الفهرس 
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Abstract
This study was performed in Internal Medicine & Infectious and Fish Diseases Dept. Faculty of Veterinary. Medicine, Mansoura University: This study was carried out on 5683 examined animals (from six farms and small holders groups) in Dkahlia, Domiatta, and Behara governorates where 270 clinical mastitic cases (4.75%) and 1560 subclinical cases (27.4%). The effect of lactation season, milk production rate, hygiene, feeding on the incidence of mastitis for each animal group was studied and we noticed that the incidence of mastitis changed from one animal group to another according to risk factors specific for each group. A bacterial cause of mastitis especially environmental causes (E coli) and contagious causes mainly Staph. aureus ,Strept. agalactia remain a major causes of bacterial mastitis in dairy farms where Staph aureus organisms represent more than 35% of bacterial causes of mastitis in certain localities .Results of antibacterial sensitivity tests revealed that the main bacterial pathogens isolated from clinical cases of mastitis were highly sensitive to gentamycin, chloramphenicol, enerofloxacin, amoxicillin, amoxicillin +claveulenic acid and cefotaxime but they were resistant to ampicillin and penicillin G. Vaccination of dry cows by bactrin locally prepared from isolates of Staph aureus and its evaluation after parturition by Somatic Cell Count ,Bacteriological examination and determination of antibody titer in the sera of vaccinated cows and results showed that it was of beneficial value in the control of sub clinical mastitis infected herds.