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

Abstract

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 'na&#xef;ve fish' were defined as fish added to tanks once temperature in water reached <26&#xa0;&#xb0;C at 21&#xa0;weeks post-challenge. Fno genome equivalents (GEs) in water were similar in all treatments 1&#xa0;h post-challenge; however, significantly lower Fno GEs were detected 2&#xa0;weeks post-challenge in all tanks, and the only treatment with detectable Fno GE after 4&#xa0;weeks of challenge were the O.&#xa0;mossambicus tanks. Twenty-one weeks post-challenge, na&#xef;ve fish were stocked with 'resident' cohorts. Over a 4-week period, mortalities occurred consistently only in O.&#xa0;mossambicus na&#xef;ve cohorts. Overall presence of granulomas in spleen of survivors was similar (>55%) in all resident populations; however, in na&#xef;ve populations, only O.&#xa0;mossambicus presented granulomas. Similarly, only O.&#xa0;mossambicus presented viable Fno in the spleen of survivors, and Fno GEs were only detected in O.&#xa0;mossambicus, and in resident O.&#xa0;aureus. In conclusion, the results of this study suggest different susceptibility of tilapia species to piscine francisellosis.