Dual-Copy VP2 expressed with CTA1-DD in transgenic Eimeria acervulina confers partial protection against infectious bursal disease virus

Infectious bursal disease (IBD), caused by the infectious bursal disease virus (IBDV), remains a major threat to global poultry production. The viral capsid protein VP2 is the primary protective antigen; however, conventional vaccines frequently provide only limited efficacy. In this study, we generated recombinant Eimeria acervulina strains expressing VP2 fused to the molecular adjuvant CTA1-DD (Cholera Toxin A1-D-domain dimer). Two transgenic strains were established: Ea-CVD (single-copy VP2) and Ea-VCVD (dual-copy VP2). Stable expression of the fusion protein was confirmed across multiple developmental stages, and both strains exhibited reduced fecundity relative to the wild-type parasite. Immunization trials in chickens showed that Ea-VCVD induced significantly stronger VP2-specific antibody responses than Ea-VP2 after booster immunization, whereas Ea-CVD displayed only a modest enhancement of VP2-specific immune responses. Challenge experiments demonstrated that Ea-VCVD conferred partial protection, as evidenced by reduced histopathological lesion scores and preserved bursa-to-body weight indices, comparable to those induced by the commercial vaccine. These findings indicate that increasing the VP2 gene copies enhances antigen expression and protective immunity, supporting the potential of Eimeria-based recombinant vaccines as an effective strategy against IBDV.