The Immune Response Of Gift Oreochromis Niloticus And Its Susceptibility To Streptococcus Iniae Under Stress In Three Environmental Factors

Tilapia is an important commercial fish in china. In recent years, it has experienced disease problems with production declines due to Streptococcus iniae. Immunity, pathogen and environmental conditions affect the happening of the disease. So we studied the immune response of GIFT oreochromis niloticus and its susceptibility to streptococcus iniae under stress in three environmental factors, in order to reduce the incidence of streptococcus disease by controlling environmental factors.This thesis studied the immune parameters (SOD, AKP, lysozyme and complement C3) of GIFT oreochromis niloticus and its susceptibility to S.iniae in different temperatures (25℃,28℃,31℃,34℃), ammonia-N (control,1.0mg/L,3mg/L,5mg/L,10mg/L) and nitrite (control,0.75mg/L,1.5mg/L,3mg/L,5mg/L).Temperature could affect the immune index significantly. In25℃group, SOD activity had no obvious difference compared to other sampling time (p>0.05), but in31℃group, SOD activity increased, and then decreased to normal level. In25℃group, AKP activity decreased at3h significantly compared to control, maximum suppression ratio achieved to12.54%. In31℃and34℃groups AKP activity increased at12h significantly, maximum induction ratio achieved to21.38%and22.59%respectively. LSZ decreased, and then recovered when transferred to25℃and31℃, but in25℃group LSZ suppress all the time. In25℃group complement C3activity decreased after3h significantly, maximum suppression ratio achieved to22.22%. In31℃group, complement C3activity increase11.85%after12h, and in34℃group complement C3activity increased significantly after12h and24h, and achieved to10.99%and13.40%. The mortality were significantly affected by temperature. The cumulative mortality of S.iniae injected-fish held in34℃achieved to56.67%, the cumulative mortality was significantly higher than that of injected-fish held in25℃,28℃and31℃. The resistance of tilapia to S.iniae decreased significantly under the condition of high temperature, and increased the susceptibility to S.iniae.There was an induction trend of SOD activity when tilapia fish were exposed to1.0mg/L ammonia-N, but instead of suppression when the tilapia were exposed to3mg/L,5.0mg/L and10.Omg/L group, maximum suppression ratio achieved to18.85%,23.47%and38.46%respectively. In1.0mg/L group, AKP activity decreased, and then recovered to normal level. AKP activity always decreased when ttilapia fish were exposed to5.0mg/L and10.Omg/L ammonia-N. There was no significant difference in LSZ when tilapia fish were exposed to1.Omg/L, And with the elevation of ammonia-N, the LSZ activity decreased in some high concentration groups. In lmg/L group, complement C3was always higher than control in every sampling time, but decreased when tilapia fish were transferred to5.0mg/L and10.0mg/L ammonia-N. The cumulative mortality of S.iniae injected-fish held in3.0mg/L,5.0mg/L and10.0mg/L achieved to43.33%,46.67%and63.33%respectively, the cumulative mortality was significantly higher than that of injected-fish held in0.1mg/L (control) and1.0mg/L. It was concluded that more than3.0mg/L ammonia-N in water caused suppression of the immune response and mortality incidence of tilapia by the S. iniae infection.The SOD activity was induced when tilapia fish were exposed to0.75mg/L nitrite after24h,48h and120h, and decreased when exposed to3.00mg/L and5mg/L nitrite, maximum suppression ratio achieved to27.89%and30.06%respectively. AKP activity was induced when tilapia fish were exposed to0.75mg/L nitrite, and1.50mg/L group had no significant difference compared to control, but AKP activity always decreased when tilapia fish were exposed to3.00mg/L and5.00mg/L group, maximum suppression ratio achieved to30.83%and30.64%respectively. There was no significant difference in LSZ when tilapia fish were exposed to0.75mg/L and1.50mg/L group, And with the elevation of nitrite, the LSZ activity decreased in some high concentration groups. In0.75mg/L and1.50mg/L groups complement C3was always higher than control in96h, but always decreased when tilapia fish were transferred to nitrite3.00mg/L and5.00mg/L group, maximum suppression ratio achieved to14.43%and11.1%respectively. Results showed that the cumulative mortality of S.iniae injected-fish held in3mg/L and5mg/L achieved to43.33%and46.67%respectively, the cumulative mortality was significantly higher than that of injected-fish held in control,0.75mg/L and1.50mg/L group. It was concluded that more than3mg/L nitrite in water caused suppression of the immune response and mortality incidence of tilapia by the S. iniae infection.The results showed that temperature, ammonia and nitrite could affect the immune index of talipia significantly and its susceptibility to S.iniae. High temperature (34℃), high ammonia-N (>3.0mg/L) and high nitrite (>3.0mg/L) could increase susceptibility to S.iniae and decreased its resistance against S.iniae. Effective measures should be carried out to regulate environment factors in order to avoid the outbreak of streptococcus disease.