It is possible that the pathogen gains entry via all these sites or

It is possible that the pathogen gains entry via all

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It is possible that the pathogen gains entry via all these sites or additionally through the lateral Hne and/or skin (Hodgkinson et ai, 1987). Perhaps, the most significant observation resulting from these experiments was the rapidity by which Aer. salmonicida entered the rainbow trout. Other investigators have not sampled so close to the initial time of challenge. McCarthy (1980) reported uptake to occur from the oral route within 5 h, with the organism found in the kidney. Tatner et al. (1984) first sampled the fish at 24 h post-challenge. However, all these studies indicate the localisation of Aer. salmonicida principally within the reticulo-endothehal systems of fish (Tatner et al, 1984). Notwithstanding, evidence continues to be firmly pointed at the role of gills and skin/mucus in the uptake of Aer. salmonicida into fish (Ferguson et al, 1998). Another route of infection that has been proposed is via the gastro-intestinal tract, due to intake of contaminated food. However, there is disagreement as to whether or not this in fact occurs. Plehn (1911) and Blake and Clark (1931) reported success in experimentally infecting fish by feeding contaminated food. However, Krantz et al. (1964a, b) and McCarthy (1980) failed to infect brown trout by feeding with food containing the pathogen. Klontz and Wood (1972) reported cHnical fur- unculosis in the sable fish apparently caused by ingestion of carrier coho salmon. Evidence has been published that shows Aer. salmonicida may translocate across the intestinal epitheha (Jutfelt et al, 2006). Here, the authors exposed intestinal segments of rainbow trout for 90 minutes to isothiocyanate-labelled cells of virulent Aer. salmonicida and demonstrated translocation. In addition, in laboratory-based experi- ments that compared various methods designed to induce the carrier state of Aer. salmonicida in juvenile Spring chinook salmon, Markwardt and Klontz (1989) observed that gastric intubation (of ca. 1 x 10^ bacteria) resulted in a 65% carrier state. This result was a significantly higher percentage than those recorded for exposure to a broth culture as a bath, ingestion of broth culture coated food, and exposure to intraperitoneally injected fish (40, 20 and 10% carrier rates, respectively). Markwardt and Klontz (1989) also commented that the exposure to clinically diseased fish and bathing in broth cultures most probably simulated the natural routes of infection (Paterson, 1982). Aeromonas salmonicida transmission; the role of eggs Vertical or transovarian transmission of Aer. salmonicida as a possible route of infection has been widely studied with inconclusive results. Smith (1939) claimed
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266 Bacterial Fish Pathogens the pathogen could be carried on the egg surface, in contrast to Plehn (1911) and Mackie et al. (1930), who were of the opinion that the organism was unable to infect fish eggs. The possibility that the pathogen could be transmitted at fertihsation was examined by the Furunculosis Committee in the U.K. The experimental evidence gathered for their report indicated that furunculosis was not transmitted in such a fashion. Lund (1967), however, contended that since the conclusions were based on
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