Comparison of success of antibiotic therapy during lactation and results of antimicrobial susceptibility tests for bovine mastitis.

Antimicrobial susceptibility testing was conducted on a variety of mastitis pathogens. The infected quarters were subsequently treated during lactation with a commercially available product containing penicillin and novobiocin that was designed for lactating cows. Cows were treated as per the recommendations of the product manufacturer, and cures were determined by the absence of bacteria in both sets of duplicate quarter milk samples that were collected at 28 d posttreatment. Comparisons were made between the susceptibility of the bacteria and the therapeutic success or failure. All isolates tested were considered to be susceptible to the penicillin and novobiocin combination. Bacteriologic cure rates for newly acquired Staphylococcus aureus intramammary infection (IMI) (< 2 wk in duration) at 28 d posttreatment were 70%. Cure rates for chronic Staph. aureus IMI (> 4 wk duration) were much lower (35%), reaffirming previous reports of the intractable nature of chronic Staph. aureus IMI. Cure rates for subclinical IMI caused by other organisms were 90% for Streptococcus agalactiae, 91% for Streptococcus uberis, 90% for Streptococcus dysgalactiae, 77% for other Streptococcus spp., and 71% for Staphylococcus spp. other than Staph. aureus. In vitro testing was considered to be a predictor of therapy outcome for IMI caused by Staphylococcus spp., newly acquired Staph. aureus, Strep. uberis, Strep. dysgalactiae, and Strep. agalactiae, but was not considered to be a useful predictor of efficacy for chronic IMI caused by Staph. aureus.