Synergistic infection of Ichthyophthirius multifiliis and Francisella noatunensis subsp. orientalis in hybrid red tilapia (Oreochromis sp.).

Francisella noatunensis subsp. orientalis (Fno) and Ichthyophthirius multifiliis (Ich) are deadly infectious pathogens in farmed tilapia, particularly during cold season when the water temperature drops to under 25 °C. We hypothesized that infection of the ectoparasite Ich might enhance susceptibility of hybrid red tilapia (Oreochromis sp.) to the facultative intracellular bacterium Fno. To prove the hypothesis, the experiment was designed as follows. Hybrid red tilapia naturally infected by Ich at 9 ± 6 theronts/fish gills and 4 ± 3 theronts/fish skin were distributed into 5 distinct groups exposed to different concentrations of Fno. In parallel, the same number of Ich-free tilapia were challenged to only Fno in the same manner. The results showed that cumulative mortality in the Fno single infection with 2.88 × 106 CFU mL-1 of water was 25 ± 7%, whereas 100% mortality was found in the coinfection treatment at dose of 1.93 × 105 CFU mL-1 of water. No mortality was observed in both control groups (Ich-infected and Ich-free fish). The coinfected fish revealed typical clinical signs and histopathological manifestations of francisellosis and ichthyophthiriasis. This study revealed synergistic effect of the Ich and Fno infection in hybrid red tilapia leading to the exacerbated mortality. Thus, farming management of fish to be free from the Ich ectoparasite might reduce risk of francisellosis and probably other bacterial diseases in farmed tilapia.