Initially it was considered likely that the animals were infected with Ren

Initially it was considered likely that the animals

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Initially, it was considered likely that the animals were infected with Ren. salmoni- narum, the causal agent of BKD, but the presence of Gram-negative bacteria and complete absence of any Gram-positive micro-organisms precluded this possibiHty (Mcintosh and Austin, 1990b). In a separate development during 1990, the organism was attributed to low-level mortalities in turbot, farmed in floating cages in France. With this outbreak, the disease signs included swelling and liquefaction of the kidney and spleen, which were also characterised by the presence of yellowish nodules (Vigneulle and Baudin-Laurencin, 1995). In 1999, Arctic charr in the U.S.A. were also found to be infected with the pathogen (Starliper, 2001). The external signs centred on a slight redness and swelling around the anus, but internally there was evidence of severe hemorrhaging with bloody ascites. Serratia marcescens It should be emphasised that the organisms were recovered only from apparently healthy fish, which were devoid of overt signs of disease (Baya et al, 1992c). Serratia plymuthica Nieto et al. (1990) noted that diseased fish did not display any external or internal cHnical signs. However, in Scotland the diseased fish possessed extensive skin lesions over the entire flank, from the operculum to the tail (Austin and Stobie, 1992b). In Poland, the organism has been recovered from 42 Atlantic salmon, rainbow trout and sea trout farms since 1996 (Grawinski and Antychowitz, 2001). Yersinia intermedia Affected Atlantic salmon were of 40-50 g in weight, and were held at a water temperature of 5°C. Disease signs included lazy movement with the fish congregating
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32 Bacterial Fish Pathogens at the surface of the water, darkening of the body pigment, tail erosion, haemorrhag- ing on the flank and abdominal inflammation (Carson and Schmidtke, 1993). Yersinia ruckeri The disease is mostly restricted to salmonids (Bullock and Snieszko, 1975) within the geographical locations of North and South America (Ross et ai, 1966; Stevenson and Daly, 1982; Bravo and Kojagura, 2004), Denmark (Dalsgaard et ai, 1984), Great Britain (Austin, 1982; Roberts, 1983), France (Lesel et al, 1983), Germany (Fuhr- mann et al, 1983), Italy (Busch, 1978), Ireland (McCormick and McLoughlin, 1993), Norway (Richards and Roberts, 1978) and Australia (Bullock et al, 1978a; Llewel- lyn, 1980). With the recent upsurge in the number of cases of the disease, particularly in Europe, it would appear that the disease is still spreading. So far, it has been a severe problem mostly in rainbow trout, although outbreaks of disease have been reported among populations of brown trout, brook trout, chinook salmon and coho salmon (Dulin et al, 1976) and Atlantic salmon and Pacific salmon (Bullock et al, 1978a). A few non-salmonid fish species have also been reported to harbour the pathogen, and these include channel catfish (Danley et ai, 1999), emerald dace (Notropis atherinoides; Mitchum, 1981), goldfish (McArdle and Dooley-Martin, 1985), carp (B. Austin, unpublished observation), lake herring or cisco (Coregonus artedii; Bullock and Anderson, 1984) and minnows (Pimephales promelas) (Michel et ai, 1986a). Also, sea bass and turbot appear to be susceptible (Vigneulle, 1990). In
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