To further elucidate the mechanisms of attenuated YM001 vaccine, we also traced the infected YM001 in vivo and assessed the
antibody levels in the body post immunization. The results (Table 5)showed that YM001 reached the brain, liver, spleen, and kidney
tissues at 6 h after the oral immunization, and multiplied into a large number; 3 days later, YM001 disappeared in the brain and
reduced in number in liver, spleen, and kidney tissues; the number of bacteria were further reduced after 7 days; However, bacteria
were isolated from the spleen even at 15 days after the immunization. These results not only supported to certain degree the observation that oral immunization of YM001 provided a relatively long term of strong immune-protection (30 days), but also
demonstrated that YM001 was not pathogenic to tilapia. The results of ELISA (Fig. 3) showed that YM001 elicited the production of
specific antibodies in tilapia, and the antibody levels of the 2 doses (1.0 109 and 1.0 108 CFU/fish) at 4 time points (7, 14, 21, and 28
days) post immunization were all significantly higher than those in the control group.