Dr. Biruk Tesfaye, Dr. Tesfaye SisayDANIEL, FISSEHA2020-06-182023-11-082020-06-182023-11-082014-06http://etd.aau.edu.et/handle/123456789/21727Mastitis is a major disease in dairy cattle of Ethiopia. The objectives of this study were to identify aerobic bacterial pathogens, estimate the incidence of mastitis, identify the associated risk factors and assess drug resistance patterns of bacterial isolates in heifers mastitis. The study was conducted for eight months and a total of 124 quarters were examined to detect clinical mastitis and subclinical mastitis by udder physical examinations and the California mastitis test (CMT) respectively. California mastitis test positive milk samples were used to isolate and identify bacteria. The study revealed that the incidence rate of mastitis per gland month at risk was 39.45%. The incidence of mastitis was found to be affected by management system, udder hygiene, dry cow therapy and lactation stage (p < 0.05). The percentage of CMT positive glands that remained positive after a month and the percentage of CMT negative glands that remained negative after a month were 57.14% and 79.91% respectively. Staphylococcus aureus (25.11%), Coagulase negative staphylococci (CNS) [14.70%] and Streptococcus species (14.70%) were the predominant bacteria. In the present study, out of eight in vitro antimicrobials used; gentamycine (0%), kenamycin (0%), ampicillin (62%), penicillin G (89%), chloramphinicol (8%), Cefoxitin (16%), tetracycline (84%) and trimethoprim (8%), showed resistance to heifer mastitis pathogens. Gentamycine and chloramphinicol were the most effective antibiotics among all the tested ones. Finally due to the high resistance level detected in the study, it was believed that it is necessary to set up antimicrobial resistance surveillance programmes in the country. It was also concluded that the prevalence of Staphylococcus aureus warrants serious attention, the application of antibacterial agents earlier before calving may help to reduce the incidence of mastitis during the early lactation period.enHeifer mastitisincidence raterisk factorsbacterial isolatesThesis