Study on pathogenicity of WSSV to Pacific white shrimp (Litopenaeus vannamei) under ammonia stress

To evaluate the harmfulness of ammonia on Pacific white shrimp (Litopenaeus vannamei), we investigated the effects of ammonia (NH₄-N) on mortality of the shrimps infected with WSSV (white spot syndrome virus), WSSV proliferation and activity of immune-related enzymes in serum. The shrimps were infected intramuscularly with 2×10⁵ and 2×10⁶ viral particles under the stress of 15.6 mg · L^-1 NH₄-N. The results show that the shrimps with 2×10⁵ WSSV got mortality of 53.3% at 144^th hour, which was significantly higher than that of the control group (stress-free). Meanwhile, the real-time relative PCR shows that ammonia stress accelerated WSSV proliferation in shrimp gills. In addition, it is showed that, under ammonia stress, the activities of immunity-related enzymes such as phenoloxidase (PO), acid phosphatase (ACP) and alkaline phosphatase (AKP) in shrimp serum continued to decrease after a brief rise. The results indicate that ammonia stress accelerates WSSV proliferation in shrimps and causes higher mortality, which is related to the decrease of activities of immune-related enzymes under ammonia stress.