The disease was considered to be sufficiently serious to warrant inclusion in

The disease was considered to be sufficiently serious

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The disease was considered to be sufficiently serious to warrant inclusion in the list of notifiable fish diseases, as defined by the (British) Diseases of Fish Act, 1937. How- ever, columnaris was omitted from the 1983 Act. Infection with Fla. columnare may result in several discrete disease conditions. In young fish, there is often negligible pathology before death ensues. The gill is usually the major site of damage. Typically, congestion (blockage) of the blood vessels supplying the gills occurs, with dissociation of the surface epithelium of the lamellae from the capillary bed. There may be scattered areas of haemorrhaging (Pacha and Ordal, 1967). In adult fish, the lesions may occur on the gills, skin and/or in the musculature. Systemic infections may develop (Wolke, 1975). Skin discoloration/ fading and muscle lesions have been documented on neon tetra (Paracheirodon innesi) (Michel et ai, 2002). Gill lesions consist normally of yellow-orange areas of necrosis. These start usually at the periphery of the gill, and extend towards the base of the gill arch. Eventually, extensive erosion may completely destroy the gill filament (Pacha and Ordal, 1970). On the body, small lesions start as areas of pale discoloration at the base of the dorsal fin or occasionally at the base of the pelvic fin, and lead to deterioration of the fins. These areas increase in size and may become as large as 3-4 cm in diameter, covering as much as 20-25% of the total surface area of the fish. This may have the characteristic appearance of a saddle, and hence the descriptive term, "saddleback". Frequently, the skin becomes completely eroded away, exposing the underlying muscle. Large numbers of bacteria are present at the advancing edge of the lesion. It is not uncommon for the fish to die within
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34 Bacterial Fish Pathogens 48 h of the appearance of the skin discoloration (Pacha and Ordal, 1970; Becker and Fujihara, 1978; Morrison et aL, 1981). Flavobacterium johnsoniae In the initial outbreak in Queensland, Australia, 2-5% mortalities occurred in farmed barramundi, Lates calcarifer, over a 2-week period when the water temperature was 27-28°C. The fish were Hstless, anorexic, and displayed elevated scales and superficial erosion of the skin (particularly on the posterior flank). Some fish presented eroded pectoral fins and lower jaws (Carson et aL, 1993). Flavobacterium psychrophilum Although this organism is mostly associated with infections in salmonids, other fish species have been infected, for example sea lamprey (Elsayed et aL, 2006), and ayu has been challenged successfully by immersion (Miwa and Nakayasu, 2005). Mostly, this organism causes skin lesions, which are often described as saddle-Hke lesions, con- taining myriads of organisms, near the dorsal fin. The fish may darken and, in advanced cases, develop bacteraemia with the pathogen ramifying throughout the animal (Wood and Yasutake, 1956; Winton et aL, 1983; Lehmann et aL, 1991).
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