الفهرس 
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Abstract
The study was conducted on three poultry farms (breeder, layer and broiler) and three hatcheries, located in Dakhlia govrnenorate. The work was divided into four parts: I: Investigation the biosecurity status and the possible risk factors for the occurrence of pathogens of biosecurity concern in different poultry production units. II: determining some epidemiological pattern of some pathogens of biosecurity concern (Salmonella and E.coli strains) in poultry operations and this was achieved by collection of samples from birds, hatchery and the environment. III: The in vitro sensitivity of isolated strains of Salmonella and E.coli to certain antibiotics and disinfectants. IV: Studying the efficacy of antibiotics versus probiotics as a vital component of biosecurity measures for control of Salmonella in newly hatched chicks. from the results: 1: Biosecurity levels were 2, 1 and 0 in breeder, layer and broiler farms, respectively. There was a seasonal variation in the isolation rate of Salmonella and E.coli from the examined farms, where the highest isolation rate was at winter. Season, age &environment measures were considered the main risk factors. The most common isolated serotypes were S.kentucky, S.newport, S.kottobas, S.typhi, S.tallahatse and S.derbi. as 90, 65, 29.9, 25, 20 and 6.255, respectively. All identified strains were resistant to nalidixic acid, amoxycillin/clavulinic acid and ciprofloxacin. Meanwhile, they were sensitive to amikacin, nitrofurantion, chloramphenicol, polymyxin B and enrofloxacin. The efficacy of Probiotic versus antibiotic resulted in slightly increase in body weights (9.86.8 ± 1.7; 985.1 ± 1.88 and 850.0 ± 1.87, respectively. Mortality rate was the lowest in Probiotic 12% followed by 16% in antibiotic group and the highest was in control group 40% at 5th day post-challenge, meanwhile lower mortality rates were recorded in the groups at 9th days post-challenge as 6, 8 and 20%, respectively. Probiotic and antibiotic treatments increased heterophils levels as 12.72 ± 1.61 and 10.13 ± 1.05, respectively at 5th days post-challenge, and slightly decreased at 9th day to 6.73 ± 0.36 and 8.15 ± 0.56 in probiotic and antibiotic groups. Salmonella was not recovered from any of –ve groups throughout study period.