat cases over many
years. Possibly, such organisms constitute contaminants or even innocuous saprophytes. However, it is readily apparent that there is great confusion about the precise
meaning of disease. A defi nition, 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 specifi ed 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…”
(Campbell et al. 1979 )
This defi nition is certainly complex, and the average reader may be excused for
being only a little wiser about its actual meaning. Dictionary defi nitions 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 among scientists about just
what constitutes a disease. Fortunately, infection by micro-organisms is one aspect
of disease that fi nds ready acceptance within the general category of disease.
For his detailed treatise on diseases of marine animals, Kinne ( 1980 ) considered
that disease might be caused by
– genetic disorders
– physical injury
– nutritional imbalance
– pathogens
– pollution.
This list of possible causes illustrates the complexity of disease. An initial conclusion is that disease may result from biological (= biotic) factors, such as pathogens, and abiotic causes, e.g. the emotive issue of pollution. Disease may also be
categorised in terms of epizootiology (Kinne 1980 ), as:
Sporadic diseases, which occur sporadically in comparatively small members of a fi sh
population;
Epizootics , which are large-scale outbreaks of communicable disease occurring temporarily in limited geographical areas;
Panzootics, which are large-scale outbreaks of communicable disease occurring over
large geographical areas;
Enzootics, which are diseases persisting or re-occurring as low level outbreaks in certain
defi ned areas.
1 Introduction