An attenuated bacteria has been made by an insertion mutation in the iglC gene of Francisella asiatica, by allelic exchange. The attenuated strain proved to be an effective vaccine by providing protection against an infection of F. asiatica in tilapia, and is believed would at least partially immunize fish from other species of Francisella. The vaccine of the attenuated Francisella asiatica ΔiglC mutant can also serve as vectors to present antigens from other pathogens to the fish, thereby serving as vaccines against other pathogens as well. In addition, a highly sensitive and specific assay that can be used for the specific identification of F. asiatica in fish has been developed.