Concentrated mannan oligosaccharide supports grass carp (Ctenopharyngodon idellus) health during Ichthyophthirius multifiliis infection: Histopathological and immunological study

Ichthyophthirius multifiliis is a highly pathogenic ciliate parasite that infects nearly all freshwater teleost fish and causes significant economic losses. However, none of the current treatments for ichthyophthiriasis are effective. Therefore, it is necessary to explore new effective, alternative, and environmentally friendly products for the control of I. multifiliis infection. Concentrated mannan oligosaccharide (cMOS) is widely used in aquaculture as a functional additive to enhance fish immunity and disease resistance. In this study, 1 ‰, 2 ‰, and 4 ‰ cMOS were included into the diet of grass carp ( Ctenopharyngodon idellus ) for 6 weeks while observing growth performance and immune response in order to evaluate the concentration and time of cMOS supplementation needed to support I. multifiliis infected grass carp. The results showed that feeding 2 ‰ and 4 ‰ cMOS significantly increased the body weight of grass carp ( P  < 0.05), and feeding 4 ‰ cMOS increased the total length and specific growth rate ( P  < 0.05). The histological results showed that grass carp fed with 4 ‰ cMOS for three weeks and 2 ‰ cMOS for four weeks had more mucous cells and T/NK cells in their gills ( P  < 0.05). Further qRT-PCR revealed higher expression of Muc-2 , TCR and IL-1β , as well as lower expression of IL-10 in cMOS fish ( P  < 0.05). After the I. multifiliis challenge, feeding 4 ‰ cMOS for 3 weeks and 2 ‰ cMOS for 4 weeks resulted in significantly less severity of I. multifiliis infection, lower mortality, and lessened pathological damage to gill tissue ( P  < 0.05). In addition, qRT-PCR revealed the expression of IL-1β , Muc-2 , MHC-II , IgM , and TCR in cMOS groups was significantly lower than that of the control-infection group, while the expression of IL-10 was significantly higher ( P  < 0.05). The results of this study contribute new knowledge for the formulation of commercial feed for grass carp and provide both theoretical support and practical guidance for research on the prevention and control of fish ichthyophthiriasis.