Avaliação da patogenicidade de Edwardsiella tarda, Chryseobacterium sp., Streptococcus agalactiae e Francisella orientalis para Tambaqui (Colossoma macropomum)

Abstract

Tambaqui (Colossoma macropomum) is the most produced native fish species, in Brazil. Currently, the susceptibility of tambaqui was confirmed only for one bacterial pathogen, Aeromonas hydrophila. Therefore, this study aims to evaluate the susceptibility of C. macropomum to isolates of Edwardsiella tarda, Chryseobacterium sp., Streptococcus agalactiae and Francisella orientalis. E. tarda and Chryseobacterium sp. strains were isolated from internal organs of healthy tambaquis, while S. agalactiae and F. orientalis strains were isolated from Nile tilapia (Oreochromis niloticus) showing clinical signs. These bacteria were evaluated for their pathogenicity to tambaqui through experimental infection. To this end, each of these bacterial strains and sterile bacterial broth, for the control groups, was inoculated into six healthy tambaqui juveniles by intraperitoneal injection after which the occurrence of clinical signs and deaths were observed for a defined period of time. Animals that died during this period and survivors at the end of experimental period were necropsied and analyzed for bacteriology and histology. Fish infected by S. agalactiae were the only ones who showed clinical signs with progression to the death of 83.3% of the tambaquis in that group. The isolation in pure culture of the recovered bacteria as well as histopathological findings were observed for animals of the groups infected by S. agalactiae, F. orientalis and E. tarda. Were observed histopathological findings in the spleen, liver, brain and heart of animals infected by S. agalactiae, which were related to neutrophilic and fibrinonecrotic inflammation. Meanwhile, animals infected by F. orientalis and E. tarda developed histopathological findings related to granulomatous nephritis, splenitis and hepatitis. Finally, we conclude that Koch's postulate was fulfilled for the interaction of E. tarda with tambaqui, whereas for the interaction of this fish with Chryseobacterium sp., Koch's postulate was not fulfilled. As for the bacterial strains originally isolated from tilapia, S. agalactiae and F. orientalis, the susceptibility of tambaqui in experimental conditions was demonstrated.