Enhanced immune responses and protection against the secondary infection in mud crab (Scylla paramamosain) primed with formalin-killed Vibrio parahemolyticus

Mud crab (Scylla paramamosain) is an economically important marine crustacean species commonly distributed in China and Indo-west Pacific countries. However, the disease outbreaks caused by Vibrio parahemolyticus have resulted in high mortality and disastrous economic losses in the mud crab culture industry. Immune priming has shown the capacity of protecting invertebrates against pathogens in the second exposure, with profound implications for the prevention of infectious diseases. In this study, the survival and immune responses of mud crabs received primary exposure to formalin-killed V. parahemolyticus (immune priming, IP) and secondary challenge with live counterparts (lethal challenge, LC) were investigated. The priming dose of 2 × 10⁵ cells·g⁻¹ of inactivated V. parahemolyticus showed the highest survival rate (75.7%). There was no significant difference among the primed groups that received lethal challenge at different time points (P > .05). The cumulative survival of the primed (FVP) group (73.5%) was significantly higher than that of the control (SA) and naïve groups after the second challenge with live V. parahemolyticus. The efficiency of pathogen clearance, total hemocyte count, and phagocytic rate were significantly increased in FVP group compared to SA group after the IP and LC treatments. By contrast, no significant changes in antibacterial activity, phagocytic index and phenoloxidase activity (excepting at 6 hpi) were found in the comparison between FVP and SA groups. These results suggested the occurrence of priming phenomenon in mud crab, primary immunization with inactivated V. parahemolyticus can improve survival following infection and enhance cellular immune responses, the particularly phagocytic rate that contributes to the elevated phagocytosis. This finding provides a novel direction in protecting mud crab against vibriosis.