The intensification of aquaculture, combined with management failures such as underfeeding or overfeeding, inadequate water quality, accumulation of organic matter or pollution, in addition to promoting the increase of pathogens in the system, compromise the well-being and health of fish, favoring the proliferation and infection by bacteria as Francisella orientalis, a Gram-negative, facultatively intracellular, non-motile, strictly aerobic and highly infectious coccobacillus. This bacterium can affect fish during different stages of development in fresh and brackish waters, mainly when the water temperature is below 24°C. The impact of disease outbreaks on fish farming is high, increasing the cost of production. Brazilian fish farmers only have the option of using two molecules approved for fish farming, florfenicol and oxytetracycline. These two products have been used to control outbreaks of francisellosis in fish farming, but due to therapeutic failures, the number of carrier animals is high. Therefore, preventive strategies to manage disease problems are essential to reduce mortality outbreaks and reduce operating costs. Among these strategies are nutritional additives, to be administered in feed, as they can prevent bacterial diseases and promote fish health during specific challenging periods. Thus, this experiment allowed feeding for 30 days with the addition of a dietary additive (Quillaja Saponaria and Yucca schidigera extract 2kg/ton) compared with one diet control (without additive) and were tested to check their effect on the survival of Nile tilapia challenged with Francisella orientalis. The experimental diets were prepared at the Fisheries Institute in São José do Rio Preto, São Paulo. The diet was produced by extrusion into 3 mm pellets. Three groups (G1 diet supplemented with Quillaja Saponaria and Yucca schidigera extract 2kg/ton and challenged with FO, G2 = diet without supplementation - Positive control and challenged with FO and G3 = diet without supplementation and inoculated with PBS - Negative control) were used in this experiment, each contained 5 tanks and each tank contained 30 fish (16.97 ± 0.49 g). Nile tilapia were used in the bacterial challenge with the selected dose LD50 (3.6 × 105 CFU/ml). Survival on fish fed with additive was higher (48%) than control (36%), however, no statistical differences were found. This study is relevant because even without statistical difference, an additive was able to increase the survival of Nile tilapia to francisellosis infection. The next steps are discover the ideal doses of additive to control to franciselosis.
FAPESP: 2021/11955-8