Monitoring of fifty broiler flocks suffered from high mortalities in baby chicks revealed that there were differences in breed susceptibility as well as age of birds. In the same time difference in the mortality rates and there are differences in governorates.The isolated bacterial agents incriminated in induction of that problem revealed that 92% of the examined flocks were positive to aerobic bacterial infection.After cultural, biochemical and serological identification revealed that 94.6% of isolates were Gram negative while 5.4%of the isolates were Grampositive. The biochemical characters revealed that E.coli isolates was the predominant strains 62.3% followed by P. vulgaris of 9.7%,K. pneumoniae 8.6%, Salmonella 6.4%,S. aureus5.3%,P.aeroginosa 4.2% and C. freundi3.2%.On serological tests revealed 14 serogroups of E.coli were identified and the most predominant was O158 and in Salmonella revealed 4 isolates of S. enteriditis and 2 isolates of S. infantis.In-vitro testing of the pathogenicity of different serogroups of E.coli after Congo-red test and molecular detection of virulence gene (iutA,iss) which proved that all E coli serogroups were pathogenic.The isolated and identified strains were inoculated into susceptible chicks by two routes (oral and s/c)and observation of clinical signs,P.M lesions, histopathological lesions, performance profile of each subgroup. The results revealed that high chick mortality were recorded with severeclinical signs and Septicemic picture of most inoculated chicks in different subgroups. Histopathological lesions proved signs of severe congestion in liver, heart, and other tissue samples. Performance profile proved that significantly lower mean live body weight all over the experimentation period (six weeks) in comparison with control group.

(Key Words:    High chick mortality,  bacterial infections, experimental infection,)


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