Dynamics of piscine francisellosis differs amongst tilapia species (Oreochromis spp.) in a controlled challenge with Francisella noatunensis subsp. orientalis.

A 25-week immersion challenge was conducted exposing Oreochromis mossambicus, Oreochromis aureus and Oreochromis urolepis hornorum to Francisella noatunensis subsp. orientalis (Fno). Two populations were compared for each fish species; 'resident fish' were defined as fish maintained in tanks since week 0 of challenge, whereas naive fish' were defined as fish added to tanks once temperature in water reached < 26 degrees C at 21 weeks post-challenge. Fno genome equivalents (GEs) in water were similar in all treatments 1 h post-challenge; however, significantly lower Fno GEs were detected 2 weeks post-challenge in all tanks, and the only treatment with detectable Fno GE after 4 weeks of challenge were the O. mossambicus tanks. Twenty-one weeks post-challenge, naive fish were stocked with 'resident' cohorts. Over a 4-week period, mortalities occurred consistently only in O. mossambicus naive cohorts. Overall presence of granulomas in spleen of survivors was similar (> 55%) in all resident populations; however, in naive populations, only O. mossambicus presented granulomas. Similarly, only O. mossambicus presented viable Fno in the spleen of survivors, and Fno GEs were only detected in O. mossambicus, and in resident O. aureus. In conclusion, the results of this study suggest different susceptibility of tilapia species to piscine francisellosis.