The aim of this work is to determine the concentration of Aeromonas hydrophila to be inoculated in pacu (Piaractus mesopotamicus), so as to induce bacterial infection but not death during a period of at least ten days and, therefore, enable the development of treatment protocols. The tested concentrations were established through the Mac Farland scale and fish were subjected to three A. hydrophila infection concentrations (1.2 × 107, 2.4 × 107 and 1.5 × 108 UFC.mL-1) through intracoelomic, intramuscular and subcutaneous injections. The clinical exam was done 24 h after inoculation, and the clinical signs suggested aeromonosis in all three groups. In the lowest concentration, animals demonstrated few clinical signs of disease, and in the highest concentration, all animals died within 24 h of aeromonosis induction with acute infection. In the intermediate concentration (2.4 × 107 UFC.mL-1), all animals presented clinical signs of aeromonosis and kept living at the beginning of the time of treatment. Therefore, 2.4 × 107 UFC mL-1 concentration was defined as viable for the study of experimental infection in pacu by A. hydrophila, as thefishes were kept alive at the beginning of the time of treatment protocol with pharmochemicals.
Key words: Bacterial infection, fish, disease, antibiotics.
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