Evaluation on the Pathogenesis of Streptococcus agalactiae in Nile Tilapia (Oreochromis niloticus)

The pathogenesis of a Streptococcus agalactiae was evaluated in a three-period experiment. Two groups of 40 fishes were intraperitoneally (i.p.) challenged in each experimental period with different infective doses of the pathogen. Doses varied from 1.0 x 10(6) to 1.5 x 10(8) CFU/fish. One group of 40 tilapia i.p. injected with tryptic soy broth (TSB) was used as a control group in each period. Mortalities varied from 67.5% in group 8 (infective dose 1.0 x 10(6) CFU/fish) to 90.0% in group 1 (infective dose 1.5 x 10(8) CFU/fish). Significant differences in mortalities were found only between group 8 and each of the other groups, except group 5 (infective dose 6.0 x 10(6) CFU/fish; mortality 75.0%). The highest mortality coefficients were observed in days 1-2 after inoculation (accumulated mortality 44.4%), and a second peak of mortality occurred at days 6-7. Challenged fishes from all the groups showed alterations in behaviour and similar clinical signs. These were anorexia, lethargy, erratic swimming, exophthalmia and ascites. Macroscopically, skin hemorrhage, splenomegaly, hepatomegaly with organ paleness and visceral adherences were observed. S. agalactiae was re-isolated from all the fishes from the experimental groups submitted to bacteriological examination. The illness observed in tilapia naturally infected with S. agalactiae was experimentally reproduced in this study, and the clinical signs produced were similar to those reported from the natural infections