亚硝酸盐氮胁迫下凡纳滨对虾对副溶血弧菌的易感性

Susceptibility of Litopenaeus vannamei to Vibrio parahaemolyticus: The influence of environmental nitrite nitrogen

为探讨亚硝酸盐氮胁迫对凡纳滨对虾（Litopenaeus vannamei）急性肝胰腺坏死综合征（Acute Hepatopancreatic Necrosis Syndrome, AHPNS）发生的影响,设置1个凡纳滨对虾对照组和4个不同亚硝酸盐氮质量浓度（2.0、4.0、6.0和10.0 mg/L）胁迫组,亚硝酸盐氮暴露20 d,于凡纳滨对虾后腹肌注射副溶血弧菌（Vibrio parahaemolyticus）进行感染。观察到在感染后9~24 h各实验组出现死亡高峰,濒死对虾表现出与AHPN相同的临床症状,48 h后,各实验组对虾不再出现死亡,对照组和各实验组对虾累计死亡率分别为0、4%、8%、16%和24%。其中,4.0 mg/L实验组对虾累计死亡率与2.0 mg/L实验组差异不显著（＞0.05）,但显著低于6.0和10.0 mg/L实验组（＜0.05）。感染副溶血弧菌后,对照组、2.0和4.0 mg/L组凡纳滨对虾血清ALT活性先上升后下降,6.0和10.0 mg/L实验组血清谷丙转氨酶（ALT）活性持续上升。对照组和2.0 mg/L实验组对虾血清谷草转氨酶（AST）活性先上升,96 h后开始下降,4.0、6.0和10.0 mg/L实验组AST活性持续升高。对照组、2.0和4.0 mg/L实验组对虾ALP活性呈现先升高后降低的趋势,6.0和10.0 mg/L实验组对虾碱性磷酸酶（ALP）活性持续降低。对照组对虾溶菌酶（LSZ）活性在各取样时间点显著高于6.0和10.0 mg/L实验组（＜0.05）,而与2.0和4.0 mg/L实验组差异不显著（＞0.05）。结果表明,亚硝酸盐氮胁迫能降低凡纳滨对虾的免疫力,增加其对副溶血弧菌的易感性。为预防AHPNS的暴发,养殖水体中亚硝酸盐氮质量浓度应控制在4.0 mg/L以下。

Acute Hepatopancreatic Necrosis Syndrome （AHPNS） or Early Mortality Syndrome （EMS）, first reported officially in China in 2010, has caused large-scale die-offs of cultivated shrimp in several countries in Asia. The causa-tive agent of the disease is Vibrio parahaemolyticus, a strain of a bacterium commonly found in brackish coastal waters around the globe. To reduce the impact of the disease, it is important to understand how environmental factors, such as nitrite nitrogen levels, affect the pathogenicity of V. parahaemolyticus to Litopenaeus vannamei. We evaluated the effect of environmental nitrite levels on immune-related enzymes （glutamic-pyruvic transaminase, ALT;glutamic oxalacetic transaminase, AST;alkaline phosphates, ALP;lysozyme, LSZ） in L. vannamei exposed to V. parahaemolyticus. L. van-namei were reared in 2.0, 4.0, 6.0, 10.0 mg/L nitrite for 20 d. The shrimp in the treatment and control groups were then challenged by V. parahaemolyticus. The moribund shrimp were anorectic and the hepatopancreas of deceased shrimp was pale to white and atrophied. The cumulative mortality peaked 48 h after exposure to V. parahaemolyticus and was 0, 4%, 8%, 16%, and 24%under exposure of 0.0, 2.0, 4.0, 6.0, 10.0 mg/L nitrite, respectively. ALT activity increased ini-tially in the control, 2.0, and 4.0 mg/L groups then decreased, whereas activity continued to increase in the 6.0 and 10.0 mg/L groups. The activity of AST in the control and 2.0 mg/L groups increased and then decreased, whereas levels con-tinued to increase in the 4.0, 6.0, and 10.0 mg/L groups. In the control, 2.0 and 4.0 mg/L groups, the activity of ALP increased initially before returning to normal levels, whereas levels continued to decrease in the 6.0 and 10.0 mg/L groups. LSZ activity was significantly higher in the control than in the 6.0 and 10.0 mg/L groups. Our results suggest that exposure to high nitrite nitrogen concentrations can inhibit the immune response of L. vannamei and increase its susceptibility to V. parahaemolyticus, resulting in an increase in mortality. Thus, it is important to regulate the concen-tration of nitrite nitrogen in the aquatic environment to prevent the break out of AHPNS.