Abstract
Flexibacter columnaris was isolated from 13 cultured Oreochromis niloticus showing respiratory disorders. The isolates developed typical swarming rhizoid colonies on Cytophaga agar medium. Antibiotic sensitivity test revealed the susceptibility of F. columnaris isolated to oxytetracycline, chloramphenicol and erythromycin. A marked difference in the pathogenicity of seven tested isolates was observed: two were highly virulent, one was moderately virulent and four were avirulent. No experimental infection could be induced with the highly virulent isolates except after injuring one of the natural barriers of the fish body. The severity of the disease and the increased median death time shortened by keeping infected fishes with injured gills in water containing ammonia. In naturally infected O. niloticus, the disease became chronic as indicated by the presence of excessive proliferative and necrotic changes. On the other hand, severe dilatation of branchial blood vessel, oedema and round cell infiltration proved that, the disease among experimentally infected tilapias was acute.
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