Immune responses and susceptibility to Vibrio parahaemolyticus colonization of juvenile Penaeus vannamei at increased water temperature

Outbreaks of acute hepatopancreatic necrosis disease (AHPND), which was reportedly caused by a strain of Vibrio parahaemolyticus, resulted in a significant decline in the world shrimp production of the previous years. Cultured shrimp are subjected to seasonal changes and changes due to culture management that can influence the physico-chemical quality of the rearing water. Elevated water temperature is assumed to have significant influence on the health status of crustaceans as well as on the pathogenesis of opportunistic pathogens such as Vibrio species. Hence, the present study evaluated the influence of abrupt exposure to higher temperature on selected immune parameters of Pacific white shrimp Penaeus vannamei as well as on the initial step in the pathogenesis of AHPND-causing strain V. parahaemolyticus which is colonization in the host's gut. Respiratory burst activity (RBA) and phenoloxidase (PO) activity of shrimp exposed to 29°C (control temperature), 32°C and 35°C were measured at 0, 24, 48, 72, and 96 hour post exposure (hpe). Results showed that RBA of shrimp transferred to 35°C significantly increased at 24 hours hpe and started to decline although comparable with the control at 48 to 96 hpe. Shrimp transferred to 32°C displayed higher PO activity than those in control throughout the 96 hour trial with significant increase at 48 hpe. No significant changes were observed in PO activity of shrimp transferred to 35°C. On the other hand shrimp kept at the test temperatures for 24, 48, 72 and 96 hours and exposed to V. parahaemolyticus for 24 hours were found to be colonized by the bacteria on the gut. Highest colonization was documented in shrimp exposed to 35°C for 96 hours which indicated that prolonged exposure to high temperature can cause shrimp to become more susceptible to pathogen invasion which may result in diseases.