Dr.Yilkal Asfaw, Dr. Kelay BelihuNIBRET, MOGES2020-06-172023-11-082020-06-172023-11-082007-06http://etd.aau.edu.et/handle/123456789/21634A cross _ sectional study was conducted on 322 local and crossbred lactating hand milked small holder cows in and around Gondar from September 2006 to March 2007 to determine prevalence, the causal agents of infection, associated risk factors and tests susceptibility to drugs using California Mastitis Test, clinical inspection of udder, and bacterial culture. Of the total cows examined, 32.6% (322) had mastitis, 0.93 % (3) was clinical and 31.67% (102) sub clinical mastitis. Out of 1249 quarters examined 164 (13.13%) were found to be infected, 7(4.27%) clinically and 157 (95.73%) sub clinically from 1288 quarters examined 39 (3.02%) were blind. Clinical prevalence at cow level was 0.93% in crossbreds and none in local zebu breeds. Sub clinical mastitis at cow level based on CMT was 28.57% in crossbreds compared to indigenous zebu 3.10% (P<0.05). Quarter sub clinical prevalence based on CMT was 16.05% and 5.12% for crossbreds and local zebu, respectively. Among potential risk factors considered breed, age, parity and stage of lactation were found to affect the occurrence of mastitis significantly (p< 0.05). The prevalence of mastitis was significantly higher in Holstein _Friesian crossbreds than indigenous zebu, in the early lactation stage than in the mid lactation stage. Of 176 CMT and clinically positive udder quarter samples analyzed microbiologically 164 were culturally positive for known mastitis pathogens and 1085 were negative. Of the 164 positive samples isolation rate of Staphylococcus aureus 16.5%, Coagulase negative Staphylococci 31.1%, Streptococcus agalactiae 15.9%, Streptococcus dysgalactiae 14.0%, Streptococcus uberis 6.7%, Micrococcus spp 7.3%, Corynebacterium bovis 2.4%, Actinomyces pyogens 1.2%, Bacillus cereus 0.6% and Escherichia coli 4.3%. Staphylococcus aureus, Coagulase negative Staphylococcus, Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, Micrococcus species, Corynebacterium bovis, Actinomyces pyogenes, Bacillus cereus and Escherichia coli were isolated from cases of sub clinical mastitis. Generally, it was found that Sulfisoxazole was the most effective antibiotic VII where 91.07% of the total isolates were found susceptible, followed by clindamycin and kanamycin (89.3%), (88.4%) The least effective antibiotics were streptomycin (45.5%) and ampicillin (49.1%). Tetracycline, erythromycin, chloramphenicol, oxacillin, 65.2%, 59.8%, 64.3%, 58.04% respectively, were categorized as drugs of weak efficacy. Inadequate sanitation of dairy environment, poor animal health services and lack of proper attention to health of the mammary glands were important factors contributing to high prevalence of mastitis. Some recommendations were forwarded to reduce the tend of mastitis in the study area.enclinical mastitissub clinical mastitisprevalencebacterial isolateThesis