Comparative Analysis of Cross-Protective Immunity Among Three Geographically Distinct Isolates of Eimeria kongi

Coccidiosis is one of the most significant diseases affecting the rabbit industry and is caused by Eimeria. In a previous study, we identified a new species of Eimeria kongi (E. kongi-ZJK) from the northern region of China (Zhangjiakou, Hebei Province) and studied its pathogenicity and immunogenicity. The aim of this study was to evaluate the pathogenicity, immunogenicity, and cross-immunogenicity from different geographical isolates of E. kongi for vaccine development. Two geographical isolates of E. kongi-QD from Qingdao, Shandong Province (eastern China), and E. kongi-CD from Chengdu, Sichuan Province (southwestern China), were isolated and identified. The pathogenicity, immunogenicity, and cross-immunogenicity among the three geographical isolates were evaluated. The pathogenicity results showed that after infecting rabbits with doses of 1 × 10³ or fewer sporulated oocysts of E. kongi-QD and E. kongi-CD, the rabbits exhibited clinical symptoms but recovered quickly, indicating lower pathogenicity. Immunogenicity studies revealed that after immunizing rabbits with 1 × 10³ sporulated oocysts of E. kongi-QD and E. kongi-CD for 14 days, followed by challenge with 1 × 10⁴ homologous sporulated oocysts, the oocyst reduction rates in the immunized groups were 99.39% and 99.12%, respectively, compared with the non-immunized groups, demonstrating good immunogenicity. In cross-immunogenicity studies, rabbits were immunized with 1 × 10³ sporulated oocysts of E. kongi-ZJK, E. kongi-QD, or E. kongi-CD and then challenged with 1 × 10⁴ sporulated oocysts of heterologous isolate. The immunized groups showed no significant clinical symptoms, and the oocyst reduction rates ranged from 55.9% to 98.4%. E. kongi-CD exhibited cross-protection against E. kongi-ZJK and E. kongi-QD, making it an ideal candidate formulation for vaccine development.