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Mastitis caused by Staphylococcus (S.) aureus and Streptococcus agalactiae (Str.) agalactiae is economically important in the dairy industry. These microorganisms are transmitted mainly during milking. The chronically infected cows are the major reservoirs of infection. Dairy herds in urban and periurban dairy production systems in Ethiopia were examined for the prevalence of S. aureus and Str. agalactiae mastitic cases. Based on the number of milking cows, the dairy herds were categorized into "large" (>30), "medium" (11-30) and "small" (up to 10). The herds were visited every six weeks beginning May 1999 until November 1999. Data for May were used to calculate the prevalences and risk factors for both microorganisms and incidence rates were calculated from subsequent longitudinal data. Isolates from the first and last sampling periods were finger-printed with the Pulsed Field Gel Electrophoresis (PFGE) in order to substantiate whether new strains have emerged either through changes in the existing population or through introduction of new strains. Antibiotic sensitivity profiles of S. aureus were also investigated. A total of 140 S. aureus and 299 Str. agalactiae isolates were obtained from mastitic milk samples during five sampling periods of 6-weeks interval. The prevalence of S. aureus mastitis was 27.1%, 26.5% and 7.1% in large, medium and small herds, respectively. The corresponding quarter prevalences for these herd categories were 10.3%, 8.1% and 2.6% and were significantly different (p=0.001) between categories. Proportionaly, clinical, subclinical and non-clinical mastitis did represent 7.8%, 47.1% and 45.1%, respectively. Apparently, 19.1% of clinical and 11.5% of subclinical mastitis were caused by S. aureus. Incidence rates of 498, 173 and 215 cases per 1000-cow-years at risk were obtained for large, medium and small herds, respectively. The most predictive risk factors for S. aureus mastitis were large herd size, the presence of floor gradient, teat washing and traditional treatment practices. In large, medium and small herds, the corresponding prevalences of Str. agalactiae mastitis were 20.5%, 19.0% and 10.7%. There was a wide variation of prevalences among herds ranging from 0 to 36.7% but hese differences were not significant among herds or herd categories. Udder quarter prevalences were 9.4% each in large and medium herds and 4.8% in small herds. The majority (65.5%) of Str. agalactiae udder infections were subclinical, 27.1% non-clinical and only 9.4% of infections were clinical. The mean incidence rate in large, medium and small herds were 921, 328 and 292 cases per 1000-cow-years at risk. The most predictive risk factors were size of the herd, older cows, large herds and teat washing. Four clusters of S. aureus isolates were identified on the basis of PFGE profiles. More than 73% of them were grouped into one cluster (cluster I) which did occur 90% clinical, 71.7% subclinical and 66.7% non-clinical mastitis. The remaining 3 clusters did contribute further 5% to 12.6%. The same number of clusters was also identified in Streptococcus agalactiae isolates. Half of them belonged to cluster I which was more homogenous than the remaining three clusters. It was isolated from 23.1% of clinical, 46.2% of subclinical and 30.9% of non-clinical mastitic cases. Although infection of three herds with new clusters were found, it could not be verified whether these infections were due to genetic changes from existing populations or due to introduction of new strains from outside. Antibiotic resistance levels of S. aureus isolated during the initial sampling period were as follows: 82.4%, 60.8% and 37.8% to penicillin, tetracycline and streptomycin, respectively. The enzyme ß-lactamase was produced from 76.4% of the isolates indicating that the mechanisms of resistance are mainly due to he production of this enzyme. Most isolates (67.5%) were resistant to multiple drugs.