Francisella noatunensis subsp. orientalis infects striped catfish (Pangasianodon hypophthalmus) and common carp (Cyprinus carpio) but does not kill the hosts

Francisella noatunensis subsp. orientalis (Fno) is the etiological agent of francisellosis in tilapia (Oreochromis spp.). It is unclear whether other aquaculture freshwater fish species, which share the same ecosystem with tilapia, are susceptible to Fno infection and francisellosis disease manifestation. Here, we investigated the susceptibility of striped catfish (Pangasianodon hypophthalmus) and common carp (Cyprinus carpio) to Fno comparing to red tilapia (Oreochromis sp.), a francisellosis-susceptible fish. Healthy fish individually received 1.5×106 CFUs of Fno through intraperitoneal injection and were monitored for 21days. By the end of the experiment, no mortality or histopathological features of francisellosis disease were observed in both Fno-treated striped catfish and common carp groups. However, 5/10 (50%) of the surviving striped catfish and 10/10 (100%) common carp were positive by Francisella-specific PCR analysis. In contrast, Fno-infected red tilapia exhibited high cumulative mortality (90%) and the presence of typical granulomas in the spleen and kidney, the histopathological sign of francisellosis. All moribund tilapia were also PCR positive for Francisella. Fno could be re-isolated from 30% of the infected tilapia but was not successfully recovered from two treated host groups at the end of the test (day 21). In situ hybridization revealed weak Fno-positive signals in the spleen of infected striped catfish and common carp, but displayed strong reactivity in the infected tilapia. The findings suggested that striped catfish and common carp are not susceptible to francisellosis. The authors strongly believe that this study provides significant understanding on the susceptibility of three aquaculture fish species to Francisella noatunensis subsp. orientalis especially to that of the red tilapia (Oreochromis sp.), striped catfish (Pangasianodon hypophthalmus) and common carp (Cyprinus carpio). •Comparative susceptibility of 3 freshwater fish to Fno was determined.•Fno infected RT, CC and SCF but killed only RT.•Histopathological lesion of francisellosis found in RT but not in SCF and CC.•Presence of Fno in experimental fish was detected by PCR and in situ hybridization.•The result suggests that SCF and CC may serve as natural reservoir hosts for Fno.