The Effect of Low-Fishmeal Feed Including an Olive Leaf Extract (Olea europaea) Additive on Growth, Intestinal Health and Disease Resistance in Litopenaeus vannamei

This study evaluated the effects of olive leaf extract (OLE) on growth, intestinal health, and resistance against Vibrio parahaemolyticus challenge in juvenile Litopenaeus vannamei. To a basal diet containing 12% fishmeal, OLE was added at 0%, 0.10%, 0.20%, 0.40%, and 0.80%, forming diets with equal nitrogen and lipid contents. The diets were divided into five groups and named OLE0 (control group), OLE1, OLE2, OLE4, and OLE8. The findings revealed that relative to OLE0, the weight gain rate (WGR) and specific growth rate (SGR) increased in the OLE1 group, whereas the feed conversion ratio (FCR) declined (p < 0.05). The crude protein (CP) content for whole shrimp in the OLE8 group was significantly higher (p < 0.05), and the crude lipid (CL) content in the OLE1, OLE2 and OLE8 groups significantly decreased compared with OLE0 (p < 0.05). In serum indices, the OLE8 group had notably higher total protein (TP) content than the control (p < 0.05). Compared with the OLE0 group, high-density lipoprotein cholesterol (HDL-C) levels in the OLE1 and OLE2 groups were increased, whereas low-density lipoprotein cholesterol (LDL-C) levels in the OLE4 and OLE8 groups were significantly decreased compared with the control (p < 0.05). Hepatopancreatic malondialdehyde (MDA) in all experimental groups was significantly decreased compared with the control (p < 0.05). The superoxide dismutase (SOD) activity in all experimental groups increased (p < 0.05) and was the largest in the OLE1 group. The OLE1, OLE2 and OLE4 groups had significantly increased glutathione peroxidase (GPX) activity (p < 0.05). Catalase (CAT) activity in the OLE1 group was significantly higher than the control (p < 0.05). The toll, lzm, imd, cru a and propo mRNA expression levels in the hepatopancreas were significantly upregulated in each experimental group (p < 0.05). Intestinal microbial analysis indicated that the relative abundance of Bacteroidetes was elevated in the OLE4 and OLE8 groups, accompanied by a reduction in Vibrio proportion (p < 0.05). Post-challenge assays demonstrated that the OLE4 and OLE8 groups achieved remarkably higher cumulative survival rates than OLE0 (p < 0.05). In summary, OLE in a low-fishmeal diet can promote growth performance, intestinal health and disease resistance for shrimp. According to the broken-line regression model of WGR against OLE dosage, 0.08% was the optimal dietary OLE inclusion level of the OLE additive in juvenile L. vannamei.