الفهرس 
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Abstract
This study aimed to evaluate the most predominant strain of avian pathogenic Escherichia coli (APEC) causing colibacillosis with respiratory manifestation in broiler chickens in Egypt. Isolation and serotyping identification for the isolates of E. coli were performed. Prevalence of APEC was made by detection of the 8 virulence-associated genes “astA, iss, irp2, papC, iucD, tsh, vat, and cva/cvi” using multiplex PCR. Antibacterial sensitivity test for antibiotics disc and oregano at different concentrations (20%, 15%, 10%, and 5%), were estimated for the most important isolates of E. coli. After that, evaluating the efficacy of oregano oil 20% (in drinking water) and oregano powder 10% (on diet) for treatment and prophylaxis of broiler chickens experimentally infected with APEC O27-local isolate was performed. A total of 300 chicks -aged 19 days old- were randomly allocated into equal 6 groups; normal control (G1), Infected (G2), oregano treated (G3), antibiotic treated (G4), oregano control (G5), and oregano treated then infected (G6). Chickens of (G5) and (G6) were supplemented with oregano powder that contains 10% oregano essential oil in the diet (250 g /ton diet) from the 1st day old till the end of the experiment. At the age of the 19th, 20th, and 21st day, G2, G3, G4, and G6 were infected with 0.2ml of APECO27 (1×107 CFU/chick) via the intratracheal route. On the 22nd, 23rd, and 24th day of age, chicks of G3 & G4 were treated with oregano 20% (2 ml per 5 liters) and difloxacin (10 mg/kg b.wt.) in drinking water, respectively. On the 3rd, 6th, 10th, and 21st day post-infection, blood and tissue samples were collected from all groups for laboratory examination of the hemogram,serum biochemistry, and histopathology of the vital organs. Results revealed that E. coli O27 was the most predominant serotype strain (11 out of 35 isolates). One isolate of APEC O27 out of 11 isolates has 6 virulence-related genes “astA, iss, papC, iucD, vat, and cva/cvi”. Difloxacin and oregano at a concentration of 20% have the highest percentage (100%) of sensitivity against 11 isolates of E. coli O27 strains. In G2 (infected only), chicks showed ruffled feathers, gasping, dullness, and loss of appetite, while mild symptoms were shown in birds of G3, G4 and G6. In G2, G3, G4, G5 and G6 chicks, the mortality rate recorded was 30%, 8%, 4%, 0% and 4%, respectively. Chicks of G2 and G4 developed macrocytic hypochromic anemia, leukocytosis, heterophilia, and lymphopenia. Values of AST, ALT, uric acid, and creatinine were increased, while total protein profile was decreased. After treatment with oregano, alterations were returned to the normal levels. In oregano supplemented groups, no significant alterations were detected. In birds of G2, G4 and G6, histopathological alterations were noticed in the internal organs (Lungs, trachea, liver, spleen and kidneys). In conclusion, E. coli O27 was the most predominant serotype infecting farms of broiler chickens where it caused alterations in blood picture, liver and kidney functions as well as histopathology of the internal organs. Oregano essential oil (solution 20% in drinking water or powder 10% in diet) could be used for the treatment or prophylaxis against APEC O27 infected broiler chichens.