Effects of low-dose dietary antimicrobial peptide Scy-hepc on disease resistance in Larimichthys crocea under secondary infection

Secondary infection by Vibrio parahaemolyticus, triggered by Cryptocaryon irritans, has become one of the most difficult diseases to control in Larimichthys crocea aquaculture due to its complex pathogens and diverse infection mechanisms. Traditional antibiotic therapies often face challenges such as resistance and low efficacy. Previous studies have shown that the antimicrobial peptide (AMP) Scy-hepc has potential as an immune-enhancing alternative to antibiotics. This study identified typical features of a natural outbreak of V. parahaemolyticus secondary infection triggered by C. irritans in a marine aquaculture farm. Under these practical farming conditions, the overall protective efficacy and potential mechanisms of Scy-hepc were further evaluated. Compared to the same dosage of Vector, Florfenicol, Bacillus subtilis, and a basal diet (Control), Scy-hepc significantly improved the survival rate and increased the total body weight of L. crocea. qPCR results showed that Scy-hepc downregulated pro-inflammatory genes (IL-1β, TNF-α) and upregulated anti-inflammatory genes (TGF-β, IL-10) as well as the AMP genes (Hepcidin, NK-lysin). 16S rRNA sequencing revealed that although all groups experienced gut microbiota disruption due to infection, the gut microbiota in the Scy-hepc group was enriched with more specific microorganisms, characterized by an increased abundance of potential probiotics (Tenericutes, Bacillaceae, Bacteroidales) and a decreased relative abundance of opportunistic pathogens (Clostridiales, Fusobacteriaceae, Spirochaetes, and Vibrionaceae). Scy-hepc also enhanced microbial functions related to metabolism and immunity. Network analysis showed higher complexity and stability in the Scy-hepc group than in the Vector and Control groups. Mantel test analysis revealed that gut microbiota composition was significantly correlated with health-related indicators (survival rate and body weight) as well as immune-related indicators (NK-lysin, Hepcidin, GPx-1a, and CAT) in the Scy-hepc group. Overall, this study provides the potential evidence of Scy-hepc’s protective effects and potential mechanisms against secondary infection under practical farming conditions. These findings support its potential as a promising antibiotic alternative to promote the sustainable and healthy aquaculture of L. crocea.