Abstract:As the scale of grass carp (Ctenopharyngodon idella) aquaculture continues to expand, there is an urgent need to cultivate superior breeds with strong disease resistance to enhance economic efficiency. This is particularly crucial in addressing the weakening ability of grass carp to resist the small melonworm (Ichthyophthirius multifiliis) disease. In this study, grass carp infected with the small melonworm disease were categorized into disease-resistant and susceptible groups. Subsequent comparative research on gill histological characteristics, disease-resistant enzyme activity indicators, and amino acid composition was conducted. The results showed that the gill filaments of healthy and disease-resistant groups had intact structures, rich and orderly arrangement of capillaries, while the susceptible group exhibited typical pathological features, such as a significant decrease in capillaries and disorderly arrangement of gill filaments. Activity tests for various immune-related antimicrobial enzymes revealed that the disease-resistant group"s AKP, SOD, CAT, ACP, and LSZ activities were significantly higher than those of the healthy and susceptible groups (P < 0.05), while MDA activity was significantly lower than that of the susceptible group (P < 0.05). Amino acid composition analysis showed significant differences (P < 0.05) in 12 amino acid components between the disease-resistant and infected groups, and 15 amino acids showed significant differences (P < 0.05) between the disease-resistant and healthy groups, with phosphoethanolamine and arginine being common differential components. Therefore, this study focused on grass carp infected with the small melonworm disease, conducting a comparative analysis in terms of histology, enzyme activity, and free amino acid composition among different groups. The study revealed the harm of the small melonworm disease to grass carp aquaculture and made inferences about its pathogenic and lethal mechanisms. Additionally, it clarified the roles of related immune function enzymes and amino acid components in disease resistance, providing a theoretical foundation and reference for future efforts in the selection and breeding of disease-resistant grass carp strains in aquaculture.