255
gen was isolated only from the kidney of infected trout held at 4 °C for 28 days.
McCarthy ( 1980 ) pointed out that the prolonged survival of Aer. salmonicida in
dead diseased fi sh shows the risk of using scrap fi sh, which might be infected with
a chronic form of furunculosis, as food, particularly in view of the fi nding of Cornick
et al. ( 1969 ) on the survival of the pathogen at low temperatures. Some investigations, which have examined the survival of Aer. salmonicida in water, have also
commented on the potential of the non-culturable cells, detected microscopically in
the experimental systems, to infect fi sh. However, these studies have generally concluded that Aer. salmonicida in this form does not appear to be pathogenic, as reinfection of fi sh does not occur (Rose et al. 1990a ; Morgan et al. 1991 ).
Trash fi sh used in the production of feed may well be implicated in the spread of
furcunculosis. In this respect, Kim et al. ( 2013 ) attributed outbreaks of disease in
Korean rockfi sh ( Sebastes schlegeli ) to the low-value fi sh, which included big head
croaker, Japanese anchovy and Konoshiro gizzard shad, used in the production of
feed.
Aeromonas salmonicida – Osmotically Fragile Forms/L-Forms Another line of
investigation examined L-forms (spherical, fi lterable cells) of Aer. salmonicida. The
L-forms were induced experimentally, and were found to be unculturable by conventional plating methods. Subsequently, a stable L-form was induced with benzylpenicillin, and determined to contain more OMP of ~40 kDa molecular weight than
its parental cell (Gibb et al. 1996 ). This stable L-form did not require specialised
media, and could grow on BHIA at 0–5 °C (Gibb and Austin 1994 ). It could be
argued that the development of an L-form state could enable it to persist in tissues
of infected fi sh, although not causing clinical disease (McIntosh and Austin 1991b ).
However, attempts to infect fi sh using L-forms did not result in recovery of the
pathogen from fi sh, even after the administration of immunosuppressants. In addition, it was reported that small numbers of ‘natural’ L-form colonies were observed,
albeit infrequently, on specialised medium (containing horse serum and high
quantities of sucrose), which had been inoculated with kidney and spleen samples
taken from farmed Atlantic salmon suffering with furunculosis. This suggests that
L-forms may have implications in the disease process. Such fi ndings certainly merit
further investigation to more conclusively establish the role of this form of Aer.
salmonicida in disease processes. Obviously, if Aer. salmonicida, in a dormant or
NCBV or indeed in any other altered state it may undergo outside a fi sh host, is
genuinely unable to transmit infection, then control of the diseases is vastly simplifi ed and rendered less diffi cult. It is not possible, on the basis of the limited data
available, to draw grand, defi nitive conclusions about this crucially important aspect
of the pathogens behaviour. Further work will, hopefully, clarify the situation.
Aeromonas salmonicida – The Role of Carriers The second mechanism by
which fi sh may provide a source of infection is by becoming carriers of Aer. salmonicida. The existence of such fi sh and their role in transmission of the disease
was recognised early in the history of furunculosis. In a study of furunculosis in
trout in the River Kennett, Horne ( 1928 ) found that Bacterium salmonicida was
Aeromonas salmonicida
gen was isolated only from the kidney of infected trout held at 4 °C for 28 days.
McCarthy ( 1980 ) pointed out that the prolonged survival of Aer. salmonicida in
dead diseased fi sh shows the risk of using scrap fi sh, which might be infected with
a chronic form of furunculosis, as food, particularly in view of the fi nding of Cornick
et al. ( 1969 ) on the survival of the pathogen at low temperatures. Some investigations, which have examined the survival of Aer. salmonicida in water, have also
commented on the potential of the non-culturable cells, detected microscopically in
the experimental systems, to infect fi sh. However, these studies have generally concluded that Aer. salmonicida in this form does not appear to be pathogenic, as reinfection of fi sh does not occur (Rose et al. 1990a ; Morgan et al. 1991 ).
Trash fi sh used in the production of feed may well be implicated in the spread of
furcunculosis. In this respect, Kim et al. ( 2013 ) attributed outbreaks of disease in
Korean rockfi sh ( Sebastes schlegeli ) to the low-value fi sh, which included big head
croaker, Japanese anchovy and Konoshiro gizzard shad, used in the production of
feed.
Aeromonas salmonicida – Osmotically Fragile Forms/L-Forms Another line of
investigation examined L-forms (spherical, fi lterable cells) of Aer. salmonicida. The
L-forms were induced experimentally, and were found to be unculturable by conventional plating methods. Subsequently, a stable L-form was induced with benzylpenicillin, and determined to contain more OMP of ~40 kDa molecular weight than
its parental cell (Gibb et al. 1996 ). This stable L-form did not require specialised
media, and could grow on BHIA at 0–5 °C (Gibb and Austin 1994 ). It could be
argued that the development of an L-form state could enable it to persist in tissues
of infected fi sh, although not causing clinical disease (McIntosh and Austin 1991b ).
However, attempts to infect fi sh using L-forms did not result in recovery of the
pathogen from fi sh, even after the administration of immunosuppressants. In addition, it was reported that small numbers of ‘natural’ L-form colonies were observed,
albeit infrequently, on specialised medium (containing horse serum and high
quantities of sucrose), which had been inoculated with kidney and spleen samples
taken from farmed Atlantic salmon suffering with furunculosis. This suggests that
L-forms may have implications in the disease process. Such fi ndings certainly merit
further investigation to more conclusively establish the role of this form of Aer.
salmonicida in disease processes. Obviously, if Aer. salmonicida, in a dormant or
NCBV or indeed in any other altered state it may undergo outside a fi sh host, is
genuinely unable to transmit infection, then control of the diseases is vastly simplifi ed and rendered less diffi cult. It is not possible, on the basis of the limited data
available, to draw grand, defi nitive conclusions about this crucially important aspect
of the pathogens behaviour. Further work will, hopefully, clarify the situation.
Aeromonas salmonicida – The Role of Carriers The second mechanism by
which fi sh may provide a source of infection is by becoming carriers of Aer. salmonicida. The existence of such fi sh and their role in transmission of the disease
was recognised early in the history of furunculosis. In a study of furunculosis in
trout in the River Kennett, Horne ( 1928 ) found that Bacterium salmonicida was
Aeromonas salmonicida