2
problems for determining reliable diagnostic traits. In many laboratories, identifi cation is now routinely accomplished by means of sequencing of the 16S rRNA gene;
a move that has led to greater confi dence in the outputs although this will refl ect the
accuracy of the data in the databases. However, whereas the use of new technologies
is to be encouraged, an on-going dilemma remains about the authenticity and value
of isolates. Also, many studies are based on the examination of single isolates the
relevance of which to fi sh pathology or science in general is perhaps doubtful.
Certainly, too many conclusions result from the examination of too few isolates.
Nevertheless, the study of pathogenicity mechanisms, diagnostics and disease control by means of vaccines have all benefi ted from molecular approaches.
It is apparent that there has been a progressive increase in the number of new
bacterial taxa associated with fi sh diseases, with examples including Pasteurella
skyensis and Francisella noatunensis. However, some elementary questions/concerns about bacterial fi sh diseases remain to be addressed:
– Why are so few anaerobes associated with fi sh diseases? Could this refl ect a lack
of interest/expertise/suitable methods as opposed to a lack of occurrence?
– Are the majority of diseases really caused by single bacterial taxa or could there
be many more incidences of infections caused by two or more taxa either acting
simultaneously or sequentially? [Would diagnosticians recognise infections
caused by more than one pathogen?]. Does the same organism instigate and
develop the disease situation or could there be a microbial succession with one
organism initiating an infection, and a second or third developing the disease.
Could conventional diagnoses differentiate primary pathogens from secondary
invaders/contaminants of diseased tissue?
– Unculturables, e.g. Candidatus, are becoming associated with fi sh diseases, i.e.
situations where pathogens may be detected microscopically or serologically but
not cultured. The question to be resolved is whether such organisms are incapable of growing outwith a host or if suitable media have not been developed. It is
speculative how many more of these unculturable organisms remain to be recognised. Moreover, how many times has Candidatus been missed, and secondary
invaders/contaminants labelled as the actual pathogen? Then, there is the situation, such as with red mark syndrome, whereby an organism may be detected by
serology or molecular methods, but not readily observed.
– There is the issue regarding the value of the cultures obtained during diagnoses.
The conventional dogma is that where isolation from an active disease situation
is attempted the result on laboratory media will be dense virtually pure cultures
and not a comparatively few diverse colony types; the latter being interpreted as
indicative of the presence of contaminants. This approach would miss infections
caused by two or more different organisms.
– The value of cultures is a major concern insofar as the assumption is that the pure
culture is representative of the organism in its natural habitat, and thus may be
used for meaningful studies of pathogenicity, ecology and disease control.
– Certainly, an initial weakening process to the host may be possible in the absence
of pathogens, and involve pollution or a natural physiological state (e.g. during
1 Introduction
problems for determining reliable diagnostic traits. In many laboratories, identifi cation is now routinely accomplished by means of sequencing of the 16S rRNA gene;
a move that has led to greater confi dence in the outputs although this will refl ect the
accuracy of the data in the databases. However, whereas the use of new technologies
is to be encouraged, an on-going dilemma remains about the authenticity and value
of isolates. Also, many studies are based on the examination of single isolates the
relevance of which to fi sh pathology or science in general is perhaps doubtful.
Certainly, too many conclusions result from the examination of too few isolates.
Nevertheless, the study of pathogenicity mechanisms, diagnostics and disease control by means of vaccines have all benefi ted from molecular approaches.
It is apparent that there has been a progressive increase in the number of new
bacterial taxa associated with fi sh diseases, with examples including Pasteurella
skyensis and Francisella noatunensis. However, some elementary questions/concerns about bacterial fi sh diseases remain to be addressed:
– Why are so few anaerobes associated with fi sh diseases? Could this refl ect a lack
of interest/expertise/suitable methods as opposed to a lack of occurrence?
– Are the majority of diseases really caused by single bacterial taxa or could there
be many more incidences of infections caused by two or more taxa either acting
simultaneously or sequentially? [Would diagnosticians recognise infections
caused by more than one pathogen?]. Does the same organism instigate and
develop the disease situation or could there be a microbial succession with one
organism initiating an infection, and a second or third developing the disease.
Could conventional diagnoses differentiate primary pathogens from secondary
invaders/contaminants of diseased tissue?
– Unculturables, e.g. Candidatus, are becoming associated with fi sh diseases, i.e.
situations where pathogens may be detected microscopically or serologically but
not cultured. The question to be resolved is whether such organisms are incapable of growing outwith a host or if suitable media have not been developed. It is
speculative how many more of these unculturable organisms remain to be recognised. Moreover, how many times has Candidatus been missed, and secondary
invaders/contaminants labelled as the actual pathogen? Then, there is the situation, such as with red mark syndrome, whereby an organism may be detected by
serology or molecular methods, but not readily observed.
– There is the issue regarding the value of the cultures obtained during diagnoses.
The conventional dogma is that where isolation from an active disease situation
is attempted the result on laboratory media will be dense virtually pure cultures
and not a comparatively few diverse colony types; the latter being interpreted as
indicative of the presence of contaminants. This approach would miss infections
caused by two or more different organisms.
– The value of cultures is a major concern insofar as the assumption is that the pure
culture is representative of the organism in its natural habitat, and thus may be
used for meaningful studies of pathogenicity, ecology and disease control.
– Certainly, an initial weakening process to the host may be possible in the absence
of pathogens, and involve pollution or a natural physiological state (e.g. during
1 Introduction