salmonicida in Atlantic salmon in seawater Rose et al 1989 noted that a minimum

Salmonicida in atlantic salmon in seawater rose et al

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salmonicida in Atlantic salmon in seawater. Rose et al. (1989) noted that a minimum dose of 10"^ CFU/ml by bath was required to initiate infection. Aeromonas salmonicida uptake into fish Another unresolved aspect of the transmission of furunculosis is the uptake of Aer. salmonicida into a fish host. It is possible that the pathogen may gain entry to a new host through the gills, lateral line, mouth, anus or a surface injury (e.g. Effendi and Austin, 1995b). McCarthy (1980) demonstrated that rainbow trout that resisted the disease subsequently died from furunculosis after their flanks had been abraded with sandpaper. Also, Lund (1967) found that infection was acquired by fish which had been scarified and experimentally challenged with the pathogen. However, these injuries were artificially induced. Effendi and Austin (1995b) evaluated many different routes for the possible uptake of Aer. salmonicida into fish. The data may be summarised as follows: Route Recovery of Aer. salmonicida from: Gill Blood and kidney Oral Blood Lateral line Blood and spleen (but not kidney) Ventral surface Blood and spleen (but not kidney) Flank Blood and spleen (but not kidney) Anus Blood (but not kidney or spleen) Generally, Aer. salmonicida remained at the site of administration for 24 h. The most effective route of uptake leading to mortalities was the gill and anus. In contrast, fewer deaths resulted from challenge via the lateral line, flank or ventral surface (Effendi and Austin, 1995b). Yet, despite all this work, the natural mode of uptake remains unresolved. In a study of uptake of Aer. salmonicida by rainbow trout, it was observed that the pathogen could be detected in the blood and kidney within 5 min of immersion in a suspension containing 10^ cells/ml (Hodgkinson et al., 1987). Interestingly, it was also found that uptake of the pathogen was enhanced by the addition of particulates, e.g. latex, to the bacterial suspension. If latex was indeed added, Aer. salmonicida was isolated from blood at 12 min, and from kidney, spleen and faeces at 4h post- challenge. The organism was also cultured from the skin, gills, blood and faeces for up to 48 h. In the absence of latex, the pathogen could be again recovered at
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Epizootiology: Gram-negative bacteria 265 12min, but from a wider range of sites including kidney, spleen and the lower intestine. However, by 24 h the pathogen was no longer recovered from the fish. From culturing methods alone, it may not, however, be assumed that Aer. salmon- icida had been totally removed from the animals. In fact, cultures of the pathogen were isolated from kidney, spleen and faeces within 1 to 4 h of immunosuppression of the fish at 7 days post-challenge. In addition, the method of challenge yielded different results. Thus, when the entire fish was immersed in the bacterial suspension, superior uptake occurred compared with exposing only the head or tail regions. The explanation of this phenomenon is unclear, but such results suggest that uptake may occur through several locations rather than a single site, e.g. mouth, nares, gill or anus.
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