Killing effect of fish serum and mucus on Gyrodactylus cichlidarum (Platyhelminthes, Monogenea) mediated by complement

Abstract

Gyrodactylids mainly parasitizing on the skin and fins of fishes, represent an ongoing challenge of farmed fishes. Evidences from previous studies suggested that complement system play important role in fish against gyrodactylids. Through a series of parasite killing experiments with fish sera and mucus, the present study revealed the role of Oreochromis niloticus complement in killing Gyrodactylus cichlidarum. The killing effect on G. cichlidarum was observed in serum and mucus of O. niloticus, which was markedly weakened after the complement inhibition by heat treatment and C3 antibody neutralization. Scanning electron microscopy (SEM) revealed various injuries on the tegument of G. cichlidarum exposed to O. niloticus serum. After inhibiting the alternative complement pathway (ACP) by zymosan, the killing activity of O. niloticus serum to G. cichlidarum was significantly attenuated (P < 0.01). The inhibition (by adding EGTA or l-lysine) to the classical complement activation pathway (CCP) also resulted a weak suppression on the killing effect. The results demonstrated the indispensable role of O. niloticus complement in killing G. cichlidarum, mainly via the ACP and with the involvement of CCP as well. In addition, complement in serum of an unnatural host (Ctenopharyngodon idella) of G. cichlidarum showed highly significantly weaker killing effect to G. cichlidarum than that of O. niloticus (P < 0.01), which needs further study for its mechanism. These findings provided new evidences about the immune mechanism of fish against gyrodactylids, and are reference for the study on the control of gyrodactyliasis.