Relationship between results of in vitro susceptibility tests and outcomes following treatment with pirlimycin hydrochloride in cows with subclinical mastitis associated with gram-positive pathogens.

OBJECTIVE To determine the association between results of in vitro antimicrobial susceptibility tests and outcomes in cows that received intramammary treatment with pirlimycin hydrochloride for subclinical mastitis associated with gram-positive pathogens. DESIGN Case-control study. ANIMALS 132 dairy cows (178 mammary glands with subclinical mastitis caused by 194 pathogen isolates). PROCEDURES Cows with positive results for a California mastitis test (CMT) were assigned to receive 50 mg of pirlimycin via intramammary administration into each CMT-positive mammary gland every 24 hours for 2 consecutive days or no treatment. Duplicate milk samples were collected before treatment and approximately 21 days later. Target pathogens included coagulase-negative Staphylococcus spp (n = 118 isolates), Streptococcus spp (28), Staphylococcus aureus (7), and other gram-positive cocci (30). Antimicrobial susceptibilities were determined via broth microdilution. RESULTS Overall treatment success rate was 66% (128/194) for both groups. In vitro resistance to pirlimycin ranged from 0% (0/7 isolates of S aureus) to 50% (13/26 isolates of other gram-positive cocci). For the treated group, 62 of 94 (66%) target pathogens were classified as treatment successes and 32 (34%) were classified as failures. Similarly for the control group, 66 of 100 (66%) target pathogens were classified as treatment successes, whereas 34 (34%) were classified as failures. CONCLUSIONS AND CLINICAL RELEVANCE Many target pathogens from cows with subclinical mastitis were eliminated without treatment, and treatment with pirlimycin did not improve the treatment success rate. Results of in vitro antimicrobial susceptibility tests were not useful as predictors of treatment success following intramammary treatment with pirlimycin.