Immune responses of Ctenopharyngodon idella juveniles against the ciliated parasite's reinfection induced by Ichthyophthirius multifiliis theronts at the threshold density

Abstract

Ichthyophthirius multifiliis (Ich) is an important ciliate parasitizing freshwater fishes worldwide and leads to considerable economic losses to the aquaculture industry. The present study aimed to find the threshold density of I. multifiliis theronts in grass carp juveniles and to evaluate the immune responses of the host fish infected at the density. The results showed that the threshold density was 20 theronts per liter water, and no mortality of the juveniles occurred when the theront density was equal or below the threshold density. The activities of serum acid phosphatase (ACP), alkaline phosphatase (AKP), lysozyme (LZM), and superoxide dismutase (SOD) of the juveniles increased significantly after treatment at the threshold density of theronts. The mRNA expression levels of immune factor genes (IL-1β, TNF-α, C3 and C9), and immunoglobulin heavy chain genes of IgM, IgD and IgZ as well as the protein levels of immunoglobulins IgM, IgD and IgZ of grass carp juveniles were up-regulated significantly post infection of I. multifiliis at a threshold density. The increased important immune related molecules levels of the juveniles eliminated the ciliated parasite, and the recovery grass carp juveniles had a mean survival rate of 90% when they were re-infected by I. multifiliis at a density of 10,000 theronts per fish (2000 theronts per liter water), which is 25 times as many as the threshold density, while the juveniles in control group were at 100% mortality. Thus, the present study determined the non-lethal threshold density of I. multifiliis theronts in grass carp juveniles, and clarified that the theronts infection at threshold density induced the fish immune responses to protect the recovery grass carp juveniles from re-infection of I. multifiliis. The results will be helpful for aquaculture industry to take strategy to manage and control the disease caused by I. multifiliis.