Pathological changes in ceca of experimental chickens in each group. On day 7 post infection (PI), chickens from challenged control group displayed severe typical gross pathological changes in ceca, including swelling and thickening, petechial hemorrhage on the serous membranes, and bloody cecal content (i and j, A), while chickens in the group orally fed with three phages A (c and d, A), D (e and f, A) and W (g and h, A), especially phages A and D, presented relatively moderate gross pathological changes. Chickens from unchallenged control group showed no gross pathological changes in ceca (a and b, A). The histopathological changes in cecal tissues of chickens in challenged control group were obvious, including compromised intestinal structural integrity, damaged intestinal villi, numerous red cells, and infiltration of inflammatory cells (e, B) (H.E. × 200), while chickens orally fed with phages A (b, B) (H.E. × 200), D (c, B) (H.E. × 200) and W (d, B) (H.E. × 200) presented relatively moderate histopathological changes in cecal tissues. Chickens from unchallenged control group showed no histopathological changes in cecal tissues (a, B) (H.E. × 200). Representative images were radomly selected from each group, and lesion scores in ceca were recorded by arabic numerals in (A).