Abstract:Based on the dynamic distribution and elimination patterns of Streptococcus agalactiae in Oreochromis niloticus at different water temperatures, we provide a foundation for research on S. agalactiae pathogenesis in O. niloticus, as well as a scientific basis for the prevention and control of S. agalactiae infection in O. niloticus by controlling water temperature and the rational use of drugs. We used O. niloticus infected with S. agalactiae (WC1535 strain) by artificial intraperitoneal injection. First, the mortality rates of infected O. niloticus were assessed at 25℃, 29℃, and 33℃. Then, we performed in vitro isolation, cultured and counted S. agalactiae from the infected fish maintained at three temperatures. Next, we assessed the abundance of S. agalactiae in the brain, liver, spleen, and kidney tissues of the infected fish at the three temperatures. The results showed that fish mortality rate increased with increasing water temperature, and the cumulative mortality rate was 6.67%, 25.56%, and 78.90% at 25℃, 29℃ and 33℃, respectively. Furthermore, the maximum abundance of S. agalactiae in unit mass of the brain, liver, spleen, and kidney tissues of O. niloticus increased with increasing water temperature. Thus, at a high temperature, S. agalactiae propagated rapidly in O. niloticus, whereas the residence time was short. In contrast, at a lower temperature, the propagation time decreased, but the residence time increased. Moreover, our results showed that S. agalactiae abundance was the highest in the spleen and survival time was the longest in the kidney. In conclusion, the propagation and elimination speeds of S. agalactiae in tissues of infected O. niloticus were closely related to water temperature. Our results provide important data for further study of S. agalactiae pathogenesis, ensuring effective prevention and control measures.
