The effect of different levels of Scrophularia striata extract in normal and challenged conditions with E. coli on performance, carcass characteristics, cellular immune response, blood antioxidant status, intestinal histo-morphometry, and microbial population (E.coli and Lactobacillus) of Cobb 500 broilers were evaluated. The experiment was performed in a complete block design with a 2 × 4 factorial arrangement with five replications of 10 birds each for 35 days in two separated halls [include challenge (C) and non-challenge (N)] with similar experimental conditions. Experimental treatments in two breeding halls were: 1) basal diet or control (CONT), 2) basal diet + 0.1 g/kg of herbal extract (EXTR 1),; 3) basal diet +0.2 g/kg of herbal extract (EXTR 2), 4) Basal diet +0.1 g/kg an antibiotic (Oxytetracycline) (ANTB). On the 16th and 24th days of the experiment, one dose of 1×107 CFU K99 E. Coli was gavaged to chickens in a challenged conditioned hall. The results showed that average body weight and daily weight gain in the whole period (days 35 and 1-35) in EXTR 2 was better than the control treatment (P < 0.05). The percentage of breast weight in the ANTB was significantly different from the control treatment (P < 0.05). Glutathione peroxidase (GPX) in EXTR 2 was better than the control treatment (P < 0.05). Malondialdehyde (MDA) in EXTR 1 was significantly lower than the control group (P < 0.05). The values of E. coli in the treatment ANTB were less than control treatment (P < 0.05). Lactobacillus value in treatment EXTR 2 was higher than control treatment (P < 0.05). The value of total immunoglobulin in 28 days in ANTB, EXTR 1, and EXTR 2 were significantly higher than that of control treatment (P < 0.05). The length of villi was affected by treatments (P < 0.05). In conclusion, dietary inclusion of 0.2 g/kg of Scrophularia striata extract may improve the health status of the birds during E. coli challenge.

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