الفهرس 
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Abstract
E. coli is a normal inhabitant of the intestinal tract of chickens, it is capable of producing sickness under specific stressful conditions such as overcrowding, poor ventilation, malnutrition, extreme temperature, and immunosuppression, avian colibacillosis is an infectious illness of chickens produced by Escherichia coli (E. coli) either as a primary pathogen or as a secondary pathogen. E. coli was isolated from 53.2% of broiler chicken organs with a high prevalence in lungs (26.1%). The most prevalent serotypes were O78, O26, O44, O55, O157, and O127 prevalence of 27.8, 22.2, 16.7 (each), and 5.6% (each), respectively. In experimental design, five groups (G1–G5) of birds were established. G1 served as the negative control group, while G2–G5 were challenged orally with E. coli O78, O26, O55, or O44, respectively. Chickens infected with E. coli O78 or O26 showed significant clinical signs in comparison to the other infected birds. Mortality (13.3%) was only observed in birds infected with E. coli O78. Necropsy of dead birds after E. coli O78 infection showed pericarditis, enteritis, airsacculitis, and liver and lung congestion. More severe histopathological changes were observed in intestines, spleen, liver, and lung from chickens infected with either E. coli O78 or O26 than for birds infected with other serotypes. Quantitative real-time PCR has been the method of choice for the rapid analysis and quantification of cytokine gene expression. One of the advantages of quantitative real-time PCR is that it requires only partial sequence information for the target genes. On the 2nd day post-infection, E. coli challenge, particularly with E. coli O78, displayed significantly up regulated levels of ileal interleukin-6 (IL-6) and interleukin-8 (IL-8), but ileal interleukin-10 level tended to be downregulated in comparison to the control group.