Effect of dietary propolis (PR) supplementation on (a) spleen index, (b) thymus index, and (c) bursa index of broiler chickens challenged with Escherichia coli (EC) infection. Bars express the mean (n = 20) with standard error (SE). Means with dissimilar letters significantly differ at p < 0.05. Treatment groups: C, control group without PR supplementation or EC challenge; EC, group without PR supplementation but challenged with EC; PR, group supplemented with PR but not challenged with EC; PR+EC, group supplemented with PR and challenged with EC.