Enzootics of visceral granulomas associated with Francisella-like organism infection in tilapia (Oreochromis spp.)

Between 2001 and 2004, 10 strains of Francisella species were isolated from visceral granulomas of diseased tilapia in Taiwan. In a comparison of nucleotide sequences for whole 16S rRNA gene with those of reference bacteria, the isolated strains had a high sequence similarity to Francisella philomiragia (98.6%), Francisella tularensis subsp. novicida (97.4%) and F. tularensis subsp. tularensis (96.1%). On the basis of electron microscopic examination, phenotypic characteristics and PCR assays for the 16S rRNA gene sequence, the causative agent clearly belongs to the genus Francisella.