The in vitro and in vivo growth inhibitory effects of egg yolk antibody (IgY) powder was investigated on Escherichia coli O78:K80, as the major cause of associated septicemic disease in broilers. E. coli O78:K80 was incubated with IgY (50, 100, and 150 mg/ml specific (sIgY) and non-specific (nsIgY) IgY powder) for 6 h incubation period. In vivo experiment was conducted with 392 chickens in a completely randomized design with 7 treatments (six E. coli O78:K80-oral challenged groups and one unchallenged group as negative control), 4 replicates and 14 chickens per replicate. The challenged groups received diets supplemented with 0 (as positive control), 0.1, 0.2 and 0.4% sIgY from eggs of immunized hens and 0.2 and 0.4% nsIgY from eggs laid by the nonimmunized hens. sIgY at the concentration of 150 mg/ml decreased bacterial proliferation by 1.18 log cfu/ml compared with the control group at 6 h of incubation period (P 0.01). After three weeks of feeding, the levels of 0.2 and 0.4% sIgY and 0.4% nsIgY had the most suppressive effects (P 0.01) on the ileal E. coli enumeration and intestinal secretory IgA concentration. Dietary supplementation with at least 0.2% sIgY caused respected increase (P 0.01) in the villus height to crypt depth ratio, and depression in the heterophils to lymphocytes ratio (P 0.05), the jejunal goblet cells and lamina propria lymphatic follicles numbers, and feed conversion ratio (P 0.01). Key words : broiler chick, egg yolk antibody (IgY) powder, E. coli O78:K80, oral passive
