Serious and repeated mortality recently experienced in imported neon tetra, Paracheirodon innesi Myers, by French ornamental fish traders and empirically ascribed to the microsporidian Pleistophora hyphessobryconis on the basis of clinical signs, was investigated. Although Pleistophora sp. spores were observed in a few cases, laboratory results demonstrated that similar clinical signs were generally caused by the pathogenic bacterium Flavobacterium columnare. In all cases, muscle was the main target tissue, and the most noticeable external signs were limited to fading of skin colouration and the development of white areas of necrosis. Three isolates were studied and typed by bacteriological tests and molecular techniques. Although their phenotypic characteristics were in accordance with F. columnare descriptions, except for higher optimal growth temperatures (18–30 °C), they all appeared to differ genetically from common European and American isolates and to be similar to Asian isolates recently assigned to a new genomovar by Japanese workers. Experimental infections suggested the isolates were highly virulent for ornamental species. The use of polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) for identification and detection of the agent in tissue samples, and the implications of this finding for health control management of imported fish and domestic species are discussed.
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