Abstract
Columnaris disease caused by the multi-variant Flavobacterium columnare is known to be responsible for severe mortality in wild and cultured freshwater fish species worldwide. Incidence of conditions presenting with clinical signs typical of Columnaris disease has been reported by catfish farmers in Nigeria. There is, therefore, the need to ascertain the pathogenicity and virulence of F. columnare isolates from disease conditions, as a prelude to molecular characterization of genetic variants of the bacterium in Nigeria. Four groups of 10 African catfish, Clarias gariepinus weighing 32 – 40 g were experimentally infected with 106 CFU/L of different isolates of F. columnare, identified using diagnostic PCR. Similar number of fish in a fifth group were not infected (Control). All Experimental treatments were replicated once. Percentage morbidity and mortality were observed and compared between experimental groups using descriptive statistics, while cellular structures of normal, discoloured and necrotic skin of affected fish were also studied. All F. columnare isolates used in the experimental infection were observed to be pathogenic, causing clinical signs and mortality in infected fish. Lesions observed were external, with characteristic dermatitis typically referred to as saddle-back lesion being prominent. There is gross variation in morbidity and mortality among the different isolates, with the highest being 80% (for both) and the least, 55% and 40%, respectively. There was vacuolation in the epidermal cells and reduction in melanocytes in the discoloured skin but complete erosion of the epidermis in severe dermatitis. There is the likelihood that some of the F. columnare isolates used in this study differ genetically, indicating the need for the identification of the different genetic variants available in Nigeria.

Key words: Columnaris, Infection, Virulence, Mortality, Catfish