In vitro effect of the Melaleuca alternifolia, Lavandula angustifolia and Mentha piperita essential oils (EOs) against Ichthyophthirius multifiliis and in vivo effect of M. alternifolia for treating ichthyophthiriasis in one of the most important South American fish, Piaractus mesopotamicus (Holmberg), were evaluated. The in vitro test consisted of three EOs, each at concentrations of 57 μL L(-1) , 114 μL L (-1) , 227 μL L(-1) and 455 μL L (-1) , which were assessed once an hour for 4 h in microtitre plates (96 wells). The in vitro results demonstrated that all tested EOs showed a cytotoxic effect against I. multifiliis compared to control groups (P < 0.05). The in vivo treatment for white spot disease was performed in a bath for 2 h day(-1) for 5 days using the M. alternifolia EO (50 μL L (-1) ). In this study, 53.33% of the fish severely infected by I. multifiliis survived after the treatment with M. alternifolia (50 μL L (-1) ) and the parasitological analysis has shown an efficacy of nearly 100% in the skin and gills, while all the fish in the control group died. Furthermore, the potential positive effect of M. alternifolia EO against two emergent opportunistic bacteria in South America Edwardsiella tarda and Citrobacter freundii was discussed.

The utilization of probiotic bacteria have been widely tested and applied in intestinal microflora modulation, through competitive exclusion of the pathogenic bacteria. Aeromonas hydrophila is a gram-negative bacterium, responsible for fish outbreaks in farms around the world, and is one of the major loss causes for neotropical fish farmers. 660 pacus (67 ± 7 g) were distributed in 20 tanks (n = 33), constituting five groups (four tanks for each treatment): four groups were fed with different levels (2, 4, 8 and 16 g kg −1) of Bacillus cereus and Bacillus subtilis (1:1, 10 8 CFU g −1), and the fifth group was fed with a control diet (without probiotic). Pacus fed with probiotic showed increment in the ROS production associated to elevated neutrophil and monocyte counts and increased phagocytic activity without affecting the growth parameters. Probiotic fed fish presented higher survival rates, subjected to an i.p. challenge with 10 8 CFU mL −1 of A. hydrophila. The results demonstrated a dose response effect and the ideal level of the probiotic (Bacillus cereus and Bacillus subtilis, 1:1, 10 8 CFU g −1) in P. mesopotamicus diets was around 8 g kg −1 , as the highest survival rates and immunological responses were found in groups of fish fed with this diet. In addition, too much probiotic should be avoided, as pacus fed diet with 16 g kg −1 showed partial suppression of these responses.

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