الفهرس 
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Abstract
During the period of the investigations from Feb. 2016 to the end of Dec. 2017, it was found that the prevalence rate of clinical and subclinical mastitis of cows at the area of study were 13% and 45% respectively (n = 200). Clinically, it was found that (13%) of the examined dairy cows (n = 200) were clinically positive to mastitis in variable forms, per acute and acute / subacute and chronic forms. The later form (61.54%) was more prevalent than the former (38.46%), which was associated with severe systemic reactions in form of anorexia, fever, tachycardia, tachypnea in association with severe pain in the infected quarters with lameness of the neighboring legs. These severe reactions might relate to the predominant isolated mastitis pathogen, Staph. aureus which elaborates various powerful necrotizing toxins inside the mammary tissues causing serious local and systemic illness. The relationship between the lactation numbers of the examined dairy cows and the prevalence of clinical and subclinical mastitis concluded that the prevalence rate of clinical and subclinical mastitis of the examined dairy cows was primarily increased by increasing lactation numbers until the third lactation season (peak of infection, 17.07% and 63.41%, respectively). Subsequent increasing in the lactation numbers gradually decreases, where the infection rate with clinical ranged from 17.07 % to 10.7% and with subclinical mastitis was ranged from 63.41% to 35.71%. there is a negative relationship between the lactation numbers of the dairy cows and prevalence of clinical and subclinical mastitis may refer to a build-up of acquired immunity by subsequent lactation seasons. Bacteriologically, the bacteriological examinations of the collected milk samples of cows with clinical and subclinical mastitis concluded that Staph. aureus (47.40%) was the predominant isolated mastitis pathogens followed byCNS (25.43%), Strept. agalactia(24.86%),E.coli (2.31%). This may indicates the veterinary importance of contagious mastitis pathogens as a major cause of mastitis of dairy cows. Mycoplasma examination revealed that 92.31% (12of 13) tested milk samples from clinically mastitic cases which didn’t respond to treatment were culturally positive and showing characteristic fried egg colonies under stereoscopic dissecting microscope. All (12) isolates were sensitive to digitonin so were classified as belonging to members of genus Mycoplasma and not belonging to the genus acholeplasma. Biochemical testing to characterize isolated Mycoplasma revealed that (2) isolates were Glucose negative, Arginine negative and Film and spots negative whereas (10) isolates were Glucose negative, Arginine negative and Film and spots positive. In the present study the results of antimicrobial sensitivity test for Staph. aureus strains isolated from mastitic cows, revealed that ciprofloxacin and levofloxacin were the most effective antibiotics (100%) for each, followed by cephoperazone and trimethoprim sulphathoxazole (86.70%) for each, cephradine and oxytetracycline (66.70%) for each, while amoxicillin (46.70%) and penicillin and Ampicillin (40%) for each, were the least effective drugs in this work. PCR technique was done on 10 Mycoplasma isolates which were Digitonin sensitive, glucose negative, arginine negative and film and spots positive, revealed that 70% (7 of 10) isolates were positive for M. bovis referring to the beneficial value of PCR technique.