The objective of the present study was to determine whether there was an association between the antimicrobial susceptibility
test results of subclinical mastitis pathogens and bacteriological outcomes of intramammary treatments using cefquinome. A total of 110 intramammary pathogens from 51 cows were assessed in this study. Most intramammary infections were due to coagulase-negative staphylococci, environmental streptococci, and coliforms. The antimicrobial susceptibility
was determined using the Kirby-Bauer disc diffusion method. Bacteriological cure rates for the sensitive, intermediate, and resistant isolates in the standard treatment group (three intramammary infusions of 75 mg cefquinome
at 16 h intervals) were 82.4%, 90%, and 87.5%,respectively. These figures in the extended treatment group (six intramammary infusions of 75 mg cefquinome
at 16 h intervals) were 83.3%, 100%, and 100%, respectively. Treatment outcomes were not associated with the results of sensitivity tests in the standard group. However, in the extended group, the probability of a bacteriological cure was lower in quarters from which cefquinome-sensitive pathogens were isolated than the quarters from which intermediate or resistant pathogens were isolated. Based on this study, the Kirby-Bauer susceptibility
test result is a poor predictor for the bacteriological cure of subclinical mastitis treated with intramammary cefquinome.