The level of mortality may be extremely high and figures of 60 90 are not

The level of mortality may be extremely high and

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The level of mortality may be extremely high, and figures of 60-90% are not uncommon. For example, columnaris was considered to be the most important contributing factor for 72.3-97.6% and 75.4-95.4% mortality among populations of adult sockeye salmon and adult chinook salmon, respectively (Fish and Hanavan, 1948). Similarly, Chen et al. (1982) reported 77% and 88.3% losses among groups of carp and goldfish, respectively. Certainly, there are good data demonstrating the seasonal effects of mortalities due to columnaris, insofar as Bowser (1973) reported 60% infection of bullheads in mid-May whereas only a few months later the incidence had dropped to only 10%. In addition to water temperature, the severity of columnaris is influenced by a multipHcity of environmental (stress) and host-related factors. Chen et al. (1982) described the highest eel mortality levels to be associated with stagnant water, whereas the lowest losses occurred in running water. Interestingly, with aeration, the total losses fell between these two extremes. In this respect, the mortality rate has been inversely correlated with the level of dissolved oxygen in the water. Moreover,
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Epizootiology: Gram-negative bacteria 275 with adequate dissolved oxygen, deaths increased with a concomitant rise in the level of ammonia. The susceptibility of juvenile chinook salmon and rainbow trout has been corre- lated with the age of the fish and the level of crowding, as well as water temperature (Fujihara et al, 1971). These workers concluded that rainbow trout (1 g average weight) and chinook salmon (average weight = 3 g) were less susceptible to the rigours of columnaris than smaller fish. Therefore, it was concluded that age is more impor- tant than weight in determining susceptibility to infection. Chrys. scophthalmum was found in the water, from which it is surmised that spread to turbot occurred (Mudarris and Austin, 1989). Halomonadaceae representative Halomonas (=Deleya) cupida H. cupida is a waterborne organism (Baumann et ai, 1972). Therefore, the source of infection was undoubtedly the marine environment. Moraxellaceae representatives Acinetobacter sp. Unreactive Gram-negative rods that are difficult to identify and which resemble Acinetobacter are common inhabitants of freshwater (Allen et al, 1983b) and marine ecosystems (Austin, 1982a). In particular, the organisms populate the skin and gills (Horsley, 1973) and digestive tract (Shewan, 1961; Trust and Sparrow, 1974; Roald, 1977) of salmonids. Therefore, a ready inoculum of cells is Hkely to be in continual contact with fish. Conceivably, any break in the integument of the host may lead to colonisation of the nutrient-rich tissues by components of the water-borne or, indeed, fish microflora. This may lead to the start of a disease cycle. Moraxella sp. It may be assumed that the organism constitutes part of the aquatic microflora.
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