The primary objective of this study was to determine the aetiology of bovine mastitis in ten herds of Holstein Friesian cow in Jordan, the prevalence of mastitis pathogens in dairy cows and their resistance to selected antimicrobial agents. Milk samples were collected from 220 lactating cows to determine the clinical and subclinical mastitis by white side test and confirmed by cultural tests. It was found that 138 quarters 15.7% had been diagnosed as clinical mastitis with definite gross pathological lesions and change in udder secretion, in addition to that 276 quarters 31.4% showed subclinical mastitis. The incidence of mastitis was found to be increased in older cows. Staphylococcus aurous was considered to be the most common cause of both clinical and subclinical mastitis and followed by coli forms, streptococcus spp., corynebacterium spp., proteus spp. and pseudomonas spp. Sensitivity tests was applied to different isolated strains using tertracycline, ampicillin, neomycin, erythromycin, penicillin G and sulphamethoxazol trimethoprim.

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