From January to May 2018, a cross-sectional study was undertaken on lactating dairy cows in Bishoftu town to isolate Corynebacterium bovis, determine the prevalence and risk factors, and evaluate the effectiveness of several antibiotics in lactating dairy farms. Study animals were selected randomly from selected dairy farms in the area. Collecting milk samples from mastitic cows, cultivating, and then performing an antibiotic sensitivity test were the procedures followed. A total of 384 lactating dairy cows were examined with inspection and California Mastitis Test (CMT), in which 86 of them were found to be CMT positive. Accordingly, prevalence was 3.9% and 18.5% for cows affected by clinical and subclinical mastitis respectively. The prevalence of mastitis showed statistically significant difference between, lactation stage, breed, age and washing (p > 0.05). However, there was no statistically significant difference noted in animal husbandry practice (p > 0.05). A total of 384 lactating dairy cows were examined with inspection and CMT, in which 86 of them were found to be CMT positive. Out of the 86 mastitis positive samples (sample indicates milk from one cow) sent to microbiology laboratory for microbiological examination, 7 bacterial isolates were identified as Corynebacterium bovis. The biochemical and morphological characteristics of 7 (1.8%) C. bovis isolated from bovine milk samples and the C. bovis reference strains were found to be uniform. Valuable criteria for identification were presence of catalase and oxidase, production of acid from glucose and a requirement for enriched basal media. C. bovis isolates have revealed a higher sensitivity to the kanamycin and streptomycin (71.4% each). A certain resistance has been noted to oxytetracycline (71.4%) and nalidixic acid (42.8%). Higher number of isolates showed moderate sensitivity or resistance to amoxicillin (51.1%). Regarding to multidrug resistance, the study reflects that only one isolate (14.3%) shows multidrug resistance to four drugs namely kanamycin, amoxicillin, nalidixic acid and oxytetracycline. This study demonstrated that mastitis due to C. bovis is rare in lactating dairy farms in Bishoftu. Some of the risk factors for mastitis can be addressed by practical management of dairy cows. Farm owners should selectively use the antibiotics to which the bacteria do not show resistance, such as streptomycin and kanamycin.
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