(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 causative agent of the disease is , 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 . 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 , 6nitrite for 20 d. The shrimp in the treatment and control groups were then challenged by . 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 and was 0, 4%, 8%, 16%, and 24% under exposure of 0.0, 2, 6, 10.0 mg/L nitrite, respectively. ALT activity increased initially in the control, 2 mg/L groups then decreased, whereas activity continued to increase in the 6 mg/L groups. The activity of AST in the control and 2groups increased and then decreased, whereas levels continued to increase in the 4and 10 mg/L groups, the activity of ALP increased initially before returning to normal levels, whereas levels continued to decrease in the 6 mg/L groups. LSZ activity was significantly higher in the control than in the 6 mg/L groups. Our results suggest that exposure to high nitrite nitrogen concentrations can inhibit the immune response of , resulting in an increase in mortality. Thus, it is important to regulate the concentration of nitrite nitrogen in the aquatic environment to prevent the break out of AHPNS.