A cross-sectional study was carried out to determine the prevalence and risk factors of bovine mastitis caused by Streptococcus agalactiae from farms in and around Haramaya district, eastern Ethiopia. A total of 384 lactating cows were selected from small-, medium-, and large-scale production systems. California mastitis test (CMT) was used for screening subclinical mastitis. Out of the total animals examined, 63.02% ( n = 242) had mastitis, where 6.77% ( n = 26) and 56.25% ( n = 216) were clinical and subclinical mastitis respectively. The quarter-level prevalence was 29.04% ( n = 446), from which the clinical form was 6.38% ( n = 98) and the subclinical was 22.66% ( n = 348), and the rest quarters were blind 3.32% ( n = 51). Milk samples from clinical as well as CMT positive quarters were cultured for isolation of S. agalactiae , where 10.3% ( n = 46) resulted in growth of the bacterium. The prevalence of mastitis was found to be statistically significant among the age groups ( p = 0.002), breed ( p = 0.000), and parity ( p = 0.000). Similar findings were found to the extrinsic risk factors considered; as production type ( p = 0.010), teat injury ( p = 0.02), and type of floor ( p = 0.000). The study confirmed the importance of S. agalactiae as the cause of contagious mastitis and also identified the associated risk factors in the study farms and hence warrants serious attention.

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