الفهرس 
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Abstract
A total of 612 samples of calves faeces, water, feed of calves (colostrum, raw cow milk and milk replacer) and calves houses air were collected from three investigated dairy farms at Sharkia governorate, Egypt. This study was carried out along a period from Aprill 2013 to March 2014. The occurrence of E.coli, Salmonella and Rota virus in newly born and pre-weaned calves were investigated ,in addition E.coli ,Salmonella were detected in water, feed of calves and air of calves houses. The occurrence of certain virulence genes in some isolates of Salmonella was also investigated. The obtained results were summarized under the follows:
1- The occurrence of E.coli in faeces of apparently healthy calves was 5%, 11.7% and 8.3% in farm 1 (Sami assad Farm), farm 2 (Ramsis dairyI) and in farm 3 (Ramsis dairy II) ,respectively. While, the occurrence of E.coli in faeces of diarrheic calves were 15%, 33% and 26.67% in farm 1 (Sami assad Farm), farm 2 (Ramsis dairy I) and in farm 3 (Ramsis dairy II), respectively.
2- Regarding the occurrence of E.coli in faeces of apparently healthy calves and diarrheic calves in different seasons E.coli in faeces of apparently healthy calves was peaked in winter (13.3%), followed by autumn (8.9%), then spring (6.7%) and was lowest in summer4.4%).Concerning E.coli from diarrheic calves the percentages of 53.33% in winter, 24.4 % in spring