The immune response of tilapia Oreochromis mossambicus and its susceptibility to Streptococcus iniae under stress in low and high temperatures

Mozambique tilapia Oreochromis mossambicus acclimated to 27 °C were then held at 19, 23, 27 (control), 31 and 35 °C, and were examined for non-specific cellular and humoral responses after 12–96 h. Total leucocyte count decreased significantly when fish were transferred to 19 and 23 °C after 48 and 96 h, and when transferred to 35 °C over 12–96 h, respectively. Respiratory burst decreased significantly when fish were transferred to 19, 31 and 35 °C over 24–96 h, whereas phagocytic activity and phagocytic index decreased significantly when fish were transferred to low temperatures (19 and 23 °C) and high temperatures (31 and 35 °C) over 12–96 h. Lysozyme activity decreased significantly when fish were transferred to 19 °C after 12–96 h, but increased significantly when transferred to 31 and 35 °C over 48–96 h. Alternative complement pathway (ACH 50) also decreased significantly when transferred to 19 and 23 °C after 12 h, but increased significantly when transferred to 31 and 35 °C after 24 h. In another experiment, tilapia reared at 27 °C were injected intraperitoneally with Streptococcus iniae at a dose of 1 × 10 7 colony-forming units (cfu) fish −1, and then reared onward at water temperatures of 19, 23, 27 (control), 31 and 35 °C. Over 48–168 h, the cumulative mortality of S. iniae-injected fish held in 19 and 35 °C was significantly higher than that of injected-fish held in 23, 27 and 31 °C. It is concluded that transfer of tilapia O. mossambicus from 27 °C to low temperatures (19 and 23 °C) after 12 h, and transfer of fish from 27 °C to high temperatures (31 and 35 °C) reduced their immune capability. Moreover, tilapia under temperature stress at 19 and 35 °C from 27 °C decreased its resistance against S. iniae.