Edwardsielosis is a systemic granulomatous disease that affects a wide range of terrestrial and aquatic hosts, including tilapia (Oreochromis sp.). Recently, Edwardsiella tarda was reclasiffied in three species: E. tarda, E. anguillarum and E. piscicida. To our knowledge, it is not known if there are differences in virulence and pathogenicity among reclassified bacteria. The main of this study was to characterize the pathobiology of intraperitoneal (IP) infection by E. tarda and E. anguillarum in tilapia alevins.
Methodology: Healthy experimental alevins of 2 to 5g were organized in 12 groups of 10 fish each (n = 120). Four (4) negative control groups were injected with PBS (Phosphate Buffered Solution), 4 groups were inoculated with 2.4x106CFU/ml of E. tarda (2) or E. anguillarum (2) and 4 groups more with 2.4x107CFU/ml of the same bacteria and groups, with 50ul per fish by IP way. Fish were monitored during the first 48 hours every two hours and subsequently twice a day for 13 days more. Necropsy was performed and samples for histopathology, bacteriology and molecular analysis were taken at the time of death. On day 15, the remaining fish were sacrificed.
Results: The infection by E. anguillarum in tilapia alevins showed higher virulence and pathogenicity in comparison with inoculated by E. tarda, with 100% of mortality during 1, 2, 3, 4 and 7 days post inoculation (DPI). Severity and extension of lesions were higher, starting with granulomatous inflammation and subsequent granulomas formation, mainly in the spleen. The infection by E. tarda had mortalities at 7, 8, 9, 10 and 14 DPI (n = 16); on day 15, the remaining 24 fish inoculated by E. tarda were sacrificed. At this time, fish showed less body weight and size in comparison with control. Also, pale skin and presence of granulomas in the spleen were seen. Histopathologically, fish inoculated by E. tarda developed mature granulomas. Negative controls did not show mortality, neither necropsy or histopathological lesions and all were negative for bacterial isolation and molecular detection.
Conclusion: In cases of edwardsiellosis is necessary differentiate the infection by E. tarda and E. anguillarum, in order to prevent the entrance of virulent strains with high mortality rates, as is the case with E. anguillarum disease. On the other hand, those farms that present E. tarda infections will show less morbidity and mortality, but the disease is present and can develop lately, affecting productive parameters.