Relationship between in vitroantimicrobial sensitivity of bovine subclinical mastitis isolates and treatment outcome in lactating dairy cows

The objective of the present study was to determine whether there was an association between the in vitroantimicrobial sensitivity test results of subclinical mastitis pathogensand bacteriological cure following intramammary treatment using a combination of nafcillin, penicillin, and streptomycin (NPS). Eighty-six intramammary pathogens from 43 cows were examined inthis study. Most intramammary infections were due to coagulase-negative staphylococci, coliforms, and environmental streptococci. The antibiotic sensitivity to NPS was determined using the Kirby-Bauer disc diffusion method. Bacteriological cure rates for sensitive, intermediate, and resistant isolates in the standard treatment group (3 intramammary infusions of NPS once daily) were 84.4, 88.9, and 100%, respectively. These figures in the extended treatment group (6 intramammary infusions of NPS once daily) were 100% for the 3 categories. Bacteriological cure was not associated with the sensitivity test result. Based on this study, Kirby-Bauer sensitivity test results were not useful as predictors of the bacteriological outcome ofsubclinical mastitis treated with intramammary NPS.