Efficacy of florfenicol for control of mortality associated with Francisella noatunensis subsp. orientalis in Nile tilapia, Oreochromis niloticus (L.)

Abstract

Francisella noatunensis subsp. orientalis (Fno) (syn. F.asiatica) is an emergent Gram-negative facultative intracellular bacterium. Although it is considered one of the most pathogenic bacteria in fish, there are no commercially available treatments or vaccines. The objective of this project was to determine the most efficacious concentration of florfenicol (FFC) [10, 15 or 20mg FFCkg(-1) body weight (bw) per days for 10days] administered in feed to control experimentally induced infections of Fno in Nile tilapia, Oreochromis niloticus (L.), reared in a recirculating aquaculture system. The cumulative mortality of fish that received 0, 10, 15 or 20mg FFCkg(-1) bw per day was 60, 37, 14 and 16%, respectively. Francisella noatunensis subsp. orientalis genome equivalents were detected in water from all challenged groups with slight reduction in the concentration in the florfenicol-treated groups 4days after treatment. The mean LOG of CFU Fnomg(-1) spleen was 3-5 and was present in all challenged groups at necropsy 11days after treatment (21days after challenge). Results show that florfenicol administered at doses of 15 and 20mg FFCkg(-1) bw per days for 10days significantly reduced mortality associated with francisellosis in Nile tilapia.