الفهرس 
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Abstract
A monitoring study was carried out on one hundred and two broiler flocks located in nine Egyptian governorates during the period of 2012 - 2014 in a trial for isolation of responsible aerobic bacterial agents causing high chicks mortality in such flocks. The highest mortality percent was recorded in Cobb breed on the 12th day of age in Beni-Suef while the lowest mortality percent recorded in Sasso breed on the 25th day of age in Sharkia. Bacteriological investigation revealed that the Escherichia coli isolates were the predominant organism (42.9%) followed by Salmonella (21.7%) Klebsiella pneumonia(10.1%), Proteus mirabilis(6.2%) and Pseudomonas aeruginosa (4.5%). Serological identification of E. coli isolates revealed that both O158 and O78 serogroups were the most predominant isolates (18.6%) each, followed by serogroups O27(10.8%), O26 (6.8%), O18 and O55(3.9%), each, O6, O111 and O159 (2.9%), each, finally serogroup O1, O8, O15, O44, O114, O119, O142, O153, O166 and O169 (1.5%), each while un-typable strains were (13.7%). Salmonella serological identification revealed that Salmonella Infantis (SI) was the most predominant isolates (27.2%) followed by Salmonella Virchow (SV) (23.4%), Salmonella Enteritidis (SE)( 20.8%), Salmonella Gallinarum (SG) (14.3%), Salmonella Kentucky (SK) (10.4%) and Salmonella Typhimurium (ST) (3.9%). In vitro testing of the phenotypic properties of the 204 E. coli isolates, it was found that all tested isolates were Congo red positive. Conventional and a real-time PCR assay were used for the detection of rfbS, flijB, fliC, stmm and sefA genes of SG, SI, SK, ST and SE respectively where they sequenced and submitted on gene bank with accession numbers KP730600, KP760484, KP760485, KP763723 and KP793717. The genetic diversity of the submitted genes was compared with sequences deposited in the NCBI database to infer phylogenetic relationships between them. The results of multiplex PCR for E. coli grouping were E. coli O1 was grouped in Enterotoxogenic group, E. coli O26 and E. coli O158 were grouped in Enteropathogenic, also E. coli O158 was grouped in Enterotoxogenic group, E. coli O78 was grouped in Enterotoxogenic group, E. coli O27 grouped in Enteropathogenic, Enterotoxogenic and Shiga toxin producing E. coli. The results of pathogenicity of different isolated Salmonella species in day old SPF chicks revealed that, the clinical signs and PM lesions were variable in their time of onset, severity and duration in different Salmonella spp. The mortality percent was in ST (84%), SG (66%), SE (54%), SK (16%) and SI (8%). No mortalities were recorded in chicks inoculated with SV and un-inoculated controls. Analysis of the level of fecal shedding on 21 DPI revealed 100% in Salmonella re-isolation for all serogroups except SV (90.9%) and SG (27.3%), while on 28DPI SE was the highest in fecal re-isolation rate followed by SV, SI, SK and SG with a percentage of 100%, 70.7%, 66.7%, 25.6% and 0% respectively. The microscopic lesions revealed that, the lymphoid organs (bursa, thymus, spleen and cecal tonsils) were severely affected in SV, SK and SI infected groups while the lesions in S.T, S.G and SE infected groups were mostly related to heart, liver, cecum and intestine. The pathogenicity of five E. coli isolates (O1, O26, O27, O78, O158) were investigated in day old SPF chicks by crop gavaged and subcutaneous inoculation (sc). Various clinical signs, post mortem and histopathological pictures were recorded. The mortality percent in sc inoculated group was 100% except in E. coli O1 and O27 groups in which the mortality percent was 80%. The highest mortality percentage in orally inoculated E. coli group was recorded in O78 infected group, followed by O26 and O158, O27 then O1 group with a percentage of 60%, 53.3%, 53.3%, 40% and 13.3% respectively. E. coli and Salmonella infections in chicks significantly reduced feed intake, altered growth of the whole body. Study the effect of some E. coli and Salmonella isolates on broiler immune system showed that the gross observations of bursa of fabrcious, thymus and spleen were atrophied with variable degree in the different inoculated Salmonella and E. coli serovars during 3-4 post infection, which confirmed by measuring of lymphoid organs body weight ratio and histopathological examination. Also infected birds experienced depressed responsiveness to viral vaccines (Newcastle disease vaccine (NDV), infectious bronchitis (IB), avian influenza (AI) and infectious bursal disease (IBD) in comparing with uninfected controls for 2-3weeks post vaccination, indicating that E. coli and salmonella infection have a transient effect on immune system which interferes with the ability of the immune system to respond humorally to antigenic stimuli.