4
The study of fi sh diseases has concentrated on problems in fi sh farms (= aquaculture), where outbreaks either begin suddenly, progress rapidly often with high mortalities, and disappear with equal rapidity (= acute disease) or develop more slowly
with less severity, but persist for greater periods (= chronic disease). As we move
further into the 21
st century, issues about global warming/climate change are discussed – could this impact on the emergence and spread of fi sh diseases? A situation
could easily arise in which the host becomes stressed by increasing temperature,
and more prone to disease. Clearly, the deteriorating situation in the natural environment is of increasing concern. Indeed, there is already concern about the health of
corals, worldwide, and the initial evidence that some coral pathogens may also
infect fi sh. In another example, it is curious why mycobacteria appear to have
increased in signifi cance in fi sh within confi ned areas, notably the Chesapeake Bay,
USA.
This text will deal with all the diseases caused by bacteria. Cases will be discussed where infectious disease is suspected but not proven. An example includes
red mark syndrome/disease (also known as winter strawberry disease) of rainbow
trout in the UK where the causal agent is suspected to be bacterial of which rickettsia is the possible aetiological agent.
Disease is usually the outcome of an interaction between the host (= fi sh), the
disease causing situation (= pathogen) and external stressor(s) (= unsuitable changes
in the environment; poor hygiene; stress). Before the occurrence of clinical signs of
disease, there may be demonstrable damage to/weakening of the host. Yet all too
often, the isolation of bacteria from an obviously diseased fi sh is taken as evidence
of infection. Koch’s Postulates may be conveniently forgotten.
So, what are the bacterial fi sh pathogens? A comprehensive list of all the bacteria, which have been considered to represent fi sh pathogens, has been included in
Table 1.1 . Some genera, e.g. Vibrio , include many species that are acknowledged to
be pathogens of freshwater and/or marine fi sh species. Taxa (highlighted by quotation marks), namely ‘ Catenabacterium ’, ‘ H. piscium ’ and ‘ Myxobacterium ’ are of
doubtful taxonomic validity. Others, such as Pr. rettgeri and Sta. epidermidis , are of
questionable signifi cance in fi sh pathology insofar as their recovery from diseased
animals has been sporadic. A heretical view would be that enteric bacteria, e.g.
Providencia, comprise contaminants from water or from the gastro-intestinal tract
of aquatic or terrestrial animals. Certainly, many of the bacterial pathogens are
members of the normal microfl ora of water and/or fi sh. Others have been associated
only with clinically diseased or covertly infected (asymptomatic) fi sh. Examples of
these ‘obligate’ pathogens include Aer. salmonicida and Ren. salmoninarum , the
causal agents of furunculosis and bacterial kidney disease (BKD), respectively. It
will be questioned whether or not bacteria should be considered as obligate pathogens of fi sh, at all. It is a personal view that the inability to isolate an organism from
the aquatic environment may well refl ect inadequate recovery procedures. Could
the organism be dormant/damaged/senescent in the aquatic ecosystem; a concept
which has been put forward for other water-borne organisms (Stevenson 1978 )?
It is undesirable that any commercially important species should suffer the problems of disease. Unfortunately, the aetiology of bacterial diseases in the wild is
1 Introduction
The study of fi sh diseases has concentrated on problems in fi sh farms (= aquaculture), where outbreaks either begin suddenly, progress rapidly often with high mortalities, and disappear with equal rapidity (= acute disease) or develop more slowly
with less severity, but persist for greater periods (= chronic disease). As we move
further into the 21
st century, issues about global warming/climate change are discussed – could this impact on the emergence and spread of fi sh diseases? A situation
could easily arise in which the host becomes stressed by increasing temperature,
and more prone to disease. Clearly, the deteriorating situation in the natural environment is of increasing concern. Indeed, there is already concern about the health of
corals, worldwide, and the initial evidence that some coral pathogens may also
infect fi sh. In another example, it is curious why mycobacteria appear to have
increased in signifi cance in fi sh within confi ned areas, notably the Chesapeake Bay,
USA.
This text will deal with all the diseases caused by bacteria. Cases will be discussed where infectious disease is suspected but not proven. An example includes
red mark syndrome/disease (also known as winter strawberry disease) of rainbow
trout in the UK where the causal agent is suspected to be bacterial of which rickettsia is the possible aetiological agent.
Disease is usually the outcome of an interaction between the host (= fi sh), the
disease causing situation (= pathogen) and external stressor(s) (= unsuitable changes
in the environment; poor hygiene; stress). Before the occurrence of clinical signs of
disease, there may be demonstrable damage to/weakening of the host. Yet all too
often, the isolation of bacteria from an obviously diseased fi sh is taken as evidence
of infection. Koch’s Postulates may be conveniently forgotten.
So, what are the bacterial fi sh pathogens? A comprehensive list of all the bacteria, which have been considered to represent fi sh pathogens, has been included in
Table 1.1 . Some genera, e.g. Vibrio , include many species that are acknowledged to
be pathogens of freshwater and/or marine fi sh species. Taxa (highlighted by quotation marks), namely ‘ Catenabacterium ’, ‘ H. piscium ’ and ‘ Myxobacterium ’ are of
doubtful taxonomic validity. Others, such as Pr. rettgeri and Sta. epidermidis , are of
questionable signifi cance in fi sh pathology insofar as their recovery from diseased
animals has been sporadic. A heretical view would be that enteric bacteria, e.g.
Providencia, comprise contaminants from water or from the gastro-intestinal tract
of aquatic or terrestrial animals. Certainly, many of the bacterial pathogens are
members of the normal microfl ora of water and/or fi sh. Others have been associated
only with clinically diseased or covertly infected (asymptomatic) fi sh. Examples of
these ‘obligate’ pathogens include Aer. salmonicida and Ren. salmoninarum , the
causal agents of furunculosis and bacterial kidney disease (BKD), respectively. It
will be questioned whether or not bacteria should be considered as obligate pathogens of fi sh, at all. It is a personal view that the inability to isolate an organism from
the aquatic environment may well refl ect inadequate recovery procedures. Could
the organism be dormant/damaged/senescent in the aquatic ecosystem; a concept
which has been put forward for other water-borne organisms (Stevenson 1978 )?
It is undesirable that any commercially important species should suffer the problems of disease. Unfortunately, the aetiology of bacterial diseases in the wild is
1 Introduction