32 days and for 40 days in the tank water where the dead fish had been kept

32 days and for 40 days in the tank water where the

This preview shows page 280 - 282 out of 594 pages.

32 days, and for 40 days in the tank water where the dead fish had been kept, thus providing a possible source of infection for healthy fish. Another study showed that Aer. salmonicida remained viable in infected trout tissues and internal organs (heart, liver, spleen and kidney) for 49 days when the fish were stored at 10°C (Cornick et al., 1969). However, the pathogen 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 fish shows the risk of using scrap fish, which might
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Epizootiology: Gram-negative bacteria 261 be infected with a chronic form of furunculosis, as food, particularly in view of the finding 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 fish. However, these studies have generally concluded that Aer. salmonicida in this form does not appear to be pathogenic, as re-infection offish does not occur (Rose et al., 1990a; Morgan et al., 1991). Aeromonas salmonicida L-forms Another line of investigation examined L-forms (spherical, filterable cells) of Aer. salmonicida. The L-forms were induced experimentally, and were found to be uncul- turable 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 fish, although not causing cHnical disease (Mcintosh and Austin, 1991b). However, attempts to infect fish using L-forms did not result in recovery of the pathogen from fish, 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 findings 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 fish host, is genuinely unable to transmit infection, then control of the diseases is vastly simplified and rendered less difficult. It is not possible, on the basis of the limited data available, to draw grand, definitive conclusions about this crucially important aspect of the pathogen's behaviour. Further work will, hopefully, clarify the situation.
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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