This experiment investigated the effects of seaweed polysaccharide (SP) and seaweed enzymatic hydrolysate (SEH) on the growth performance, serum biochemical indices, antioxidant capacity, and intestinal function of Muscovy ducks. A total of 240 healthy 1 day female Muscovy ducks (48.85 ± 0.45 g) were randomly divided into 3 treatment groups, with 4 replicates per group and 20 ducks per replicate. The control (CON) group received a basic diet supplemented with 20 mL/kg of water, the SP group received a basic diet supplemented with 20 mL/kg of SP, and the SEH group received a basic diet supplemented with 20 mL/kg of SEH. The experimental period lasted for 28 d. The results indicate that, compared to the CON group, the average daily feed intake (ADFI) and feed to gain (F/G) of the SP and SEH groups of ducks significantly decreased at 28 d (p < 0.05). In the SP group, serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST), as well as the concentrations of glucose (GLU), triglycerides (TG), total cholesterol (TCHO), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C), were significantly reduced (p < 0.05). In the SEH group, the activities of ALT and AST were also significantly lower (p < 0.05). Additionally, serum total antioxidant capacity (T-AOC) levels and superoxide dismutase (SOD) activity in the SEH group were significantly higher than those in the CON group (p < 0.05), while the malondialdehyde (MDA) content was significantly reduced (p < 0.05). Compared to the CON group, serum levels of immunoglobulin A (IgA), immunoglobulin G (IgG), interleukin-4 (IL-4), and interleukin-10 (IL-10) in the SP group were significantly increased (p < 0.05), whereas the levels of tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and interleukin-6 (IL-6) were significantly decreased (p < 0.05). In the SP and SEH groups, the villus height (VH) and the villus height to crypt depth (V/C) of the Muscovy ducks significantly increased (p < 0.05), while the crypt depth (CD) significantly decreased (p < 0.05). A significant increase in the abundance of Barnesiella was observed in the SP and SEH groups (p < 0.05), whereas the abundances of UCG-005 and Romboutsia significantly decreased (p < 0.05). LEfSe analysis indicated that g__Bacillus and g__Veillonella were significantly abundant in the SP group (p < 0.05), while g__Coriobacteriaceae_UCG_002 was significantly abundant in the SEH group (p < 0.05). In summary, the addition of SP and SEH to the feed can promote the healthy growth of ducks by improving intestinal morphology, regulating the structure of intestinal microbiota, enhancing antioxidant capacity and immune function, and optimizing metabolic indicators. This occurs while reducing feed intake and feed-to-weight ratio, and there is a certain specificity in their mechanisms of action.

The moderate pathogenicity coupled with high host susceptibility of Eimeria maxima has precipitated substantial economic losses in the poultry industry. Addressing challenges such as emerging drug resistance underscores the imperative for innovative vaccine strategies. This study developed a novel DNA vaccine to solve this challenge by fusing E. maxima elongation factor-1α (EmEF1α) with chicken chemokine XCL1 (ChXCL1) in the pVAX1 vector. The recombinant plasmid, designated pVAX1-ChXCL1-EmEF1α, was successfully constructed and confirmed to express the ChXCL1-EmEF1α fusion protein in vitro. Immunization of chickens with this DNA vaccine elicited a robust and balanced immune response, characterized by significantly increased proportions of CD4⁺ (11.76%) and CD8⁺ (5.58%) T lymphocytes, elevated levels of Th1-associated cytokines (IFN-γ and IL-12), and strong antigen-specific IgG and IgA antibody responses. Following experimental challenge with E. maxima, vaccinated birds exhibited substantial protection: a 66.4% reduction in oocyst shedding, a 71.7% improvement in relative weight gain, marked attenuation of intestinal lesions, and an anticoccidial index (ACI) of 170. These findings demonstrate that the ChXCL1-EmEF1α DNA vaccine effectively enhances both cellular and humoral immunity. Collectively, this study validates ChXCL1 as a potent molecular adjuvant and establishes the “antigen–adjuvant” fusion DNA platform as a promising strategy for developing next-generation vaccines against avian coccidiosis.