Comparison of seven antibiotic treatments with no treatment for bacteriological efficacy against bovine mastitis pathogens.

Milk culture results were retrospectively reviewed from 9007 cases of subclinical mastitis affecting cows housed in dairy herds located in New York and northern Pennsylvania. Cases included in this analysis had at least one mastitis pathogen isolated from the initial milk sample, were recultured within 1 mo, had permanent cow identification, and had records of whether mastitis was treated with an antibiotic or no treatment at all. Overall bacteriological cure rate for 21 mastitis pathogens was 68% (6097 of 9007). Antibiotic treated cases had a higher cure rate (75%) than did untreated cases (65%). Antibiotic treatments that significantly differed from the untreated cure rate of 65% were amoxicillin (82%), erythromycin (76%), cloxacillin (73%), and pirlimycin (44%). Cure rates for antibiotic treatments with cephapirin, hetacillin, or penicillin did not differ from the untreated cure rate. Agents for which some antibiotics were associated with increased cure rates compared with no treatment were Streptococcus agalactiae, streptococci other than Strep. agalactiae, and coagulase-negative staphylococci. The antibiotic most commonly associated with higher cure rates was amoxicillin. Most of the 21 mastitis agents showed no difference in bacteriologic cure rates between any of the 7 antibiotic treatments and no treatment.