Abstract: This study aimed to establish an enzyme-linked immunosorbent assay (ELISA) to examine antibody responses in chickens induced by a 4-valent attenuated vaccine (COCVAC) containing E. tenella, E. necatrix, E. acervulina and E. maxima against chicken coccidiosis. Three hundred Lingnan Yellow chickens were randomly divided into groups A (immunization-challenge group), B (non-immunization-non-challenge group) and C (non-immunization -challenge group), respectively; and each group had 100 broilers. Chickens in group A were immunized with the vaccine at the age of 4 days, and then boosted (second immunization) at the age of 11 days. After the second immunization chickens in groups A and C were challenged with 280000 mixed oocysts of the 4 Eimeria species/bird) at the age of 17 days. Seven days after challenge infection, the survival rate of in groups A and C was compared. After the second and the third immunizations, blood samples were collected from birds in groups A and B, and serum antibodies to the vaccine were detected using the established ELISA. The average optical density (D) values of birds in group A were 0.870 and 0.904 respectively, which were significantly higher than the average D values of chickens in group B (0.270 and 0.261 respectively). These results indicated that chickens immunized with the 4-valent attenuated coccidian vaccine can induce significant antibody responses, which correlates with the immune protection it induced, demonstrating that the established ELISA provides a useful tool for the evaluation of vaccine efficacy.

Key words: The 4-valent attenuated coccidian vaccine COCVAC; chicken; ELISA; antibody response